When the owners of an old wood-framed building say that they’re doing a “gut rehab job,” that usually means that they’re demolishing the lath-and-plaster walls to expose the studs — the first step of renovation work that usually includes new wiring, new plumbing, and new insulation.
Uninsulated stud bays are common in older homes, so workers who pull down the lath and plaster expect to see the back side of sheathing boards on the exterior side of the studs. But in some cases, there isn’t any sheathing: the workers find themselves staring at the back side of the siding (see Image #1 on the right).
So how do you insulate stud bays from the inside if the building has no wall sheathing or water-resistive barrier (WRB)?
One “solution” to this situation is to install spray polyurethane foam insulation directly against the back side of the siding. Why is this is a bad idea?
There are several possible ways to insulate this type of wall. No matter what type of insulation you choose, though, you need to create an air space between the back of the siding and the insulation. (For more information on the benefits of having an air space behind siding, see All About Rainscreens.)
If you’re planning on filling the stud bays with air-permeable insulation (fiberglass batts, blown-in fiberglass, mineral wool, or cellulose), the material you use to create the air space (in most cases, a layer of rigid foam) needs to be an air barrier.
The easiest way to create the necessary air space is to install vertical ¾” × ¾” or 1″ × 1″ “sticks” in the corners of each stud bay. These vertical sticks need to be tacked in place toward the exterior, up against the siding.
Once these sticks are in place, rectangles of rigid foam can be…