Page 54 - Green Builder Nov-Dec 2021 Issue

Key Air Barrier Details

AIR BARRIER LOCATION AIR BARRIER DETAIL
Wall Behind Shower/Tub
Wall Behind Fireplace
Insulated Attic Slopes in Unvented Attics
Attic Knee Walls
Walls Adjoining Exterior or Skylight Shaft Walls
Unconditioned Spaces
Wall Adjoining Porch Roof
Staircase Walls Adjoining Exterior
Double Walls with Air Space
Rim/Band Joists
Insulated Floor Above Garage, Basement, Crawl Space
Floors Between Conditioned
and Unconditioned Space Cantilevered Floor
Floor Framing Adjoining Garage
Duct Shaft
Shafts Between Conditioned Piping Shaft
and Unconditioned Space
Flue Shaft
Attic Access Panel
Attic Drop Down Stair
Dropped Ceiling/Soffit
Attic/Ceiling Interface Raised Ceiling
Recessed Lighting Fixtures Table 1: Comprehensive
Wind Baffles at Attic Eaves Air Barrier Details
Whole-House Fan SOURCE: U.S. ENVIRONMENTAL
PROTECTION AGENCY

infrared image shows light images are warm and dark surfaces attic and one with an unvented attic. The choice between these
are cold. It will also show extensive thermal bypass through the two roof insulation options has a significant impact on cost and
attic insulation resulting in extensive cold surfaces. performance.
In the summer, this thermal bypass will work in reverse and In the unvented attic, the interior drywall ceiling serves as the
lead to excessively hot surface temperatures at the dropped ceil- only air barrier at the attic/ceiling interface with the air barrier
ings. This can cause significant comfort issues due to the dispro- missing at the top side of the insulation. Therefore, this is not a
portionate impact mean radiant surface temperatures have on six-sided assembly with nearly half of the ceiling insulation sur-
comfort relative to ambient temperature. face area exposed to the attic (assuming wind baffles or blocking
Figure 3 shows a solution to the dropped ceiling air barrier at the eaves between rafters).
detail, where drywall has been installed at the ceiling and wall Since fibrous insulation is typically used for ceiling insulation,
adjoining an unconditioned attic and garage ahead of the soffit this top layer is exposed to air flow. Thus, any driving forces in
framing. Note that all seams are fully taped and spackled, and the attic due to wind, excessive temperatures, and high humid-
edges sealed for a complete air control layer. This will mitigate the ity can lead to more to less air, heat, and moisture flow through
type of thermal bypass problems that were noted earlier. the attic insulation. In addition, fibrous insulation in the attic is
A complete air barrier requires a wide range of special details subject to compression and reduced performance over time, as
that occur in home construction as shown in Table 1. An excel- it accumulates moisture and maintenance work involves work-
lent reference for these details is the “ENERGY STAR Qualified ers in the attic.
Home Thermal Bypass Checklist Guide.” I assume code insulation requirements are much greater for
A section through the whole house should pass a red line test attics than walls to account for the missing topside air barrier and
where the exterior and interior air barrier can each be drawn more egregious attic summer temperatures. A six-sided assembly
continuously without lifting the pen. An example of this red can easily be implemented in most climates, with an unvented
line test is shown in Figure 4 for two homes: one with a vented attic insulated at the slope with spray foam or structural insulated

52 GREEN BUILDER November/December 2021 www.greenbuildermedia.com