Energy and Buildings 72 (2014) 441–456
Contents lists available at ScienceDirect
Energy and Buildings
journal homepage: www.elsevier.com/locate/enbuild
A review of high R-value wood framed and composite wood wall
technologies using advanced insulation techniques
Jan Kosny a,∗ , Andi Asiz b , Ian Smith c , Som Shrestha d , Ali Fallahi a
a
Fraunhofer CSE, Boston, MA, USA
Prince Mohammad Bin Fahd University, Saudi Arabia
University of New Brunswick, Fredericton, Canada
d
Oak Ridge National Laboratory, Oak Ridge, TN, USA
b
c
a r t i c l e
i n f o
Article history:
Received 19 August 2013
Received in revised form
26 December 2013
Accepted 2 January 2014
Keywords:
Building energy
Building envelopes
Thermal insulation
a b s t r a c t
The main objective of this study is to indentify advanced wall frame assemblies applicable for residential
and small commercial buildings, that have or could reach R-values larger than RSI – 3.5 m2 K/W (U-value
lower from 0.29). An extensive literature review of existing and past practices is used as the main vehicle
to analyze: framing and wall insulation methods, architectural details with focus on minimizing thermal
bridges, structural adequacy aspects with respect to gravity and lateral loads, and ability to provide fire
and sound breaks. In this paper a wide selection of advance framing wall assemblies is discussed in
details with main focus on construction methods, architectural details with minimized thermal bridges,
and structural (strength) concerns. High performance wall technologies of consideration include: double
walls, Larsen truss walls, optimum or advanced framing walls, walls using distance spacers (furring) and
walls made of wood-based composites. Since wood framing for wall applications is mostly used in North
America, Scandinavia, and Central Europe, this study is focused on research studies from these regions. In
addition, field test studies are presented discussing an application of high R-value of new and retrofitted
wall assemblies in actual test houses that have been constructed and being currently monitored.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Various super insulated homes with high R-value wall frame
assemblies have been built and investigated in North America and
Europe since the energy crisis in the early 1970s. Even until today,
a number research projects focused on high thermal performance
envelopes has still been high due to increased concerns on issues
related to environmental impacts (sustainable or green construction), saving more building energy consumption (zero net energy
building), and higher thermal and occupant comfort expectation
than those in the past. Wood-based wall technologies are widely
considered as primarily building materials for low-environmentalimpact buildings [1–4]. For example, current research performed
by the Mid Sweden University demonstrated that both the primary
energy consumption as well as the CO2 emission generated during
production of materials used in building construction are lower in
case of wood-framed constructions than for concrete buildings [5].
In the past Oak Ridge National Laboratory (ORNL) Buildings
Technology Center (BTC) conducted in Oak Ridge, TN, U.S.A., a
detailed experimental and numerical analysis of over 150 wall
∗ Corresponding author. Tel.: +1 617 714 6525.
E-mail address: jkosny@fraunhofer.org (J. Kosny).
0378-7788/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.enbuild.2014.01.004
technologies including advanced wood-framed walls. For architectural details thermal performance comparisons are available base
on results of detailed three-dimensional finite difference modeling.
For each considered wall technology clear wall computer model
was validated against the hot-box test data. This information is
available as the Whole Wall R-value Database [6–9]. Downloadable
from Internet Whole Wall R-value Calculator enables direct performance comparisons of wall technologies and detailed analysis of
wall architectural components [10]. In addition, a wide selection of
high thermal performance walls constructed in actual homes have
been studied and monitored by the ORNL [11]. Some of these walls
have effective R-values exceeding RSI – 4.4 m2 K/W. The key wall
technologies used in ORNL research houses include structural insulated panel (SIP), optimum value engineering (OVE), double wall
with composite framing, and exterior insulation and finish system
(EIFS).
In recent study conducted for the U.S. Department of Energy
Building America Program by Straube and Smegal [12] summarized high R-value of walls that have been tried out in North
American residential buildings, ranging from double stud walls to
exterior insulation finish wall system. The main analytical method
in their study was two-dimensional steady-state heat flow model
to quantify thermal performance of wall assemblies with incorporated some effects of integral structural components. Additional
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J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
quantitative measure related to durability was also imposed using
a simplified one-dimensional hygrothermal model to estimate
potential risk of condensation of those various walls [13–15]. One
of the main conclusions obtained in that study was that adding
exterior insulation to most wall assemblies has many advantages
in term of thermal performance aspect due to continuous thermal
break provided. They also concluded that new wall frame assemblies with high R-value insulation materials could have durability
issue due to high condensation risk in outer layers of building envelope (e.g. sheathing), and careful detailing has to be practiced to
avoid it.
During last two decades research from Fraunhofer IBP (Institute
for Building Physics), Germany has investigated different ways of
constructing high energy efficient walls while providing excellent
long term durability [14]. It was demonstrated that from long term
durability perspective, in heating dominated climates, destructive temperature or humidity conditions caused by poor insulation
and inappropriate material selection can be easily avoided. It was
also found that for many constructions working well in specific
climates, changes in application location or building operation
schedules may lead to hygrothermal and energy performance problems within the structure [15].
Several high performance insulation options including vacuum
insulations and aerogels have been recently analyzed and tested
by Fraunhofer CSE, U.S.A. in wall retrofit applications [16,17]. In
2010 a demonstration project was initiated with one story wood
framed building insulated with vacuum insulation. In this case
Vacuum Insulation Panels (VIPs) were sandwiched with expanded
polystyrene (EPS) foam for mechanical protection and installed on
the exterior face of the wall.
In Canada thermal performance of residential and other buildings has become a dominant focus of changes to construction
practices with special requirements for prescriptively built housing and small buildings being approved in 2012 (Part 9 of the
National Building Code), and a more generalized National Energy
Code of Canada for Buildings published in 2011 (http://www.
nationalcodes.nrc.gc.ca/eng/necb/index.html).National Research
Council of Canada (NRC) has conducted several projects developing
high thermal performance wall assemblies that can be applied in
extreme northern climates [18]. Several construction criterions
applicable in the northern Canada including constructability,
cost, durability, sustainability, socio economic have been imposed
before selecting suitable building envelope systems. Based on this
study, NRC has recommended using double stud wall, standoff
truss wall, structural insulated panel, or structural insulated
concrete wall system in wall housing assemblies to reach R-value
larger than RSI – 5.3 m2 K/W. Some of these walls have been tested
in the NRC laboratory for further verification.
In principle, thermal performance of wall frame assemblies can
be increased by either: applying thicker and wider insulation space
in wall cavity; installing insulating sheathing; improving thermal
resistance of insulation materials; reducing or eliminating thermal
bridging; or applying airtight construction. Combination of these
methods is normally applied in practice to reach high R-value and
sometime to improve other (wall or building) performance aspects
such as durability, constructability, and costs. The main objective
of this study is to indentify wood frame or composite assemblies
applicable for residential and small commercial buildings that have
or could have R-values larger than RSI – 3.5. In this paper, double
wall, Larsen truss wall, optimum or advanced framing wall, European walls, and walls with furring and composites are considered.
Advance framing assemblies amongst other walls will be discussed
in details with main focus on construction methods, architectural details with minimized thermal bridges, structural (strength)
concerns, and capability of providing fire and sound break. Since
the scope of this work is mostly on wood-frame or wood-based
composite assemblies, other types of high R-value walls such as for
example Structural Insulated Panels (SIP) are not covered. Advantages of using balloon framing construction in minimizing thermal
bridges between floors are discussed in certain applications. Utilization of engineered wood composites to deal with strength
requirement and reduced material sizing to accommodate insulation placement is also presented. Case studies are discussed using
actual test houses constructed using selected high thermal walls
performance to demonstrate their effectiveness in reducing building energy consumption.
2. Prospect reductions of whole building energy
consumption as a function of improvements in R-value
Building envelopes play an important role in the heat transfer
between the exterior and the interior spaces of the building. From a
thermal perspective, a well-performing wall is one that contributes
to thermal comfort inside the building with minimum consumption
of space conditioning energy [1,19]. During the last two decades,
in the U.S. hundreds of building envelope technologies have been
evaluated using a hot-box testing and advanced numerical thermal analysis. This collection of technical information, field and
lab test thermal performance data, and three-dimensional thermal
analysis, enables an objective evaluation of the existing building
envelope technologies. R-values or U-values have been used for
decades as measures of thermal performance of building envelope components. For most wall systems, the part of the wall that
is traditionally analyzed, that is, the flat part of the wall that is
uninterrupted by details (clear wall), comprises only 50–80% of the
total area of the opaque wall. The remaining 20–50% of the wall
area is not analyzed nor are its effects incorporated in the thermal
performance calculations. For most of the wall technologies, traditionally estimated R-values are 20–30% higher than overall wall
R-values [6,10]. Such considerable overestimation of wall thermal
resistance leads to significant errors in building heating and cooling
load estimations.
The DOE-2.1E and Energy Plus computer codes were utilized
to simulate a single-family residence in representative U.S. climates. A reason for selection of this computer tool is coming
from the fact that this building model had been extensively validated in the past over the field experimental data [20,21]. The
standard building selected for this purpose is similar to a singlestory ranch style house that has been the subject of previous
energy efficiency modeling studies [22]. U.S. residential buildings’
standards, including ASHRAE 90.2 [19], are based on the whole
building energy analysis performed with the use of the same
house. In this study ten lightweight wood-frame walls of R-values
from 0.4 to 6.9 m2 K/W were simulated in ten U.S. climates. The
heating and cooling loads generated from these building simulations are used to estimate the relation between wall R-value and
whole building energy consumption in conventional wood-framed
house.
Numerical results demonstrated that wall R-value change
between most-commonly used today in North American residential buildings RSI – 2.2 m2 K/W and expected in the close future RSI
– 3.6 may generate between 5% and 8% changes in the buildingenvelope-generated whole building energy consumption (Fig. 1).
Considering that in most of North American residential buildings, walls may generate in average up to 25% of total loads
associated to building envelopes (U.S. DOE Building Energy Data
Book – http://buildingsdatabook.eren.doe.gov/), it is a substantial
improvement of the whole building performance generated by only
a single building enclosure component (like wall). In that light, wall
framing improvements can be considered as an important source
of future energy savings in residential buildings.
J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
Fig. 1. Total whole building energy consumption for twelve lightweight woodframe walls calculated using whole building energy simulations for one story
rancher.
3. Thermal effects of wall framing improvements and
cavity insulation enhancements
In the current practice of testing and theoretical analysis of
building envelope assemblies, the amount of structural members
incorporated into the total wall area is commonly called a framing
factor. It used to be expressed as a percent of the total wall area [23].
Traditionally, for wood-framed walls, the wall area represented
by framing members (framing factor) has been considered to be
between 10% and 14%. According to the report prepared in 2002 by
Enermodal Engineering for California Energy Commission, there is
27% framing in current residential walls in California [24]. A similar
study performed by the American Society of Heating Refrigeration
and Air-conditioning Engineers (ASHRAE) in 2003 has concluded
an average 25% of framing factor for all U.S. residential buildings
[25].
It is well known that a presence of framing members (like wood
or steel profiles) will reduce the nominal R-value of a wall system.
The measure of this effect is known as the framing effect coefficient
‘f’ of a wall. One of the methods to calculate f is using the following
simple expression
f = 1−
Rcw
Rn
× 100%
(1)
where Rcw is the simulated clear wall R-value for the considered
wall configuration, Rn is the nominal R-value (in-series center of
cavity R-value) for the considered wall configuration.
Five wall configurations containing different framings are
numerically analyzed to study the effect of framing percentage on
wall R-value. For comparison, wood framed walls are compared
with walls containing light gage metal studs. Clear wall R-values
for each wall configuration were computed using finite difference
code. A 28 ◦ C temperature difference with a mean temperature of
24 ◦ C was maintained between the two external surfaces. For these
configurations, the nominal in-cavity R-value, which is the R-value
of a system without considering any framing, was calculated and
found to be RSI – 2.3 m2 K/W. Next, framing effect coefficient was
calculated using equation 1. The simulation results are shown in
Table 1.
In both the wood and steel frame walls it can be observed that
increasing the framing in wall reduces the R-value. The increase in
framing effect coefficient in wood frame wall is almost consistent
with increase in percentage of framing whereas in steel walls this
effect is more prominent. It can be noted from these results, that
steel framing generates 3–4 times higher framing effect comparing
to wood stud walls. When using steel framing, a building needs
443
to be carefully designed due to possible strong thermal bridging
effects.
One of the most common problems in North American wood
framed buildings is precision of installation of structural components. Very often the studs offset by ±25 mm. or situations where
the wood studs are twisted because of moisture loading and later
drying effects. In addition, some structural members can be buckled under structural, unsymmetrical moisture, and thermal loads
(one side of the stud more effected). When installing batt insulation in places where stud distances are not equal to nominal, it is
very common to find un-insulated air gaps or locations where batts
are compressed. Poorly installed insulation can significantly impair
the thermal performance of building components.
At present, in North American residential retrofit applications
a use of cavity fiber insulations is a most popular, low-cost insulation option [26]. However, the R-value of these applications is
very limited due to the cavity space restrictions. Fiberglass batts
are the most popular wall cavity insulation. In addition, cellulose is the best-known blown application; however, it is not the
best-performing option. Other alternatives involve applications
of foam cavity injections. These applications may damage cavities (expanding foams), or create undesired air voids (shrinking in
time, so-called non-expanding foams). In that light, blown aerogel
insulation with thermal conductivity as low as 25 mW/m K can be
considered as an attractive potential alternative to the above-listed
methods. It is expected that conventional 2 × 4 wood stud wall
(0.14-m thick) may reach RSI -value of about R – 4.4 m2 K/W. Blown
aerogel for existing stud walls is not fully commercially available
yet. Fraunhofer CSE is currently working with industrial partners
to begin test evaluating this technique as a potential wall retrofit
measure [16].
At this stage of technology development, we assume that the
blown aerogel is produced from chopped aerogel blankets – see
Fig. 2. In the future, it is anticipated that an aerogel product
developed specifically for this application, using the cost reducing approaches outlined in Fraunhofer CSE 2013 report [16], would
reduce material costs by 30–50%. Moreover, the blown aerogel
insulation could be easily blended with other fiber insulations –
bringing lower-cost alternatives to the U.S. building insulation market.
4. Insulating sheathing – a simple and effective way of
increasing wall R-value
It is widely known that installing continues insulating sheathing is one of the simplest ways to improve a thermal performance
of wall systems. Addition of sheathing insulation is not a new
approach. During last two decades of 20th century, thermal efficiency of insulating sheathing was analyzed by numerous authors
including Barbour et al. [27], Trethowen [28], Strzepek [29], and
Christian and Kosny [10]. Rigid foam sheathing can enhance building air-tightness as well. Foam sheathing can be located on either
the exterior or interior wall surfaces. It is important to remember that insulating sheathing is changing mean temperature for the
cavity insulation. That is why, for high R-value walls, different nominal thermal conductivities of the cavity insulation may need to be
considered for different insulation configurations. Framing effect
is a very convenient measure of thermal efficiency of the insulating sheathing. It represents the R-value reduction generated by the
framing members (in case of framed wall technologies – studs and
tracks) [7].
Whole building energy simulations presented above demonstrated that whole building energy consumption can be effectively
controlled by wall R-value changes. Annual energy loads for twelve
lightweight wood-frame walls were calculated using DOE-2.1E
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J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
Table 1
R-values and framing effect coefficients for different percentage of framing in wood and steel framed walls.
R-value (m2 K/W)
Percentage of framing
5% framing
8% framing (∼610 mm o.c.)
11% framing (∼400 mm o.c.)
11% framing (∼610 mm o.c. with track)
14% framing (∼400 mm o.c. with track)
Framing effect coefficient, f (%)
Wood
Steel
Wood
Steel
2.13
2.05
1.97
1.97
1.90
1.64
1.34
1.21
1.19
1.09
7.13
10.75
14.27
14.07
17.16
28.79
41.49
47.40
48.21
52.70
Table 2
Expected thermal load savings in conventional 2 × 4 wood-framed house after 25 mm. EPS foam sheathing was analytically added to the exterior walls.
Location
Atlanta
Bakersfield
Boulder
Chicago
Fort Worth
Miami
Minneapolis
Phoenix
Seattle
Sterling
% load savings
8.4
11.8
8.3
7.2
7.6
3.8
6.8
6.8
4.7
7.8
energy simulations. Regression analysis was performed to evaluate the relationship between steady-state clear wall R-values (of
wood-stud walls) and the total building loads (associated to building enclosure) for ten U.S. climates. A strong correlation (r2 of
about 0.98) was found between wall R-values and whole building
loads.
After taking into consideration that for wood-framed houses,
the whole wall R-value is about 8% lower from the clear wall R-value
[10], 25 mm of EPS foam sheathing gives in average 7.3% of savings
in that part of the whole building energy consumption which is
generated by building enclosure. Summarized data generated by
described-above energy simulations is shown in Table 2 for ten U.S.
locations. Comparisons are made against conventional 2 × 4 wood
framing assembly.
EIFS (Exterior Insulation Finish System) is a wall technology that
utilizes rigid insulation sheathing and plaster finish on the exterior wall surface. Current EIFS walls typically consist of expanded
polystyrene (EPS) board attached adhesively or mechanically to
the structural sheathing boards and covered with a lamina composed of a modified cement base coat with woven glass fiber
reinforcement and a textured colored finish coat (Fig. 3). In principle, the strategy to minimize thermal bridging is similar to that of
the advanced wall frame with exterior insulation sheathing [30].
Thermal performance of EIFS wall is heavily dependent on the
thickness of the exterior insulation applied. For example, using
100 mm thick EPS foam board with empty 2 × 4 wall stud cavity
yields R-value of around RSI – 3.5 m2 K/W. If cellulose or fiberglass
insulation is added into the 2 × 6 wall stud cavity in addition to the
100 mm EPS foam board, the overall wall R-value of RSI – 5.3 m2 K/W
can be obtained [12]. It should be noted however that building
codes in most North American jurisdictions have limited the maximum exterior foam insulation thickness to 100 mm due to fire
performance issue emerged from fuel contribution of the insulation
material.
A major concern using EIFS in the past was a moisture performance issue due to poor detailing practice related to water
drainage. However, over the years, EIFS walls have been further
developed and upgraded to overcome this issue. In fact, field
monitoring and laboratory tests results have indicated that EIFS,
being one of the most tested wall assembly, demonstrated positive performance with respect to moisture management system
and thermal control [31]. After exposing these wall systems to real
weather for 30 months, it was found that the best performing wall
cladding was the EIFS wall with 100 mm of EPS insulation and a
fluid-applied water-resistive barrier. It was also found from that
study, that EIFS drainage assemblies using vertical ribbons of adhesive provide a drainage path and air space that contribute positively
toward hygrothermal performance of walls. These key EIFS features
are implemented in the walls of the ORNL test houses, which will
be described later in this paper.
5. Double walls
One obvious solution to increase R-value of wall is to use
thicker insulated space via two sets of wall framing, either via
two 2 × 4 studs, a set of 2 × 4 and 2 × 3 studs, or a set of
2 × 6 and 2 × 4 studs). In practice, this type of wall is called
double wall or double stud walls. Sometimes it is called party
wall because it can be used as interior bearing walls that can
reduce sound transmission in multi-family buildings through
small gaps provided between the interior and exterior wall frame
s. Because of these wide wall cavities (empty or filled), double wall
system can provide more fire resistance relative to the traditional
wall frame assemblies.
Fig. 2. Shredding of aerogel blankets: (a) finely chopped pieces of aerogel blanket as they exit the shredder and (b) final insulation product.
J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
445
Fig. 3. Typical cross section of EIFS wall.
Fig. 4. Double wall construction – Riverdale Net Zero Project, 2007 [32].
Figs. 4 and 5 present examples of double wall systems being
assembled for constructing zero net energy housing in Canada [32]
and in southern U.S. (ORNL test house) [11], and Fig. 6 shows a
typical framing configuration of double wall. Depending on the
insulation thickness provided, a whole R-value larger than RSI –
5.3 m2 K/W) can be reached. It should be noticed that whole-wall
R-value is used here (rather than clear-wall R-value) to indicate
the effects of framing elements and interfaces or junctions within
wall assembly. For example, double (2 × 4) wall with 240 mm RSI –
6.0 m2 K/W insulation has whole-wall R-value of RSI – 5.3 m2 K/W
[18,21,33]. Any type of insulation materials can be used to fill
out the wall cavity, but blown cellulose insulation is preferred
for better durability performance due to capability of storing and
redistributing small amounts of moisture in addition to environmentally friendly materials. Double wall system can be used to
retrofit an existing building, and this can be done via adding interior or exterior non load bearing walls to the existing load bearing
wall.
As shown in Figs. 4–6, the studs in double wall can be placed
either in-lined or staggered order (the studs in the first wall
are offset from the studs in the second) with studs spacing
600 mm o.c. on each side [14]. By placing studs in staggered way,
Fig. 5. Construction details of double wall framing system designed by Jan Kosny for Oak Ridge, TN, U.S.A. field experiment [11]; (a) internal layer of framing and (b) two
layers of framing separated with fiber fabric.
446
J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
Fig. 6. Cross section of double wall framing system.
thermal bridging is reduced further due to wider space available
for insulation. In addition, multiple types of thermal insulations
can be used in double walls. As shown in Fig. 5, two layers of framing can be separated with fiber fabric to enable an application of
two different blown thermal insulations. Either interior or exterior
stud-frame wall can be designed as load bearing (structural) wall,
depending on the architectural need (e.g. interior space dimension).
In general, structural design procedure for the load bearing wall
is similar to that of the standard wall system. Gravity and lateral
loads (wind or earthquake) should be transferred fully to the structural wall by designing adequate nailing between the sheathing to
stud frames. Single or double top-plate can be used for the structural wall depending on adequacy of the wall chord (i.e. top plate)
in carrying and transmitting in-plane lateral load to the wall and
foundation. For the exterior non load bearing wall, structural adequacy with respect to out-of-plane wind load should be checked
including the cladding and components attached to it. Normally
the wall cladding and component system will be the determining
factors instead of the wall frame. On top of the top-plate, gusset
plate made of plywood or OSB (9.5 mm thick) can be used to connect the interior and exterior wall frame (Fig. 6). As for framing
members, single or double top plate would have less impact on the
whole R-value because significant thermal breaks can be provided
between the exterior and interior frames. One of the concerns of
this wall with respect to durability is that the sheathing is kept
very cold, and has little drying potential if the sheathing is wetted
by air leakage or rain water penetration.
For two- or more storey constructions, platform system can be
used with insulation material inserted inside the rim joist to minimize thermal bridging (Fig. 7a). As one can see, in the platform
system thermal bridge is reduced into a very small area, i.e. gusset
plate. Balloon frame system also can be constructed for the exterior
non load bearing system to eliminate the work of insulating the rim
joist as in the platform system (Fig. 7b). Stronger and stable engineered wood composite such as Laminated Veneer Lumber (LVL) or
Laminated Strand Lumber (LSL) can be used for constructing long
and slender studs in balloon framing. Using balloon framing can
increase R-value of the double wall system due to elimination of
thermal bridging suffered in un-insulated rim joists of the platform
framing. The main constraint about reaching higher R-value is interior space demand, and if this is not an issue for architect or home
owner, R-value larger than RSI – 8.8 m2 K/W can be reached simply
by increasing the space between the two stud lines and filling it out
with insulation.
6. Truss walls
Other high R-value wall frame assembly that has been tried
out in North America is called truss walls or standoff truss wall
system. Sometime it is referred to the Larsen truss wall system
because it was developed first by John Larsen in Canada in the
early 1980s [10,34]. Fig. 8 illustrates an example of Larsen truss
wall construction. In principle, Larsen truss wall is a double wall
assembly plus vertically 19 mm gusset plates (OSB or plywood)
spaced at 600–900 mm o.c. that tie the exterior and interior studs
frame. This creates stiffer out-of-plane wall frame system. Unlike
the double wall system, the interior wall frame of Larsen truss
wall is designed as a load-bearing wall, which does not compromise interior space demand. Fig. 9 shows typical cross sections of
the Larsen truss wall assembly. The truss wall cavity is normally
filled with environmentally friendly dense blown cellulose insulation. The exterior frame can be extended below the rim joist in the
wall-to-foundation junction to provide insulation space that eliminates thermal bridge which is commonly occurred in this junction.
Due to reduced impacts of thermal bridges associated with structural intersections and wall opening details, Larson truss walls,
Fig. 7. Inter-storey junction in double wall system (platform vs balloon framing).
J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
Fig. 8. Larsen truss wall construction [34].
demonstrate lower differences between clear wall and whole wall
R-values comparing to other wood-based technologies [6].
Depending on the insulation thickness provided, whole-R-value
higher from RSI – 6.2 m2 K/W can be reached. For example, truss
wall with 0.30 m RSI – 7.6 m2 K/W cellulose insulation has wholewall R-value of RSI – 6.4 m2 K/W [12]. The three layers of low
permeable materials (drywall, dense pack cellulose, and housewrap) make the walls virtually impermeable to infiltration. As in
the double wall, a significant cavity thickness plus cellulose insulation in the truss wall provide a good fire resistance and sound
insulation.
The structural design procedure for the load-bearing wall subjected to gravity and lateral loads is similar to that of the standard
wall system. Because of laterally stiffer relative to the double wall
system, the exterior non-load bearing wall can carry heavier out-ofplane wind load. Depending on the structural load demand, single
or double top plate can be used for the interior bearing wall with
horizontal plywood or OSB plate closure tied to the exterior wall
frame for each storey. The exterior wall also can be balloon-framed
to minimize thermal bridging between the floors (Figs. 7 and 9).
The studs’ frames can be spaced 0.40 m or 0.60 m o.c. depending
on the load requirement, since the whole-wall R-value is not influenced by this spacing requirement due to significant thermal break
447
provided between the interior and exterior wall frames. Window
framing on the interior load bearing wall can be designed as in the
standard wall construction, i.e. using double header, since again
this will not notably affect the wall R-value due to thermal break
applied between the walls. To make it airtight in opening areas, plywood is applied to tie typically deep window box frames. To anchor
each truss at the top of the walls of the house, the upper most gusset plate is extended 38 mm and nailed to adjacent roof trusses or
rafters. The ceiling joists are cantilevered outward to form the soffit and meet the rafter tails (ceiling joists and rafters are lapped
to opposite sides of each stud) with a 50 mm block in between to
create a thrust-resisting triangle.
As in the double wall system that has a very thick insulation
layer, in winter conditions, a very cold area can be easily developed
on the interior side of the exterior sheathing. This fact could lead to
moisture deposition issue if drying system ability is not sufficiently
facilitated. However, OSB exterior sheathing can be eliminated and
replaced with t-bracing and 19 mm drop siding over house wrap.
Let-in metal t-bracing in exterior and interior load-bearing walls
and wooden under-rafter diagonal bracing sufficiently stiffens the
structure, particularly once the sealed drywall is installed. Due to
the amount of framing involved and time need it to assemble them,
the Larsen truss can be prefabricated off-site. For retrofitted wall
envelopes, a vapor barrier could be applied over the wall sheathing
before installation of trusses without fear of condensation as long
as 2/3 of the R-value of the envelope is outside of the barrier.
Other variation of Larsen truss wall is to use ‘an actual’ standoff
truss system [6,35]. Instead of using gusset plate to tie the interior and exterior wall frames, 2 × 2 diagonal braces are applied to
the cords (exterior and interior studs) with metal gang-nail plate
connectors at certain spacing and angles. It is claimed by Hefner
[36] that this truss wall could reach RSI – 8.8 m2 K/W or more with
0.30 m insulation cavity.
7. Optimum framing wall technologies
Reducing or optimizing placement of lumber framings to
minimize thermal bridging and replacing them with insulation
materials is another way to improve thermal performance of wall
Fig. 9. Typical cross sections of Larsen truss wall assembly.
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Fig. 10. OVE construction – wall system designed by Jeff Christian for Oak Ridge, TN, U.S.A. field experiment [11].
envelopes. In residential construction, this technique is known
as advanced framing or optimum value engineering (OVE). Historically, the main objective of OVE is merely to safe lumber
consumptions without compromising other important aspects of
construction. The OVE practice includes: using wider spacing for
wall studs (0.60 m o.c. rather than 0.40 m o.c.), using single top
plate rather than double plates, optimized corner design using less
lumber framing (wall-to-wall junction or double stud corner), and
optimized lumber framings in window/door openings. Fig. 10 illustrates an example of OVE construction build in Oak Ridge, TN, U.S.A.
[11]. Figs. 10 and 11 show typical OVE wall frame assembly. OVE
practice is normally combined with placing externally insulation
system outside structural sheathing to provide continuous thermal break on the wall. Furring strip can be provided to facilitate
placement of the exterior insulation. Overall-wall R-value of about
RSI – 6 can be obtained for OVE wall using 2 × 6 studs installed
on 0.60 m o.c. with RSI – 3.3 m2 K/W – fiber glass insulation plus
102 mm-thick RSI 3.5 m2 K/W extruded polystyrene foam sheathing [33]. Because of smaller thickness, the OVE wall has lower fire
and sound performances compared to the double and truss walls.
Note: without external insulation, the overall-wall R-value of OVE
wall is around RSI – 2.7 m2 K/W for 2 × 6, and RSI – 2.0 for 2 × 4 stud
walls. It is clearly beneficial to exploit utilization 2 × 6 studs in OVE
wall due to better structural and thermal performances relative to
2 × 4 studs.
2 × 6 studs (0.60 m o.c.) with single top plate: As shown in Fig. 11,
single top plate is structurally possible if the wall frame assembly
follows what so called stack framing technique, in which the framing from one floor is lined up directly with the framing above and
below to create a continuous vertical load path. In addition to this,
a single top plate should be capable of transmitting lateral forces
from floor diaphragm to the wall frames, and this is normally done
in practice by checking the tensile strength of the plate and by
designing adequate metal splice connector. One of the practical
challenges using single top plate is adjusting total building height
that is originally based on double plate system [37]. Trimming standardized size of sheathing materials (e.g. drywall, OSB) is needed to
make up for the adjusted building height, and this work could add
labor costs. Compensation for this extra labor effort can be anticipated from saved materials and long term energy consumption.
Double stud corner: The presence of building corners structurally
improves lateral resistance of wall frame regardless the type of
corner joining system used. For majority of residential structures,
double stud corner method joining two wall frames is adequate to
provide integrity of the structural system. The structural integrity
between two intersecting walls is provided by nailing two studs
together, a connector plate at the top of wall, and nailing into the
floor sheathing at the bottom of the wall. Since insulation material
replaces the (un-necessary) third stud, thermal bridging which is
very profound in the buildings’ corners can be reduced. Practical
issue dealing with two-stud corner is associated with the installation of drywall, and this basically can be overcome by installing
drywall clips in one of the studs (Fig. 11b). Alternatively, insulated
three-stud corner can be used in OVE frame [37].
Fig. 11. OVE practice; (a) wall cross section and (b) double stud corner.
J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
449
Fig. 12. Cross laminated timber (CLT) construction.
Optimized framings for wall and door openings: Building envelope
opening is important location to look at with respect to thermal bridging, since normal standard construction practice applies
excessive lumber framing around window and door openings leading to excessive thermal bridge issue. Depending the opening width
(span), double or triple (or more) headers with triple vertical studs
(jack and king studs) are used to adequately carry and transfer gravity load to lower storey or foundation. In-lining at least
one side of the frame opening with stack frame arrangement can
reduce lumber framings around it. Sizing headers is prescriptively
given in the building code, and is given as a function of supporting types (roof, ceiling, and floor), span of the openings, building
widths (usually 3.7–11 m), and range of ground snow loads (from
1.4 to 3.4 kPa). Lateral loads such as wind also play important
role in determining the header size. Based on typical wind load
calculations, in regions where the 3-s gust basic wind speed is
170 km/h or less, gravity loads may control. While in regions with
the 3-s gust basic wind speed is larger than 170 km/h, wind load
may control depending on the header span. Table 2308.9.5 of the
2006 International Building Code [38] provides header sizes ranging from double 2 × 4 to quadruple 2 × 12 of lumbers. None is
given to a single header size. Therefore, design calculation must
be performed to check whether single header is adequate to carry
gravity load, lateral loads, and combinations of them. Alternative
way to increase the strength of single header is to utilize engineered
wood composite such as Laminated Veneer Lumber (LVL). Similar
table as in IBC 2006 for sizing header can be obtained in engineering specification provided by wood composite manufacturers,
e.g. Trus JoistTM , [39] specifier’s guide for headers and beam using
LVL.
8. Other innovative European and North American
wood-based wall systems
Unlike in North America, there are various types of wood-based
wall technologies used in residential buildings in Europe ranging
from light frame to heavy and massive wood assemblies. The most
recent one is an application of massive timber elements called cross
laminated timbers (CLT) for low-rise building envelopes. CLT was
first developed in Austria in early 1990s, and was intended to utilize
low value timber boards. CLT is made by laminating timber boards
while alternating direction of the wood grain in adjacent layer
(Fig. 12a). Fig. 12b illustrates CLT application for building envelopes
in residential construction in Europe. Fig. 13 shows typical layers of CLT wall assembly. The CLT plates are normally connected
to floor and roof diaphragms via self-taping and long screw fasteners with metal angle connectors. Further technical information
about CLT materials and their application in residential construction can be seen, for example, in KLH, 2010 [40]. Since there is no
wall cavity as in the light-frame wall system, insulation materials are assembled in the interior and exterior of CLT (load bearing)
wall eliminating thermal bridging issue. Two types of insulation
are used, sound and thermal insulations. Sound insulation material can be put on the interior or exterior side of the CLT plate
for damping sound transmission. Fire would not be a big issue
for this massive wall system, but for normal application interior
finish such as gypsum wall board is added after the sound insulation layer or wall cavity intended for wiring or utility. Designing
higher thermal resistance for this wall system depends heavily
on how thick insulations are provided, since CLT plate itself contributes only to a small portion of the wall R-value. For example,
Fig. 13. Typical CLT (exterior load-bearing) wall assembly [40].
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Fig. 14. Other European wall frame assemblies.
exterior wall with total thickness of 391 mm including 142 mm CLT
panel, overall-wall R-value of RSI – 6.3 is obtained [26,41]. Note: for
some applications, interior CLT walls are left without additional
protection layers for esthetic purposes. Major concern anticipated
in applying CLT panels in residential building envelope is massive
consumption of wood materials, since it requires approximately
4 times more wood materials to construct wall envelope relative
to the North American light-frame wood stud wall systems. Canadian NEWBuildS research program is performing a 5-year project
to develop design and construction practices for CLT in buildings
covering structural and building science aspects, including thermal
mass benefit CLT can offer [18,32,34].
Conventional high thermal performance wall frame assemblies
in Europe usually are utilizing deeper stud frames to provide more
insulation space in the wall cavity relative to those normally used
in North America. For example, Swedish wall frame assemblies
typically use 45 mm × 195 mm solid wood spaced 600 mm o.c.
(equivalent to U.S. 2 × 8 studs) for load bearing exterior envelopes
(Figs. 14 and 15a) Considering double insulation layers and wall
cavities provided, this type of wall is estimated to have clear wall
thermal resistant of RSI – 5.9 m2 K/W [42], and its whole R-value of
is estimated around RSI – 4.4 m2 K/W after taking into account framing factors and wall junctions. In term of sound and fire resistance,
this type of wall has better performance relative to the standard
North American walls due to the presence of air gap and sound insulation layers. In some other European regions, these load bearing
studs are replaced with engineered wood I-joists of equivalent
strength to minimize thermal bridging (Fig. 14).
There are some other ways to improve thermal performance
of wall assembly than were just described above. Utilizing furring
strips is a relatively easy practice to increase thermal performance
of new or existing wall assemblies. Using this technique 2 × 2 horizontal furring is added and connected to the 2 × 4 or 2 × 6 stud
frame with the main intention to limit thermal bridging to the
points where these two (furring strips and studs) are in contact
(Fig. 15). It is estimated that the thermal bridge effect is reduced
around 30% from that of the standard 2 × 4 or 2 × 6 walls. Dense
pack cellulose insulation is normally used to fill out 180 mm wall
cavity created using 2 × 6 studs and 2 × 2 furring strips. Insulation
mesh glued to the face of the 2 × 2 furring is needed to facilitate
blowing the dense-packed cellulose into the wall cavity. Thermal
breaks provided in this type of wall also means providing soundtransmission break, decoupling the sound insulation transmitted
from the studs.
For new construction, this wall can be combined with OVE technique such as insulated (single) header, stack framing, and two-stud
corner, to achieve higher R-value. Wall using 2 × 6 on 0.60 m o.c.
plus 2 × 2 furring strips with OVE practice and cellulose insulation
is estimated to have clear wall R-value of RSI – 4.8 m2 K/W, with
whole-wall R-value of around RSI – 3.5 m2 K/W after discounting
with isolated thermal bridging and junction factors – see Fig. 15.
Overall-wall R-value of around RSI – 6.2 can be obtained assuming
Fig. 15. Wall with furring strips technique [50].
J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
451
9. ORNL field experiments with high-R-value wall
assemblies
Fig. 16. Engineered wall with composite studs [43].
using filled insulation cavity, XPS foam sheathing, and 2 × 3 furring
strips on 2 × 6 studs on 0.60 m o.c. [35].
Other high thermal performance wall configuration that has
been practice in North America is called engineered walls (EW).
The main feature of EW is substituting 38 mm standard stud
width with thinner engineered wood composites called oriented
strand wood (OSW) Fig. 16. Like other engineered wood composites, OSW is manufactured from small-fast growing trees that are
shredded into 0.30 m strands. These strands are saturated with
adhesive and packed into a steam-injection press to create billet of 10.7mx2.4mx1.7 m. This billet is then cut into lumbers with
different thickness and widths. Because stronger than traditional
lumbers, the OSW studs can be manufactured into 32 mm width
making more room for insulation in the wall cavity. OSW furring
strips are attached to the inside face of the framing to support
installation of drywall around window and door openings, creating a 38 mm air space behind the drywall that increases R-value
slightly and improves sound and fire performance relative to the
standard wall system. EW uses a continuous header mounted to
the inside face of the studs. Due to smaller width, framing-to-wall
ratio for EW is around 9%, compared to the conventional wall which
is between 13–30%. As with the furring strip technique, EW is normally combined with OVE practice and exterior insulation. The
overall insulating value of the EW discussed here is RSI – 4.5 m2 K/W.
Fig. 16 shows EW designed at Davis Energy Group as part of ACT
Squared demonstration home in California [43]. Nowadays, OSW
has been rarely used as studs in residential construction due to
emerging applications of other structural composite lumbers (SCL)
such as LSL, LVL, and Parallel Strand Lumber (PSL).
Various high thermal performance walls constructed in actual
homes have been studied and monitored in Oak Ridge, TN, U.S.A. as
part of a collaborative research project called ZEBRAlliance or Zero
Energy Building Research Alliance [44]. This project is a partnership
between ORNL and building materials and construction companies
and sponsored by Tennessee Valley Authority and the U.S. Dept
of Energy to demonstrate and develop new energy efficient technologies for homes. It is expected that these homes are 50–60%
more energy efficient than homes of similar size and style built to
local building code [11]. Thermal performance of wall envelopes is
clearly one of the crucial factors in the building system to achieve
this target. Four homes were constructed using four different wall
envelope systems. The key wall envelopes used includes structural insulated panel (SIP) in house #1, optimum value engineering
(OVE) wall in house #2, double wall with phase change material
(PCM) insulation in house #3, and exterior insulation and finish
system (EIFS) in house #4. Around 400 sensors have been installed
for data collection related to energy efficiency under standard living condition. Earlier in this paper the OVE wall #2 was discussed –
Figs. 10 and 11. In this section the discussion is focused on the last
two walls (in houses #3 and #4) because of their high clear wall
R-values of around RSI – 5.3 m2 K/W.
Double wall with Phase Change Material (PCM): Figs. 5 and 17a
show the test house #3 constructed with double wall filled with
PCM-enhanced cellulose insulation. The house is two-story with
total floor area of 253 m2 . The wall studs are made of 2 × 4
Laminated Strand Lumber (LSL) and are spaced 0.60 m o.c. (Fig. 17b).
In case of the LSL profiles, the dimension (width and depth) is
6.4 mm larger than those of the traditional lumbers. For example,
2 × 4 LSL actual dimension is 44 mm × 95 mm instead of conventional 38 mm × 89 mm. This creates stiffer wall frames with wider
stud spacing that provides more room for insulation relative to the
traditional systems. In the test house #3 the studs from one wall
are offset by 0.30 m from the other wall’s studs (Fig. 18). The interior framing is supported on top of the bottom plate that is fastened
through floor sheathing and floor truss, while the exterior framing
is supported on the sill plate and is fastened to the floor truss. A top
plate was used to tie the two walls together for lateral strength. It
is anticipated that this double wall system function as composite
system. The interior frame will acts as the structural wall component responsible for transmitting gravity and lateral force to the
foundation. However, since the two double top plates are mechanically connected and wall sheathing is provided in the exterior wall,
a portion of lateral load is carried by the exterior walls as well.
Fig. 17. ORNL #3 double wall house and internal layer of the double wall designed by Jan Kosny for the Oak Ridge field experiment [11].
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Fig. 18. Cross sections of wall frame assembly designed by Jan Kosny for the double wall house [11].
As shown in Figs. 5b and 18, a fiber fabric is stapled between
the two sets of 2 × 4 studs to separate and hold two different types
of blown cellulose insulation. The wall interior cavity is insulated
with conventional dense-packed cellulose while 20% by weight of
microencapsulated PCM was added to blown cellulose fibers in the
exterior framed cavity [11]. Since 2002, the ORNL and Fraunhofer
CSE research teams have analyzed several configurations of wall
insulations blended with microencapsulated and shape stabilized
PCMs [33,45–49].
In double walls thermal bridging at the corners is minimized by
applying double stud corner practice with insulation surrounding
it (Fig. 18). The exterior wall OSB sheathing has a built-in protective
weather resistive barrier (WRB) overlaid at the factory to eliminate
the need for house wrap. All joints were taped to also make the
sheathing air tight. A high-density polyethylene sheet having about
a 6.4 mm high dimpled profile was also installed on the exterior of
the sheathing to ventilate the exterior walls. It provides drainage
of transient moisture migrating through the wall and creates two
Fig. 19. EIFS wall assembly designed by Achilles Karagiozis for the ORNL #4 house [11].
Fig. 20. Wood framed building retrofitted in Brunswick, ME, U.S.A. with vacuum insulation; (a) building view before retrofit and (b) view after retrofit.
Source: Fraunhofer CSE.
J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
453
Fig. 21. Wall retrofit using VIPs in Brunswick, ME, U.S.A.; (a) an application of the wall liquid sealant on the structural sheathing and (b) installation of the VIP-foam
sandwiches.
Source: Fraunhofer CSE.
independent air flow streams to dry out both the cladding and
the concealed wall cavities. The product eliminates the impact of
solar driven moisture problems, and reduces the impact of interior
moisture loading at the same time.
Exterior Insulation and Finish System (EIFS) wall: Fig. 19 shows the
two-story 253 m2 house constructed with EIFS wall, which utilizes
an insulated cladding made of 127 mm of EPS foam insulation on the
outside of the exterior wall. The EPS insulation (RSI – 3.5) effectively
mitigates framing-generated thermal bridges that are a major contributor to framed walls’ energy losses. The system is lightweight,
highly energy efficient and vapor permeable. The EPS foam insulation extends from about 0.30 m above the ground up to the soffit of
the roof. EIFS wall system was selected because of its potential for
energy efficiency, costs, and smaller environmental impacts [11].
Fig. 19 shows the cross section of EFIS wall around the wallto-foundation junction. In general, when installed properly, EIFS
can perform very well [31]. However, despite of showing good
appearance, EIFS is one of the wall assemblies that needs to be
carefully detailed in term of water (moisture) management. Even
small short-cut in the installation standard and quality component
procedure can lead to an issue in the longer term. To overcome this
issue, a flexible polymer-based membrane was manually applied
in a liquid form over all of the plywood sheathing. This membrane
resists water penetration and eliminates air infiltration to make the
home air tight. Afterwards, a fiber-reinforced cementitious adhesive was trowel applied to the weather resistive membrane to
adhere the EPS insulation. The trowel application forms rows of
the adhesive with each row about 5–6 mm high. The rows provide
a small drainage cavity between the WRB and the EPS insulation
board through which incidental water can weep to the outdoor
ambient. The exterior is finished in an acrylic-based coating finish
over the stucco.
10. Vacuum insulation used as exterior insulating
sheathing
In recent years, the target R-value of framed wall assemblies for deep energy retrofits is typically RSI – 5.30 m2 K/W or
greater in colder northern U.S. climates. As described in earlier
sections, advanced framing and exterior insulating sheathings can
significantly improve wall thermal performance to achieve high
R-values. However, there are space constraints for wall cavities
Fig. 22. Modeling geometry and the temperature map of the simulated VIP-foam sandwich.
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J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
or exterior installations of sheathing insulations, particularly in
retrofit projects. Also, very thick building envelopes are not desirable due to several issues, for example, reduction of internal floor
area for internal insulation applications, zoning regulations in cases
of the exterior foam sheathing, a common need for alteration of
window and door openings, architectural restrictions, and material
usage. To achieve the highest possible thermal insulation resistance
with existing space limitations of retrofit projects, new thermal
insulation materials with low thermal conductivity, such as vacuum insulated panels (VIPs) and aerogels, are decent alternatives to
conventional insulation materials. The main barriers to application
of this new material are low production volume and cost.
VIPs are promising alternatives to conventional building thermal insulations due to their ultra-low thermal conductivity. Their
thermal conductivity is 4–8 times less than foam insulation materials, resulting in a substantially thinner solution to the building
envelope relative to conventional insulation. A typical VIP consists
of a core panel enclosed in an air-tight envelope, to which a vacuum
is applied. The common core materials are fumed and precipitated
silica, open cells PU and several types of fiberglass. The core is
wrapped in a metallic or mylar foil, and then the vacuum is applied.
The metallic film is sealed to maintain the vacuum for a long period
of time; although there may be some loss of insulation value as the
panel ages, depending on the design of the installation.
During last decade VIPs came through a serious of transformations bringing a cost of vacuum panels to the level which
justifies large scale applications [16]. A novel wall retrofit strategy, utilizing vacuum insulation, has been studied and monitored
in the North Eastern U.S. coastal climate as part of a collaborative
field test project between Dow-Corning, Dryvit, and Fraunhofer
CSE. This project was sponsored by the U.S. Dept. of Energy to
demonstrate a new VIP-based energy efficient wall retrofit strategy. Figs. 20 and 21 shows a single-story wood-framed building
in Brunswick, ME which was currently retrofitted using VIP-based
EIFS and high performance windows. The building in this study
is a 2 × 6 wood-frame wall construction with empty wall cavities.
Opaque part of the wall was insulated using vacuum insulation system made of 25 mm thick VIP panels sandwiched with 25 mm layers
of EPS foam on each side. As presented in Fig. 21, VIPs were packed
for mechanical protection with EPS foam and later covered with
stucco exterior finish. Retrofit windows used in this experiment
are highly insulating windows with a whole-window RSI – value
of 0.88 m2 K/W (a U-factor of around 1.14), and are the top tier
of energy-efficient windows for cold and mixed climates available
today on the U.S. construction market.
A detailed thermal model of the VIP-foam sandwich had to be
developed to enable thermal simulation of the opaque part of the
wall assembly. A three-dimensional finite difference heat transfer program was used to predict conductive heat flux through the
VIP-foam sandwich. As depicted in Fig. 22, VIP was covered with
∼25 mm. thick layers of EPS foam on top and bottom sides. In addition, each edge of the VIP was protected with ∼6 mm. thick layers
of the foam. Since foam is significantly more conducting from the
VIP, a three-dimensional thermal analysis was necessary to estimate the effect of thermal bridging generated by the foam and to
calculate the effective system R-value.
Fig. 22 shows the representative insulation sandwich panel as
used during the Brunswick, ME field experiment. It is composed
of three interconnected rectangular VIPs with 6-mm. thick foam
protection edges. The original VIP panel is wrapped in a 12-␮m
aluminized foil and sandwiched between two layers of XPS foam
for protection against lateral damage. The wrapping foil and foam
protection edges, representing 7.3% of the assembly area, cause
heat loss by thermal bridging effects across the VIP edges. The
purpose of thermal modeling was to quantify this loss by predicting conductive heat flux and effective assembly R-value needed
for further thermal analysis. The simulated assembly presented
in Fig. 22 represents similar aerial proportions as the whole wall
installation in the test building. Right side of Fig. 22 shows a detailed
temperature profile in place of connection between two VIP panels. For the purpose of this model, a temperature gradient of 27.7 ◦ C
was assumed across the VIP by assigning 21.1 ◦ C interior air and
−6.6 ◦ C exterior air temperatures. Fig. 22 illustrates the temperature map of the simulated fumed silica VIP core wrapped with
12-␮m aluminized foil and sandwiched with 25 mm. thick layers
of plastic foam. An effective thermal resistance of RSI – 6.7 m2 K/W
was calculated for the VIP only while considering thermal bridges
through the wrapping foil. The effective thermal resistance of the
whole assembly when VIP is sandwiched between two layers of
EPS foam was calculated to be RSI – 7.4 m2 K/W. The corresponding numerically generated clear wall RSI – value was of 8.5 m2 K/W
for wall configuration as used in Brunswick, ME, U.S.A. field
experiment.
11. Conclusions
It can be observed that many different high R-value woodbase wall assemblies have been developed and successfully tried
out using many different insulation materials with different wall
thicknesses and framing techniques. Extensive literature review of
existing and past practices of residential wall construction was conducted to identify wall assemblies of high thermal performance.
A special attention was paid to wood and wood composite framing technologies. This included double stud walls, truss walls,
advanced framing (with optimum value engineering – OVE) wall,
walls with exterior insulating sheathing, EIFS wall, European high
R-value walls, walls with composite frames, and walls with furring
strips. Different insulation methods were analyzed as well. For all
theoretically analyzed wall systems, clear wall R-values were ranging between RSI – 3.3 m2 K/W for the OVE wall system and RSI –
8.8 m2 K/W for double walls and truss walls. In addition to basic
factors for increasing the R-value (like insulation and framing),
other important approaches have been explored to mitigate thermal bridging, secure air tightness and to improve overall durability
performance of those walls.
Traditionally several configurations of double walls and truss
walls have been considered as energy efficient solution for lowenergy buildings. Current research demonstrated that R-values of
these walls can easily exceed RSI – 5.3 m2 K/W. Conventional wall
frames with exterior rigid foam insulation have been identified
by numerous researchers and construction practitioners to be the
easiest to apply and best performing due to efficiency in reducing
thermal bridging and relatively inexpensive assembly, if window
door opening and roof overhang alterations are not necessary. It
was also found common a usage of wood based composites for wall
framing to improve thermal performance without compromising
structural safety while at the same time promoting in optimizing
utilization of wood natural resources as part of efforts toward sustainable (green) construction practice.
Additionally, high R-value wall assemblies should be designed
to suit difference climate, difference availability of materials costs
and labor, and difference expectation of building performance
objectives (e.g. reduced energy consumption buildings, durability,
indoor air quality, etc.). Experimental research work is underway
at ORNL and Fraunhofer CSE, to support this conclusion based on
the field monitoring results and constructor’s documentations on
design and construction of the test houses. Four test houses have
been constructed and they are currently studied and monitored
in Oak Ridge, TN, U.S.A. as part of a collaborative research project
with industrial partners and construction companies. The following
three key wall envelopes discussed in the paper includes optimum
J. Kosny et al. / Energy and Buildings 72 (2014) 441–456
value engineering (OVE) wall, double wall with cellulose insulation
and phase change material (PCM), and exterior insulation and finish system (EIFS). The clear wall R-values of last two walls were
around RSI – 5.3 m2 K/W with 216 mm and 254 mm of the total
wall thicknesses, respectively for the double wall and EIFS wall.
State-of the art wall vacuum insulation was tested in Bruswick, ME,
U.S.A. by Fraunhofer CSE and a team of industrial partners. Conventional 2 × 6 wood-framed wall with empty cavities was insulated
using VIP-based EIFS (25 mm thick vacuum panels sandwiched
with 25 mm layers of EPS foam on each side). The clear wall R-value
of the Brunswick wall using VIPs was around RSI – 9.0 m2 K/W with
254 mm of the total wall thickness. VIPs are the highest R-value
insulation option tested in field conditions for wood framed walls.
Acknowledgements
Financial supports provided by Oak Ridge National Laboratory
(ORNL) through the Oak Ridge Institute for Science and Education
(ORISE) and by Frunhover CSE are greatly acknowledged. Thanks
are also extended to ORNL and Fraunhofer CSE research staffs for
providing technical information about ZEBRAlliance test houses
and materials used in the Brunswick, ME test house.
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