Thursday, April 9, 2009

About Attics, Air Leaks, and Foam-in-a-Can...

This photo was taken in the attic, behind the knee wall:

The masonry to the rear is a chimney. The seam where the chimney meets the framing has some gap-filling foam applied (the pale yellow stuff in the upper right corner), but it's poorly done and clearly not finished yet. The slats in the foreground are plaster lath for the ceiling below. The one-by-six plank to the left of the image is an attic floor joist. In the center of this image, parallel to the joist and running perpendicular to the lath strips, is wall top plate (that is, the top of a wall which is located in the room immediately below). All the literature points to wall top plates as a source of air leakage from conditioned space (below) into the attic. The medicine here is to seal the top plate with expandable foam. The foam, when still wet, is also a great adhesive; one can "glue" down a plank wide enough to form an air-tight cap over the top plate. A similar cap needs to be placed over the exterior wall top plate, which can be seen here running from left to right in the back of the image, immediately in front of the masonry.


  1. I like your blog. We are doing similar things (although our house is from, um, 1938 I think). We have an organization in Portland that will come to do a free blower test (I think that is what it is called). They did an assessment for us of air leaks and possibilities for more energy efficiency. Fun stuff.

  2. Hi, I am an energy auditor located in the Northern Illinois / Chicago land area, and just ran across your blog.. great job! I want to chime in and give you and anyone else reading this some tips; covering both safety and other items. I see there is some expanding foam used to seal the bypass where the chimney extends into the attic: both masonry and sheet metal chimneys should have a minimum of 2 inches of clearance between them and any combustible materials. A good way to seal chimneys is to cut a flange of sheet metal to fit the gap, and use fire-rated caulk to seal the metal to the chimney and other surrounding materials. Do note that 'fire block' expanding foam is a bit of a misnomer; it helps to prevent fire spread by containing air movement, but if you take a piece of that foam and hold a match to it, look out!!