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)?
Maybe I can I install spray foam against the siding…
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?
- Spray polyurethane foam — especially closed-cell spray foam — will cling tenaciously to the siding, gluing everything together. This will make future siding repairs — for example, replacing a cupped or split piece of siding — almost impossible.
- The spray foam may ooze out of cracks between the siding as it expands, forcing the siding pieces apart. If this happens, you’ll end up with an unsightly, hard-to-clean-up mess.
- The spray foam will limit the ability of the back side of the siding to dry. When a rainy day is followed by a sunny day, the exterior side of each piece of siding will be dry, while the interior side will be wet. This condition leads to cupping and splitting.
You’ve got to have an air space
There are several possible ways to insulate this type of wall. No…