Sunday, June 21, 2009

Air Sealing the Attic Knee-Wall Area

The latest episode of my adventure involved improving the insulation of the attic. Once upon a time, I converted this from unfinished to finished space, using knee walls to maximize the occupied space under the hipped roof (the roof slopes at about 45 degrees on all four sides-- a characteristic of four-square house design). At the time, I was not motivated to research proper insulation techniques for attics. I've come to regret that, because in finishing the space the way I did, I lost a number of opportunities to make the house more energy efficient than what it could be. Nevertheless, the cause is not totally lost-- I am making a number of improvements behind the kneewall, in the areas where the sloping roof intersects with the soffit and the top sills of the walls.

Ten years ago, I attached batts of fiberglass insulation backed with kraft paper to the rafters (the sloped beams supporting the roof sheathing). The batts were then faced with drywall:

As you can see, this left a gap at the bottom in the bays between the floor joists. During the winter, heated air simply flowed through these passages as it pleased. One consequence of that is the formation of ice dams and some pretty mean ice cycles whenever we get an appreciable snowfall. The medicine for this is to fill these soffit gaps with insulation wrapped in a vapor barrier that prevents air from passing through it. This was achieved in several steps. First, I pulled back the existing insulation from the rafters and attached baffles to the underside of the roof sheathing. Baffles protect the roof's under-surface from warm air, which would lead ultimately to ice dams and a host of subsequent problems:

The second step involved sealing the top plate of walls with the foam-in-a-can product. This prevents air from escaping into the attic from heated spaces below:

Next, I manufactured a number of insulation "bricks," in this case, using unfaced R-19 fiberglass batts. The batts are sold in 24" by 48" sheets. My rafters and ceiling joists are spaced 24 inches on-center. So I cut the batts into thirds and and quarters, and wrapped them snuggly (but not compressed) in 13-gallon plastic bags, which have a 24" dimension along their bottom edge:

These bricks (or "pillows," if you prefer) are then tucked into the rafter bays:

This was also a good opportunity to fill voids in the interior walls, which are also an escape route for heated air. I'm pointing to such a void here:

This particular gap was filled with styrofoam from a packing crate:

This was then topped of with the foam-in-a-can stuff (applied after this photo was taken). By the way, those latex gloves? Absolutely worthless; they rip with little or no provocation.

It took me about four hours to complete about twenty linear-feet of rafter bays. Once again, I emerged absolutely filthy from the confines behind the kneewall-- an area that only a contortionist would love. But once again, the pay-off is the immense satisfaction of knowing that this task has been completed. I have two more sessions of this before the entire attic perimeter is finished. Then my attention turns to the floor joists in the basement. Truth be told, I can't wait.

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