Mind the Gap – Air Leakage at the Top Plates

Most houses in this part of the world are ‘stick-built.’ That is, they’re framed with 2x4s – sticks. This term came from the timber framers who were used to building with massive posts and beams. When smaller lumber became available in the 19^th century, the timber framers derisively called them ‘sticks’ and the method of constructing houses out of them, ‘stick building.’

Today, most people in the construction industry use the term stick-building (without its negative connotation) to distinguish it from other methods, such as masonry, adobe, insulated concrete forms, and structural insulated panels. In a stick built house, each framed wall has vertical framing members (studs) between the in the summer it’s worse than that because air leakage from the attic brings in hotter air than air leaking in through an open window.

If your home has this kind of air leakage, it’s easy to find. Just go up in the attic, pull back the insulation, and find the top plates. The photo at left shows a typical one – flush drywall on one side and a big gap on the other. To fix it, you just fill the gap with caulk or spray foam. I can tell you from lots of personal experience that it’s not fun work, especially at the eaves in a house with a low pitch roof.

In new construction, it’s much easier to deal with. It almost never happens, but it would be so easy. After the drywall hangers get the ceiling done, they come back and do the walls. Occasionally they’ll put adhesive on the walls, but usually they’ll glue only the studs, not the plates. If they’d simply run a continuous bead of adhesive on the top plate, too, they’d block the pathway for air to move between attic and walls. So simple a solution. So much easier than going up into the attic afterward.

Another solution would be to buy a green modular home. Modular construction avoids the top plate gap by having continuous sheetrock across the ceiling. The ceiling panel is framed and sheetrocked before being set on top of the walls. If you look carefully at the photo at right, you’ll notice a white band between the wall and ceiling framing. That’s the drywall.

Of course, the best solution is just not to have an unconditioned attic. Frame the roof with structural insulated panels, or spray foam insulation between the rafters. If the attic is inside the building envelope, those top plate gaps don’t matter.

Photo at top from centralwisconsinhabitat.org.