Cascadia Technical Bulletin #1b
Achieving ASHRAE 90.1-2007
Compliance: The Benefits of
Cascadia Fiberglass Windows
This bulletin discusses how Cascadia’s fiberglass windows and doors can help
designers comply with ASHRAE Standard 90.1-2007 “Energy Standard for Buildings
except Low-Rise Residential Buildings, while achieving higher glazing percentages
and optimizing the building enclosure thermal performance.
Background
Buildings that fall under Part 3 of the BC Building Code (BCBC2006) are required to
comply with ASHRAE 90.1-2004. However, the City of Vancouver Building Bylaw
references ASHRAE 90.1-2007. There are differences between the 2004 and 2007
versions of ASHRAE 90.1, in particular with respect to building envelope and window
requirements. ASHRAE 90.1-2007 compliance, specific to the City of Vancouver is
discussed in this bulletin #1b, while 90.1-2004 is discussed in a separate bulletin
#1a.
Key points to consider:
To date, code officials have not been strictly enforcing ASHRAE 90.1
requirements.
Code officials are beginning to enforce ASHRAE 90.1 requirements
Wall Assemblies with thermal bridges have discounted R-values that need to
be considered.
Standard aluminum frame windows will not typically meet ASHRAE 90.1
requirements – while “high performance aluminum windows” barely meet
requirements, leaving little room for achieving higher glazing areas.
Windows require NFRC certified U-values – not all manufacturers have this.
Methods of Compliance
There are 3 alternate compliance paths that can be followed to meet the building
envelope requirements of ASHRAE 90.1-2007.
1. Prescriptive Path
a. Wall R-values (or U-values) must meet the appropriate Table 5
requirements.
b. Window R-values must meet the appropriate Table 5 requirements and
the vertical glazing area must not exceed 40% of the gross wall area.
c. Skylights, roofs, floors and walls grade, opaque doors also must also
meet appropriate Table 5 requirements
2. Building Envelope Trade-off Path
a. A baseline building (the entire building envelope using minimum
prescribed R-values and prescriptive requirements for 40% vertical
glazing area) is modeled and compared to the proposed building
(using actual building envelope component R-values and glazing
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percentages). The proposed building vertical glazing percentage can
exceed the prescriptive path limit of 50% by using better thermally
performing walls and/or windows, as long as the “Energy Performance
Factor” (EPF) is lower (i.e. better) than the EPF for the same envelope
configuration with minimally compliant envelope characteristics of the
baseline building. For the most part, a lower EPF results from the
overall R-value for the proposed building being at or better than that
of the baseline building. The trade-off method also considers daylighting and solar heat gains, particularly through windows.
3. Energy Cost Budget Path
a. Whole building energy performance considering all building
components (building envelope, mechanical system, lighting, etc.) of
the proposed building versus the baseline building.
The two non-prescriptive compliance paths approximate and compare the heating
and cooling energy associated with a single square foot of building surface, where
the proposed building design must perform equal to or better than a baseline design.
For the purposes of comparing alternate window assemblies in this bulletin, area
weighted U-values are utilized to simplify the analysis.
Table 1 below includes an excerpt from Table 5.5-5 from ASHRAE 90.1-2007
(Climate Zone 5 - A, B, C Residential Buildings, for Vancouver/Victoria) indicating the
prescriptive requirements. Typically table 5.5 requirements are used for most of the
Lower Mainland and Vancouver Island Regions of BC, with table 5.5-4 for warmer
regions, or table 5.5-6 to 5.5-8 for colder regions, depending on the annual heating
degree day value of the building location.
For many buildings, the Building Envelope Trade-off or Energy Cost Budget
compliance paths are utilized:
1) to achieve greater vertical glazing areas than allowed by the prescriptive
values shown in Table 1,
2) because one or more of the other prescriptive requirements cannot be met, or
3) to realize some optimization benefits of the non-prescriptive paths.
The following sections outline building envelope considerations when using either
non-prescriptive compliance paths.
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Table 1: Adapted from ASHRAE 90.1–2007 Table 5.5-5 (Zone 5–A,B,C)1
Assembly Maximum
Wall Type:
Minimum R-value
U-value (Btu/hft2°F)
(R=1/U)
(includes all thermal
bridges)
Mass (i.e. exposed concrete,
concrete block, brick masonry etc.)
0.080
12.52
Metal Building (i.e. structural
steel components, i.e. industrial
type buildings)
0.057
17.5
Steel-Framed (i.e. steel stud wall,
but also includes curtain wall,
spandrels etc.)
0.064
15.62
Wood-Framed
0.051
19.6
Assembly Maximum
U-value
Minimum R-value
(R=1/U)
Nonmetal Framing (fiberglass,
vinyl, wood, hybrid)
0.35
2.86
Metal framing (curtainwall,
storefront)
0.45
2.22
Metal framing (entrance door)
0.80
1.25
Metal framing (all other, i.e.
aluminum punched, strip or
window-wall)3
0.55
1.82
Vertical Glazing (Windows)
1
2
3
Imperial units are used. To convert to metric divide R-values by 5.678, and multiply U-values by
5.678.
There is a ‘continuous insulation’ requirement associated with these wall types, if the overall U-value is
not compliant, based on accepted ASHRAE calculation procedures. This requirement, if compliance is
enforced, will mean that some insulation is required to the exterior of elements such as floor slab
edges, and structural steel. This represents a significant change in most conventional practices.
While fiberglass windows must have a U-value of less than 0.35 per the prescriptive requirements, for
the building envelope trade-off method a window U-value of 0.55 may be used for the baseline
building, with the U-0.35 (or the actual fiberglass U-value) being used for the proposed building. This
provides a huge incentive for the use of non-metal framing windows and for easy compliance with
ASHRAE 90.1 building envelope requirements when fiberglass windows are used.
Building Envelope Trade-off Considerations
Both the Building Envelope Trade-off and Energy Cost Budget compliance paths
provide greater flexibility for designers. The better than code minimum thermal
performance of one building envelope component can make up for another
component that may not meet the prescriptive standard. This approach allows for
vertical glazing percentages beyond 40%, the use of alternate wall assemblies, as
well as the incorporation of thermal bridge elements (i.e. balconies or eyebrows). For
the Envelope Trade-off method, compliance is achieved if the overall envelope
performance factor (building envelope U-value in simplified analysis) of the proposed
building is better than or equal to the baseline building. In the Energy Cost Budget
method, mechanical and lighting savings can also be considered to achieve
compliance beyond this simplified approach.
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The primary basis for the Building Envelope Trade-off method in our climate is an
area weighted U-value calculation. Similarly the Energy Cost Budget Compliance
path utilize area weighted U-values for components of the building envelope in
conjunction with the energy performance characteristics of other building systems to
compare with the baseline building. To assess the merits of alternate windows (or
overall window and wall combinations) during the early phases of design, it is useful
to examine the relationship between glazing percentage and area weighted U values
relative to the ASHRAE baseline performance criteria.
Window U-Values
ASHRAE 90.1 requires that fenestration U-values must be determined in accordance
with National Fenestration Rating Council (NFRC) Standard 100. Certified overall
window U-values which accurately account for different framing and IGU
combinations can be difficult to obtain from some window manufacturers who
haven’t gone through the NFRC certification process. Windows that do not have
NFRC certification (not listed in the NFRC Product Directory) are required to
reference Tables of typical product types provided in the 90.1 Standard Appendices
(and are unfavorable). Historically, NFRC certification has only been undertaken by
window manufacturers trying to qualify for EnergyStar ratings. As most aluminum
window fabricators cannot achieve EnergyStar, there has been little incentive to
carry out the costly and time consuming NFRC certification process.
NRFC U-values are based on modeled standard window sizes (validated through
laboratory testing), and may not be representative of actual window sizes and
configurations installed in the actual building. This is reasonable as determination of
certified window U-values for various window sizes and IGUs cost-prohibitive for the
window fabricator. Computer modeling to determine NFRC certified window U-values,
SHGC factors, Condensation Resistance (CR), and Visual Transmission (VT) factors is
typically performed using THERM and WINDOW software packages, developed by
Lawrence Berkeley National Laboratory (LBNL).
It is also worth noting that the BC Energy Efficiency Act contains new mandatory
requirements for labeling of window products with respect to energy performance
and NFRC product certification.
NFRC certified window U-value information is necessary in order to undertake any of
the trade-off analysis associated with the Building Envelope Trade-Off or Energy Cost
Budget compliance paths.
Wall R-values
ASHRAE 90.1 stipulates that wall R-values must consider the effect of thermal
bridging, to be representative of actual thermal performance (i.e. consider effects of
steel studs, girts, clips, slab-edges, balconies, eyebrows etc.). The one exception is
that the continuous insulation referenced by the prescriptive building envelope
option’s “minimum rated R-values of insulation” need not take into account
“fasteners and service openings”.
The minimum prescriptive R-values presented in Table 5.5 will be difficult to achieve
using wall designs that have been considered acceptable in the past. Hence the need
to explore the alternate compliance paths.
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For example, an exterior insulated metal panel wall with 3 inches of extruded
polystyrene insulation (R-15 nominal) has an effective R-value of approximately R-11
to R-13 (75 to 85% of nominal) when the intermittent metal clips and supports are
considered. A similar wall with stucco cladding utilizing continuous steel girts at 16
inches on center has an effective R-value of approximately R-7.5 (50% of nominal).
Neither of these walls meets the minimum R-15.6 requirement. Continuous
insulation, with designed cladding supports, spanning over the exterior of structural
framing members and slab edges is the most effective method of achieving
compliance with ASHRAE 90.1, and is required in order to comply with the
prescriptive path.
Opaque spandrel panels within a window assembly are considered ‘walls’ for
purposes of analysis, and are therefore considered separately from windows.
Spandrel panels typically have poor thermal performance due to thermal bridging
through frame components and the limited space available for insulation within the
window frame assembly. Depending on the size of panels, and thermal performance
of the frames, the effective R-value for a spandrel panel wall is generally less than
50%-70% of the nominal insulation value provided (with aluminum frames) – as
shown by thermal analysis and guarded hot-box testing. It is very difficult to
incorporate a spandrel panel arrangement that complies with the minimum ASHRAE
90.1 prescriptive R-value of 15.6.
Comparative Analysis of Alternate Window Systems
Determination of overall window U-values is a function of both the thermal properties
of the frame and the glazing.
For a fair comparison of alternate window frame systems, the IGU properties should
be the same for all windows being considered. Cascadia’s standard IGU is made up
of Cardinal Low-e 270 (dual silver soft-coat) with stainless steel spacers and argon
fill. This IGU is a very high performance unit and most other window manufacturers
do not carry this as their baseline option. Adding stainless steel spacers, argon fill
and a premium soft-coat dual or triple silver low-emissivity coating (emissivity
<0.04) will add up to $5.00 per square foot for most window suppliers – a point
which must be considered when comparing window types. For this analysis, all
windows considered have been assumed to utilize Cascadia’s standard IGU with a Uvalue of 0.25.
The following assumptions have been made in the analysis:
Cascadia Fiberglass Frames, U-0.27 (R-3.70)
A conservative value of U=0.27 (R-3.70) for a combination of window and
door types can be assumed if the exact mix of window and door types is
unknown at the onset of the project. If known, calculate an area weighted
overall U-value for all window/door types. Unlike aluminum windows, the Uvalues for both fixed and operable units in fiberglass are similar as the extra
framing associated with operable units does not significantly degrade the
assembly U-value
High Performance Aluminum Frames, U-0.39 (R-2.56)
The combination of better thermally performing aluminum frames with high
performance IGUs similar to Cascadia’s standard units result in overall U
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values in the range of 0.37 (R-2.70) for fixed and 0.44 (R-2.29) for operable
units. Assuming a mix of 70% fixed to 30% operable, an area weighted Uvalue of U-0.39 (R-2.56) is used for this analysis. Again, this information is
sufficient for preliminary analysis purposes, but actual area weighted
averages should be used if they are known.
Standard Aluminum Frames, U-0.57 (R-1.75)
Many aluminum window products are not ideally configured thermally, nor do
they utilize improved thermal breaks. They therefore have poor overall
energy performance characteristics. For the purpose of this analysis it is
assumed that a standard aluminum window (with the same IGU as Cascadia)
will at least meet the minimum ASHRAE U-value of 0.57 for <40% glazing
when using the building envelope trade-off method.
Walls
The walls have been assumed to be steel-stud infill with exterior insulation
with the minimum prescriptive performance level of U=0.064 (R-15.6) that
would account for some thermal bridging at balconies, etc.
Overall Building U-Values
The overall building U-values are determined using area weighted averages,
accounting for both the wall and window areas. U-values are used for the
calculations; however R-values are easier to understand and conceptualize
and are therefore plotted in the charts that follow.
Results of Analysis
Two charts are presented on the following pages. Each of the charts plots the overall
envelope R-value on the y-axis, with the percentage of the building envelope taken
up by vertical glazing (windows and doors) on the x-axis. Curves represent different
window types as described above.
At 0% window area, the overall envelope Rvalue is equal to the wall R-value and at 100% window area the overall envelope Rvalue is equal to the window R-value. Even a small percentage of window area has a
large impact due to the lower thermal performance. The second chart is the same
as the first except that it presents only a portion of the chart in order to better
facilitate a discussion the implication of alternate design scenarios.
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Figure 1: ASHRAE 90.1-2007, Effect of Window Frame Type and Window Area % on Overall
Building Envelope R-value
Figure 2: ASHRAE 90.1-2007, Effect of Window Frame Type and Window Area % on Overall
Building Envelope R-value
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Several design scenarios are illustrated in Figure 2:
1. Baseline – The baseline envelope scenario represents the minimum R-value
requirements for walls and windows utilizing 40% vertical glazed area per the
building envelope or energy cost budget trade-off methods. Window R-1.82
(U-0.55), Wall R-15.6 (U-0.064), Overall R-3.75 (U-0.267).
2. Increase Glazed Area – This scenario is applicable if the goal is to increase
the vertical glazed area above the baseline value of 40% when using the
building envelope trade-off or energy cost budget methods.
If high performance aluminum frames are used then a vertical glazing
percentage of 60% can be achieved while still maintaining the same energy
performance factor as baseline compliance with ASHRAE 90.1-2007 (overall
R-value stays the same as the baseline).
If Cascadia’s fiberglass frames are used then a vertical glazing percentage of
up to 94% can be achieved while still maintaining compliance with ASHRAE
90.1-2007 (overall R-value stays the same as the baseline). A building
envelope utilizing 94% glazing is unlikely in residential construction,
particularly considering the area of slab edges, balconies and desired opaque
walls. More realistically a maximum glazing percentage of 70% may be
utilized achieving an overall building envelope performance of R-4.8, 24%
better than the baseline.
3. Improve Energy Performance – This scenario is applicable when the goal
is to optimize annual energy costs. It also can achieve LEED credits (Energy
& Atmosphere – Optimize Energy Performance).
If Cascadia’s fiberglass frames are used then the overall envelope R-value is
76% higher than the ASHRAE 90.1 baseline (40% vertical glazed area is
maintained).
4. Optimize Initial Construction Costs and Floor Area – The flexibility
provided by the non-prescriptive compliance paths means that improved
window performance can be traded for slightly lower performance walls or
spandrels. This could mean less insulation thickness and thinner walls, a
greater amount of thermal bridging etc. This scenario also facilitates a desire
to maximize floor area or minimize initial construction costs.
As an example, if Cascadia’s fiberglass frames are used and the vertical
glazing percentage is maintained at 40% then the overall envelope R-value is
6.83 (Scenario 3). However, if the goal is to lower wall costs and gain some
floor area then the wall R-value can be reduced (less insulation, larger
balcony thermal bridging areas) while still maintaining at least the baseline
envelope performance that is mandated by ASHRAE 90.1. If 1” of insulation
is removed to create thinner less costly walls and greater floor area, then the
wall R-value of the walls is reduced to R-11.6 and the overall R-value to R6.26.
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LEED
The non-prescriptive ASHRAE compliance paths are also necessary in assessing LEED
credits. The overall U-value has a large impact on energy consumption, and a better
than ASHRAE baseline U-value will achieve more LEED points. Up to 10 points
(Energy & Atmosphere – Optimize Energy Performance credit) are available for
reducing the energy cost of the design building by between 15 and 60% over the
ASHRAE 90.1 baseline building (for LEED Canada NC 1.0).
Summary
Cascadia window and door products offer excellent energy performance. They
provide designers with great flexibility while still meeting the stringent performance
requirements of ASHRAE90.1-2007. When used in conjunction with the nonprescriptive ASHRAE compliance paths Cascadia products facilitate:
Significantly improved overall building envelope thermal performance
Increased glazed area percentages
Maximizing potential for LEED credits associated with the Energy &
Atmosphere – Optimize Energy Performance credit
Ability to optimize the balance of energy performance, glazed area percentage
and floor space.
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