Air Barriers - Exterior Wall Assembly

Saturday, October 28, 2017

Also, since insulation is a material full of air pockets, it is important to stop air ow through that material. For insulation to be effective, it must be encased by an air barrier. Air barriers function to keep air from freely  owing through insulation, allowing it

to achieve the thermal performance (R-value) at which it was rated. To be effective, insulation and air barriers should be both continuous and contiguous, meaning that every exterior building assembly is insulated and encased by an air barrier. Research has proven that installing insulation without an effective air barrier results in a huge reduction in the effectiveness of the insulation and high bills with poor comfort.

Wherever the insulation is installed, there must be an air barrier in contact with the insulation on all six sides, leaving no insulation exposed; this prevents convection currents.  e wall studs, along with the top and bottom plates, close up four sides.  e exterior sheathing encloses the outside of the cavity, and drywall normally encloses the inside, but not without exceptions.

These exceptions are because there are areas of the thermal envelope that may be insulated but o en do not have drywall installed on the inside of the cavity.  is includes  replace and HVAC chases and behind bathtubs when these features are located on an exterior wall.  is can also include a stairwell on an exterior wall, even if part of the area under the stairs is a closet. Usually the under-stair closet ceiling slopes down to a point such that the bottom few steps of the stair would create a ceiling height too low to be usable. These bottom few steps, if on an exterior wall, will usually not have that area of the wall enclosed with drywall. In these areas, it is necessary to install some other type of air barrier to encase the insulation on the inside of the wall assembly.

Note that an air barrier is shown installed on the inside and out- side of the wall common with the attic space, o en called a knee- wall or pony wall.  These are vertical walls that separate a room from an attic space. Typically, builders do not install an air barrier on the attic side of these walls. They just stuff  some batts into the cavities and call it good enough. They also don’t place air blocking in the big holes under the knee walls where the ceiling framing runs.  is leaves dozens of big holes (16 inches by 8 inches) open so that out- side attic air easily blows between the uninsulated  floors and ceilings. In cold climates, the result is o en frozen pipes between the floors of the home where you would think that cold air shouldn’t be able to go. Very o en rooms over garages are uncomfortable be- cause they su er from both of these problems.  These areas, even if insulated, are large holes in your thermal envelope when not sealed by some type of air barrier.

The exception to the air barrier installation requirement is if you are installing blown insulation on the attic  floor. For blown-in insulation in the attic  floor, significantly higher R-values are typically required by building codes to achieve the desired resistance to heat needed here. Since the insulation is not installed vertically, it is not as susceptible to convection loops and for this reason doesn’t need to be encased on the sixth side.  e depth markers that are commonly seen in this type of installation ensure the depth of the insulation achieves its stated R-value.

A common hole in attic insulation occurs at the location of an attic scuttle hole or attic stair. Any attic access that penetrates the thermal envelope should be well sealed with weather stripping, with multiple layers of rigid board insulation applied to the attic side of the board cover, or by installing an insulated stair unit.

Air barriers should be sealed at all penetrations. On the exterior side of the insulation, the house wrap or rigid foam board must have all seams taped. To complete the air barrier, it is necessary to caulk and seal all penetrations in the building envelope.  ere can be no exception to this rule. Some of the more common penetrations in wall and roof assemblies include plumbing and mechanical vents, condensation drain pipes,  replace chimneys and electrical conduits,  fixtures, and outlets. Air infiltration into the building assembly occurs wherever these penetrations are not properly  flashed or sealed.

The way to know if you have an effective air barrier is to test the house under pressure and then measure the air infiltration rate. Since 2009 this test, o en called a blower door test, is required by code, but if you do not live in an area that mandates code inspections, you should make sure your builder is aware of the blower door test and that you see the results.  e house must be tested and proven to be a very tightly sealed structure by achieving no more than 5 air changes per hour at 50 pascals of pressure (ACH50).  is standard will be restricted even further to no more than 3 ACH50 in climate zones 3–8 by the 2015 International Energy Conservation Code (IECC) (advance information June 2014).

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