Air Sealing Kneewalls

by Paul LaGrange

Any attic wall that adjoins an interior space should be insulated and air sealed!

Where air barriers (See Info Sheet #1.3) are not installed on the unconditioned (attic) side of the walls, very hot or very cold air in the unconditioned space can easily flow through and around the wall insulation.   Figure 1 depicts cold surface temperatures at an attic knee wall in winter as a result of no attic-side air barrier and poorly installed insulation.

The darker colors in the infrared image in Figure 1 indicate excessive heat loss to the cold attic through the knee wall insulation. The wood studs appear as much brighter vertical lines, indicating they are much warmer than the insulated spaces between them.  In this case, the wood studs are providing more effective thermal protection than the insulation itself. The unsealed insulation has a very low R-value, and the resulting increase in energy bills can be significant.

The solution to this problem is installing an air barrier with sheathing or rigid insulation on the attic side (Figure 2).  The diagram below shows a knee wall constructed as a “six-sided wall,” with air barriers on all sides of the insulation, including top and bottom plates and blocking at floor framing.

Figure 3 Examples of properly blocked and air sealed attic knee walls

The images in Figure 3 above show the indoor (room) side of attic knee walls that have been fully air sealed. Once these walls are properly insulated, the rooms will be more comfortable and less likely to suffer from moisture problems.

“Air sealing reduces heat flow from air movement (convection) and prevents water vapor in the air from entering the wall. In a 100-squarefoot wall, one cup of water can diffuse through drywall without a vapor barrier in a year, but 50 cups can enter through a ½-inch, round hole. In fact, sealing air leaks is 10 to 100 times as important as installing a vapor barrier.” (www.energy.gov)

Materials Needed to Address the Situation

Unsealed walls can be covered with rigid foam board.

Construction Methods

Sheets of rigid foam board should be attached to the studs with roofing nails.  All gaps and seams, including the top and bottom edges, should be sealed with expandable foam or foil tape.

This image shows a kneewall covered with rigid foam board and sealed with foil tape.

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