ASHRAE Journal - March 2022 - 58

TECHNICAL FEATURE
FIGURE 5 Triple-pane window site energy use equivalency in terms of air-sealing
tightness.
2021
Prescriptive
Trade-Off
Option 2
8
7
6
5
4
3
2
1
3C
4C
Air Infiltration: 3 ACH50
Window: U-0.30
Air Infiltration: Maximum Allowable for Equal Energy Use
Window: U-0.20
250
Prescriptive
Alternative
200
5 ACH50 = 2021 IECC Trade-Off Limit
150
100
50
4A
5B
Climate Zone
approximately 1 ACH to 2 ACH across all climate zones
under the IECC performance compliance paths.
To achieve higher performing envelopes, builders
should always strive to meet the tighter air leakage
standards; however, as previously noted, builders have
repeatedly identified the IECC air-leakage requirements
as one of their biggest challenges. Air leakage results are
also not known until after the envelope is closed, so this
uncertainty presents additional concerns for builders.
The trade-off results indicate that, across all climate
zones, upgrading to higher performing windows can
offer builders a substantial buffer or " insurance " against
the risk of not meeting air-sealing targets and could provide
some flexibility in design decisions.
Wall Insulation
Builders make wall insulation selections based on a
variety of factors including regional preferences, construction
practices and material and labor availability.
While the added cost of exterior foam board might be
acceptable for a given project, the builder might find
the potential moisture control issues and constructability
challenges with extended sills and flashing details
unacceptable for their project. These builders might
58
ASHRAE JOURNAL ashrae.o rg M A R CH 2022
6A
8
FIGURE 6 Envelope component site energy use trade-offs: windows versus walls.
2021
Prescriptive
Trade-Off
Option 2
Wall: CZ3: R-20 or R-13 +5 or 0 +15
CZ4 - CZ8 R-20 +5 or R-13 +10 or 0 +20
Window: U-0.30
Wall: CZ3: R-13
CZ4 - CZ8: R-20 or R-13 +5
Window: U-0.20
3C
4C
4A
5B
Climate Zone
see window upgrades as a welcome alternative to adding
foam insulation. Alternately, there may be other builders
in certain regions and markets who are okay with foam
board insulation but would prefer to avoid moving from
2 × 4 to 2 × 6 wall stud construction for cost and labor reasons,
along with concerns of losing interior floor area.
Figure 6 depicts the whole-house site energy use comparison
between the model home built to the 2021 IECC
prescriptive envelope requirements versus a home built
with high R-value windows (U-0.20 and solar heat gain
coefficient appropriate for climate zone) and 2 × 4 R-13
wall insulation in CZ3 and 2 × 6 R-20 in CZ 4 through
CZ 8. These comparisons are presented in terms of site
energy savings (MMBtu/yr). The results demonstrate
that the stud and cavity insulation upgrade in CZ 3 and
the +5 exterior insulated sheathing in CZ 4 through CZ 8
could be traded off for high R-value windows in all climate
zones.
In CZ 4 through CZ 8, for example, 2021 IECC requires
R-20 +5 or R-13 +10 or R-0 +20. For builders who typically
build to R-20, the high R-value window trade-off
eliminates the need for exterior insulated sheathing
(+5), which reduces material costs for the fasteners,
sill extenders and flashing as well as the insulation for
6A
8
ACH50
Site Energy Use (MMBtu/yr)
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