Kneewall Attics Part 2 - Air Sealing

Once again, prior to insulating, air sealing needs to occur. Typically in a kneewall attic, you'll find the soil stack (AKA: Stink Stack), electrical penetrations and the main area to seal - open floor cavities running the entire length of the wall on two sides.

The soil stack is almost always open to air penetration. Builders cut a square opening and run a circular pipe through it. Of course, there's air leakage all around. A quick foam fix.

Due to the fact that the attic has been turned into a living space, the most overlooked air sealing area is the floor. The joists typically run perpendicular to the vertical long wall - and run from one kneewall attic under the living space and into the second kneewall attic. There are 2 common ways to stop the cold air from running from the kneewall attic underneath the floor. One is expensive (but works the best) and the next is the least expensive (but is labor intensive).

Expensive: Two part foam in a box. This foam has a resin and a catalyst that when combined create a quick drying, dense foam. Each floor cavity is sprayed in full where the wall intersects the floor joists - stopping air from going underneath the floor. You could easily go through $300 of foam. This way is very quick and easy.

Laborious (I like this word): Using an R11 vinyl backed fiberglass, you stuff each cavity with this fiberglass (vinyl facing outwards) and then foam around the edges to create a seal using one part foam (purchased from any home improvement store). Cost on this would be the fiberglass (around $40) and foam (10-15 cans - $100).

Once the floor under the living space has been sealed off, the flat area is ready for insulation.

Next Topic: Kneewall Attics Part 3 - Sidewalls, Slopes and Crown