The impact of details on the overall heat flow through the building envelope can be determined easily using linear transmittance. The additional
heat flow through specific details can be highlighted as presented in Table
4. This example compares the “effective R-value” or overall U-value for a
total wall elevation with 40% glazing with only one type of floor slab edge
condition. Table 3 summarizes the parameters utilized in this example.
Table 3: Example building parameters
Building Parameter
Table 3: Example
building parameters
Parameter Value
Total Elevation Area 3
3000 ft2 (278 m2)
Glazing %
40%
Total Opaque Wall Area
1800 ft2 (167.2 m2 )
Brick veneer supported by brick ties with R-15 Exterior
Insulation and concrete block back-up wall
U-0.056 Btu/hrft2oF
Metal Cladding supported by horizontal girts on 24”
o.c. (610 mm) with R-15 Exterior Insulation and R-12
Batt Insulation in a Steel-Stud back-up wall
U-0.054 Btu/hrft2oF
(0.31 W/m2K )
(0.32 W/m2K )
Table 4 shows the calculated results. The difference between the standard and stand-off shelf angle is not significant for this example. This is
largely due to the continuous metal flashing over the shelf angle for both
scenarios. There is a significant difference between the common concrete
slab projection scenario and a thermally improved system incorporating
a thermal break. The difference between these two scenarios for the example wall assembly has the same impact as adding approximately 45%
more wall area or 4 feet of wall height to each floor.
“The difference between the two scenarios
has the same impact as adding 4 feet of
wall height to each floor”
The insulated slab edge with improved details has an overall U-value closest to the U-values typically assumed for whole building energy modeling
but there is still a 28% difference5.
Table 4: Example building thermal values
Parapet
Table 4: Example building
thermal values
and
Floor Slab
Scenario
Standard Shelf
Angle with
Continuous Metal
Flashing
Stand-off Shelf
Angle Continuous
Metal Flashing
Un-insulated
concrete slab
Projection
Thermally improved
concrete slab
Projection
Insulated Slab with
Standard details
Insulated Slab with
improved details
Clear Field
Heat Flow
BTU/hroF
(W/K)
101
(53)
97
(51)
97
(51)
% Heat
Flow
Slab Edge
Window
U-Value
Heat Flow associated BTU/hrft2oF
Transition
with Floor
BTU/hroF
(W/m2K)
(W/K)
Heat Flow
Slab to
BTU/hroF
Total
(W/K)
25
(13)
52
(26)
25
(13)
Effective
R-Value
hrft2oF/ BTU
(m2K/W)
80
(42)
39%
0.115
(0.65)
8.7
(1.5)
56
(29)
31%
0.101
(0.57)
9.9
(1.7)
106
(55)
46%
0.129
(0.73)
7.8
(1.4)
36
(18)
22%
0.09
(0.51)
11.1
(2.0)
0.087
(0.49)
0.072
(0.41)
11.1
(2.0)
13.3
(2.4)
8
(4)
8
(4)
6%
6%
The key take away from this example is that the relative impact of each
detail can be determined and these details should not be overlooked in
practice. Ignoring these details will likely lead to misguided conclusions
and wasted opportunities for increased energy efficiency and to reduce
material consumption. This kind of assessment combined with sensitivity analysis of insulation levels, if doing whole building energy modeling,
should lead to more rational design decisions. Recognizing the diminishing returns of increasing insulation levels combined with the impact of the
details, will in many cases lead to more attention to designing thermally
effective details rather than simply adding more insulation and believing
energy savings will be achieved.
SOLUTIONS MH Volume 2012 Issue 3
CLOSING
It is time to make the conceptual leap to linear transmittance
to evaluate building envelope details, such as floor slab,
during design. Not only is the method straightforward to use
but the data in the 1365-RP catalogue and categories in this
solution allows the relative strength of details that are often
overlooked in practice to be effectively assessed. These
concepts are not limited to floor slabs; heat flow through window transitions can be significant and should not be ignored
either.
An increased awareness of the impact of the overall thermal performance of the building envelope, by utilizing these
methods, can be incorporated in practice by the entire
design team; energy modeler, architect, contractor, HVAC
designer; to make informed decisions that consider cost,
energy efficiency, and material use.
Floor Slab Edges Count
Volume 2012 Issue 3
We think that codes and standards will eventually adopt the
linear transmittance methodology because highly insulated
assemblies with thermally poor details is not rational and this
methodology can be effectively adopted within the existing framework using several practical approaches. In the
meantime, these concepts should be part of the discussion
on any project where whole building energy simulations are
being conducted and overall thermal transmittance are being
input into a model.
This solution highlights MH’s expertise of the thermal efficiency of the building envelope. Our collective knowledge
and judgment is gained through extensive field experience
and participation on a wide variety of projects. We leverage
this experience on every project to apply solutions that are
relevant to the design, construction and operation of the
build environment.
BUILDINGS
RECOGNIZING THE IMPACT
LEADING THE WAY TO A SUSTAINABLE
FUTURE
FOOTNOTES
1
ASHRAE Research Project 1365 Thermal Performance of Building Envelope Details for Mid- and High-rise Buildings is available
through the ASHRAE website bookstore or at www.morrisonhershfield.com /ashrae1365research.
2
This idea was fostered by Belgian researchers that categorized
linear transmittances into three categories: “Business as Usual”,
“Standard”, and “Thermal Bridge Avoidance”. Janssens, A, E. V.
Landersele, B. Vandermarcke, S. Roels, P. Standaert, P. Wouters.
“Development of Limits for the Linear Thermal Transmittance of
Thermal Bridges in Buildings”. Proceedings of the IX International
Conference on the Performance of Whole Buildings, Clearwater, FL,
2007.
THE CHALLENGE
There are two challenges with evaluating the impact of thermal bridging
at floor slab details:
1. There are a variety of floor slab details that can be
integrated into a variety of wall assemblies.
2. The heat-flow paths at floor slabs can be complex and
are often three-dimensional (3D).
3
For slab edge details that are not fully insulated, the linear transmittance varies by less than 5% for increasing amounts of exterior insulation beyond R-5. This follows common sense, since regardless of
the amount of exterior insulation, heat can still bypass easily through
the thermal bridges. For these types of details, the linear transmittance can be considered constant.
4
Based on manufacturer’s data for one proprietary system
5
The overall U-value compared to the clear field U-value
If you wish to discuss how our services can help you make a
difference, please contact us at:
buildingenvelope@morrisonhershfield.com
www.morrisonhershfield.com/ashrae1365research
or through your local Morrison Hershfield office.
The prevailing North American method to account for heat flow through
parallel paths, as is often assumed when evaluating the impact of floor
slab details, uses weighted averages to combine heat flows. The main
drawback to this method is that an “area of influence” must be assigned
to the floor slab detail, which is not only often difficult to determine but
also not straightforward to use in practice. This process is not straightforward because of the 3D heat flow paths typically associated with
the details and the “area of influence” are usually unique to the specific
details and assemblies. Moreover, the true impact of the additional
heat flow through the floor slab detail is blurred by the adjacent highly
insulated wall assembly. As a result, it is difficult to effectively evaluate
options using this approach.
Thermal bridges at floor slabs are often overlooked in energy standards and common design
practice. The accepted truth seems to be that
structural members to support cladding cannot
be avoided and the area of the steel or concrete
bypassing the insulation is small compared to
the total wall area. Therefore the impact of floor
slab details on the total thermal performance
must be insignificant and there is not much you
can do about it anyway.
The inconvenient truth is much as 50% of the
heat flow through the wall might actually be due
to thermal bridging at the floor slab, if solutions to
minimize thermal bridging at floor slabs are not
implemented. This discrepancy widens as we
add higher levels of insulation into wall assemblies but don’t consider the impact of the details.
The good news is that there are viable solutions
to overcome current shortcomings in energy
standards and design practice. The outcome of
implementing these solutions will lead to more
energy efficient buildings and more efficient use
of building materials.
This SOLUTIONMH provides the motivation and direction to overcome
these challenges by showing how significant floor slab details are on
overall building envelope thermal performance and how the impact of
details, not typically considered part of an assembly, can be effectively
evaluated in practice using a linear transmittance approach.
SOLUTIONS MH Volume 2012 Issue 3
THE LINEAR TRANSMITTANCE APPROACH
The linear transmittance approach simplifies heat
flow calculations by addressing clear field assemblies
and details separately. With this approach, there are
3 ways in which heat flows are categorized:
Y = 0.35
BTU/hr ft2 oF
Un-insulated Exposed Concrete
Clear Field
Transmittance
Point
Transmittance
Linear
Transmittance
Clear field transmittance (Uo) is the heat flow per
area of an assembly, including uniformly distributed
thermal bridges that are not practical to account for
on an individual basis (ie: steel studs, brick-ties, subgirts to support cladding).
Linear transmittance (Y) is the additional heat
flow per length caused by details that are defined
by a length, L (ie: slab edges, corners, parapets, and
transitions between assemblies).
Point transmittance (c) is the additional heat flow
caused by thermal bridges that occur only at single,
infrequent locations (ie: beam penetrations).
The total heat flow through a wall is simply the addition of the clear field assembly heat flow and additional heat flow associated with details such as floor
slabs:
TOTAL
HEAT
FLOW
=
HEAT FLOW
THROUGH
CLEAR FIELD
ASSEMBLY
+
ADDITIONAL
HEAT FLOW
DUE TO
DETAILS
LINEAR
TRANSMITTANCE
RANGE
Y = 0.26 to 0.29
BTU/hr ft2 oF
Standard Shelf Angle
POOR
THE SOLUTION MH
Floor slab details were investigated by Morrison Hershfield as part of
ASHRAE Research Project 1365 (1365-RP), Thermal Performance
of Building Envelope Details for Mid- and High-rise Buildings, using a
calibrated 3D thermal model1. Floor slab details were analyzed for a
variety of wall constructions, included exposed concrete slabs, brick shelf
angles, pre-cast concrete, and exterior insulated steel framed structures.
The most practical approach to quantify the heat flow through a floor slab
detail is by linear transmittance (see left bar). This approach requires a
conceptual leap from some of the prevailing North America methods but
there are some key benefits to embracing this change:
1. Tangible values can be assigned to details, largely irrespective of
the assembly
2. We can categorize a wide range of details into broad categories
using this method
3. The level of effort to implement this method in practice is not
onerous
FLOOR SLAB EDGE CATEGORIES OF THERMAL
QUALITY
Using the building envelope thermal performance catalogue developed
for 1365-RP, we divided the floor slab details into categories of poor,
improved, and efficient2. Table 1 shows the range of linear transmittance
assigned to each category3 superscript. Graphics of some details for each
of these categories are illustrated above.
Table 1: Thermal Quality of Floor Slab Details
Categorized by Linear TransimIttance
For energy modeling or comparisons to standards
and codes, often it is useful to present the heat flow
per area or the overall U-value. The overall U-value
for total area, ATotal, is calculated as follows :
The upper limits of each category are helpful during the preliminary design
stages before details are established. The upper limits for a particular
category can be utilized in calculations to evaluate the benefit of improving the building envelope details in-conjunction with setting the optimum
assembly insulation levels.
SOLUTIONS MH Volume 2012 Issue 3
Y = 0.19
BTU/hr ft2 oF
Stand-Off Shelf Angle
Y = 0.12
BTU/hr ft2 oF
Thermally Broken Concrete Slab Extension
IMPROVED
IMPROVING THE PERFORMANCE OF
THE DETAILS
Y = 0.07 to 0.18
BTU/hr ft2 oF
Exterior Insulated Structural Steel Floor
EFFICIENT
Table 2: Summary of Linear Transmittances for Floor Slab Details
Details for standard practice often fall into the poor
category. Though it is often difficult to avoid all thermal bridging due to structural requirements, there are
viable solutions to address the worst offenders. For
example, a steel shelf angle at the slab edge is a common element for brick veneer assemblies. Shelf angles
that are attached directly to a floor slab provide a direct
heat path through the insulation. An improvement to
this detail is to attach a smaller angle with intermittent
knife edges to the floor slab and insulation between the
angle and floor slab. This detail is 40% better, in terms
in linear transmittance, and is not difficult to implement
in practice. This improvement can have some real benefit such as needing less insulation in the assembly to
meet specific overall targets as is illustrated below.
“The worst offenders are
exposed floor slabs for
interior insulated poured-inplace architectural
concrete walls”
The most notorious offender is un-insulated concrete
balconies or overhangs; often the “fin effect” of “HarleyDavidson Architecture” is cited as the worst thing you
can do in terms of energy efficiency4. However, for
exterior insulated assemblies there is not much difference between a protruding concrete slab and a flush
exposed slab face, both are bad. The worst offenders
are exposed floor slabs for interior insulated poured-inplace architectural concrete walls. Table 2 summarizes
the linear transmittance of some common floor slab
details. Solutions are designing “thermal breaks” into
the floor slab at the plane of insulation, (proprietary
systems are available) or using this information as a
good reason to exterior insulate, including at the floor
slab.
SOLUTIONS MH Volume 2012 Issue 3
The impact of details on the overall heat flow through the building envelope can be determined easily using linear transmittance. The additional
heat flow through specific details can be highlighted as presented in Table
4. This example compares the “effective R-value” or overall U-value for a
total wall elevation with 40% glazing with only one type of floor slab edge
condition. Table 3 summarizes the parameters utilized in this example.
Table 3: Example building parameters
Building Parameter
Table 3: Example
building parameters
Parameter Value
Total Elevation Area 3
3000 ft2 (278 m2)
Glazing %
40%
Total Opaque Wall Area
1800 ft2 (167.2 m2 )
Brick veneer supported by brick ties with R-15 Exterior
Insulation and concrete block back-up wall
U-0.056 Btu/hrft2oF
Metal Cladding supported by horizontal girts on 24”
o.c. (610 mm) with R-15 Exterior Insulation and R-12
Batt Insulation in a Steel-Stud back-up wall
U-0.054 Btu/hrft2oF
(0.31 W/m2K )
(0.32 W/m2K )
Table 4 shows the calculated results. The difference between the standard and stand-off shelf angle is not significant for this example. This is
largely due to the continuous metal flashing over the shelf angle for both
scenarios. There is a significant difference between the common concrete
slab projection scenario and a thermally improved system incorporating
a thermal break. The difference between these two scenarios for the example wall assembly has the same impact as adding approximately 45%
more wall area or 4 feet of wall height to each floor.
“The difference between the two scenarios
has the same impact as adding 4 feet of
wall height to each floor”
The insulated slab edge with improved details has an overall U-value closest to the U-values typically assumed for whole building energy modeling
but there is still a 28% difference5.
Table 4: Example building thermal values
Parapet
Table 4: Example building
thermal values
and
Floor Slab
Scenario
Standard Shelf
Angle with
Continuous Metal
Flashing
Stand-off Shelf
Angle Continuous
Metal Flashing
Un-insulated
concrete slab
Projection
Thermally improved
concrete slab
Projection
Insulated Slab with
Standard details
Insulated Slab with
improved details
Clear Field
Heat Flow
BTU/hroF
(W/K)
101
(53)
97
(51)
97
(51)
% Heat
Flow
Slab Edge
Window
U-Value
Heat Flow associated BTU/hrft2oF
Transition
with Floor
BTU/hroF
(W/m2K)
(W/K)
Heat Flow
Slab to
BTU/hroF
Total
(W/K)
25
(13)
52
(26)
25
(13)
Effective
R-Value
hrft2oF/ BTU
(m2K/W)
80
(42)
39%
0.115
(0.65)
8.7
(1.5)
56
(29)
31%
0.101
(0.57)
9.9
(1.7)
106
(55)
46%
0.129
(0.73)
7.8
(1.4)
36
(18)
22%
0.09
(0.51)
11.1
(2.0)
0.087
(0.49)
0.072
(0.41)
11.1
(2.0)
13.3
(2.4)
8
(4)
8
(4)
6%
6%
The key take away from this example is that the relative impact of each
detail can be determined and these details should not be overlooked in
practice. Ignoring these details will likely lead to misguided conclusions
and wasted opportunities for increased energy efficiency and to reduce
material consumption. This kind of assessment combined with sensitivity analysis of insulation levels, if doing whole building energy modeling,
should lead to more rational design decisions. Recognizing the diminishing returns of increasing insulation levels combined with the impact of the
details, will in many cases lead to more attention to designing thermally
effective details rather than simply adding more insulation and believing
energy savings will be achieved.
SOLUTIONS MH Volume 2012 Issue 3
CLOSING
It is time to make the conceptual leap to linear transmittance
to evaluate building envelope details, such as floor slab,
during design. Not only is the method straightforward to use
but the data in the 1365-RP catalogue and categories in this
solution allows the relative strength of details that are often
overlooked in practice to be effectively assessed. These
concepts are not limited to floor slabs; heat flow through window transitions can be significant and should not be ignored
either.
An increased awareness of the impact of the overall thermal performance of the building envelope, by utilizing these
methods, can be incorporated in practice by the entire
design team; energy modeler, architect, contractor, HVAC
designer; to make informed decisions that consider cost,
energy efficiency, and material use.
Floor Slab Edges Count
Volume 2012 Issue 3
We think that codes and standards will eventually adopt the
linear transmittance methodology because highly insulated
assemblies with thermally poor details is not rational and this
methodology can be effectively adopted within the existing framework using several practical approaches. In the
meantime, these concepts should be part of the discussion
on any project where whole building energy simulations are
being conducted and overall thermal transmittance are being
input into a model.
This solution highlights MH’s expertise of the thermal efficiency of the building envelope. Our collective knowledge
and judgment is gained through extensive field experience
and participation on a wide variety of projects. We leverage
this experience on every project to apply solutions that are
relevant to the design, construction and operation of the
build environment.
BUILDINGS
RECOGNIZING THE IMPACT
LEADING THE WAY TO A SUSTAINABLE
FUTURE
FOOTNOTES
1
ASHRAE Research Project 1365 Thermal Performance of Building Envelope Details for Mid- and High-rise Buildings is available
through the ASHRAE website bookstore or at www.morrisonhershfield.com /ashrae1365research.
2
This idea was fostered by Belgian researchers that categorized
linear transmittances into three categories: “Business as Usual”,
“Standard”, and “Thermal Bridge Avoidance”. Janssens, A, E. V.
Landersele, B. Vandermarcke, S. Roels, P. Standaert, P. Wouters.
“Development of Limits for the Linear Thermal Transmittance of
Thermal Bridges in Buildings”. Proceedings of the IX International
Conference on the Performance of Whole Buildings, Clearwater, FL,
2007.
THE CHALLENGE
There are two challenges with evaluating the impact of thermal bridging
at floor slab details:
1. There are a variety of floor slab details that can be
integrated into a variety of wall assemblies.
2. The heat-flow paths at floor slabs can be complex and
are often three-dimensional (3D).
3
For slab edge details that are not fully insulated, the linear transmittance varies by less than 5% for increasing amounts of exterior insulation beyond R-5. This follows common sense, since regardless of
the amount of exterior insulation, heat can still bypass easily through
the thermal bridges. For these types of details, the linear transmittance can be considered constant.
4
Based on manufacturer’s data for one proprietary system
5
The overall U-value compared to the clear field U-value
If you wish to discuss how our services can help you make a
difference, please contact us at:
buildingenvelope@morrisonhershfield.com
www.morrisonhershfield.com/ashrae1365research
or through your local Morrison Hershfield office.
The prevailing North American method to account for heat flow through
parallel paths, as is often assumed when evaluating the impact of floor
slab details, uses weighted averages to combine heat flows. The main
drawback to this method is that an “area of influence” must be assigned
to the floor slab detail, which is not only often difficult to determine but
also not straightforward to use in practice. This process is not straightforward because of the 3D heat flow paths typically associated with
the details and the “area of influence” are usually unique to the specific
details and assemblies. Moreover, the true impact of the additional
heat flow through the floor slab detail is blurred by the adjacent highly
insulated wall assembly. As a result, it is difficult to effectively evaluate
options using this approach.
Thermal bridges at floor slabs are often overlooked in energy standards and common design
practice. The accepted truth seems to be that
structural members to support cladding cannot
be avoided and the area of the steel or concrete
bypassing the insulation is small compared to
the total wall area. Therefore the impact of floor
slab details on the total thermal performance
must be insignificant and there is not much you
can do about it anyway.
The inconvenient truth is much as 50% of the
heat flow through the wall might actually be due
to thermal bridging at the floor slab, if solutions to
minimize thermal bridging at floor slabs are not
implemented. This discrepancy widens as we
add higher levels of insulation into wall assemblies but don’t consider the impact of the details.
The good news is that there are viable solutions
to overcome current shortcomings in energy
standards and design practice. The outcome of
implementing these solutions will lead to more
energy efficient buildings and more efficient use
of building materials.
This SOLUTIONMH provides the motivation and direction to overcome
these challenges by showing how significant floor slab details are on
overall building envelope thermal performance and how the impact of
details, not typically considered part of an assembly, can be effectively
evaluated in practice using a linear transmittance approach.
SOLUTIONS MH Volume 2012 Issue 3