ASHRAE Journal - March 2022 - 54

TECHNICAL FEATURE
* Ceiling: Ceiling insulation
increases from R-38 to
R-49 for CZ 2 through CZ 3
and from R-49 to R-60 in
CZ 4 to CZ 8.
* Walls: Increases wood
FIGURE 3 IECC 2021 climate zone map.
Dry (B)
Moist (A)
Marine (C)
frame wall by R-5 for CZ 4
and CZ 5. Adds additional
prescriptive wall options.
* Slab: Increases slab
insulation depth for CZ 3
through CZ 5.
* Air-Leakage: Sets
maximum air leakage to
5 ACH for trade-offs with a
prescriptive rate of 3 ACH or
less for CZ 3 to CZ 8.
As envelope insulation
requirements are raised,
the disparity between
opaque envelope and window
thermal performance
becomes more pronounced.
For example, in CZ 5 the R-value ceiling
insulation requirement has increased
from R-38 to R-60 (~60% increase) during
the past five code cycles, while the
equivalent window requirements have
increased only modestly from R-2.86
(U-0.35) to R-3.33 (U-0.30) (~16%
increase). This relative disparity in
the thermal performance of windows
draws down the total R-value for the
wall assembly.
IECC Climate Zones
0A Extremely Hot Humid
0B Extremely Hot Dry
1A Very Hot Humid
1B Very Hot Dry
2A Hot Humid
2B Hot Dry
3A Warm Humid
CZ
1
2
3
4
5
6
7/8
WINDOWS
CEILING
(U-FACTOR)
NR
0.40
0.30
0.30
0.30
0.30
0.30
When a homebuilder upgrades the
windows of a home from double- to
triple-pane, the window performance can improve by
60% or more (R-3.33 [U-0.30] to R-5.26 [U-0.19]). This
added performance can boost the total wall R-value
and ensure code compliance while improving thermal
comfort. Within the energy code frameworks, the use of
higher performance windows allows for some flexibility
in envelope construction approaches as well as tradeoffs
with other insulating measures.
Trade-Off Analysis
To evaluate possible trade-offs, the authors used
54
ASHRAE JOURNAL ashrae.o rg M A R CH 2022
(R-VALUE)
30
49
49
WOOD-FRAMED WALL
(R-VALUE)
13 or 0 +10a
13 or 0 +10
20 or 13 +5 or 0 +15
60 20 +5 or 13 +10 or 0 +15
60 20 +5 or 13 +10 or 0 +15
60 20 +5 or 13 +10 or 0 +20
60 20 +5 or 13 +10 or 0 +20
3B Warm Dry
3C Warm Marine
4A Mixed Humid
4B Mixed Dry
4C Mixed Marine
5A Cool Humid
5B Cool Dry
TABLE 1 2021 IECC selected envelope insulation and air-sealing requirements.
SLAB
FLOOR
(R-VALUE)
13
13
19
19
30
R-VALUE,
DEPTH
10, 2 ft
10, 4 ft
5C Cool Marine
6A Cold Humid
6B Cold Dry
7 Very Cold
8 Subarctic/Arctic
CRAWL SPACE WALL
(R-VALUE)
5/13
10/13
10, 4 ft 15/19 or 13 +5a
30 10, 4 ft 15/19 or 13 +5
38 10, 4 ft 15/19 or 13 +5
AIR LEAKAGE
RATE (ACH)
5
5
3
3
3
3
3
aFirst value is cavity insulation and the second is continuous insulation, e.g., 0 +10 means R-0 cavity insulation, plus
R-10 insulated exterior sheathing.
programs to model and compare energy savings for various
2021 IECC code-compliant envelope configurations
in a 2,400 ft2 (223 m2) wood-framed home with a slab
foundation insulated to code, an unfinished vented attic
and a 15% window-to-wall ratio. The modeled home has
a code-compliant 80% AFUE gas furnace and a 14 SEER
air-conditioning system, and lighting is supplied with
high-efficacy LEDs in all lighting fixtures.
To determine the air leakage trade-off impacts, the
2021 IECC prescriptive code requirements were modeled
with 0.3 U-value windows and 3 ACH at 50 Pa of
envelope leakage. The home was then modeled with
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