OPTION 1:
AIR-PERMEABLE INSULATION:

A gut rehab on an old home usually means opening up the walls from the interior so that new wiring, new plumbing, and new insulation can be installed. Uninsulated walls are common in older homes, so it’s no surprise to see empty stud bays. But in some older homes—those without any sheathing—you’ll be looking right at the siding. How do you insulate stud bays from the inside if the building has no wall sheathing or water-resistive barrier (WRB)? There are two basic approaches, and they start with creating an air gap between the back of the siding and the cavity insulation you plan to install.

Using air-permeable insulation with rigid foam
If you plan to fill the stud bays with airpermeable insulation (fiberglass batts, blown-in fiberglass, mineral wool, or cellulose), the material you use to create the air gap needs also to be an air barrier. The most common choice is rigid foam. The easiest way to create the necessary air gap is to install wooden spacers (3⁄4 in. or 1 in. sq.) in the corners of each stud bay. These vertical spacers need to be tacked to the framing, up against the siding. Once they are in position, rectangles of rigid foam can be inserted (cut-and-cobble style) against the spacers in each stud bay. The idea is to create an air gap between the siding and the rigid foam. The rigid foam functions as an exte rior air barrier and is also a substitute for the missing WRB.

Foam thickness
The rigid foam needs to be thick enough to resist deformation by whatever type of insulation is used to fill the rest of the stud bay. For studs 16 in. on center, 3⁄4-in. XPS or polyiso should suffice for fiberglass or mineral-wool batts. Wider stud spacing or dense-pack cellulose may require thicker rigid foam.

In cold climates, you need to decide whether to be concerned about condensation on the interior side of the rigid foam. There are two possible ways to approach this situation. The conservative approach (assuming that most of the stud bay will be filled with air-permeable insulation) is to make sure that the rigid foam is thick enough to prevent condensation or moisture accumulation on the interior side of the foam. If the thickness of the air-permeable insulation will be 31⁄2 in. or less, the rigidfoam layer needs a minimum R-value of R-5 in climate zone 5, R-7.5 in zone 6, or R-10 in zones 7 and 8 to prevent this type of condensation.

A less conservative approach is to use 3⁄4-in. unfaced EPS for the foam layer. Without a foil facing or a polyethylene facing, EPS is somewhat vapor permeable (1-in. unfaced EPS has a permeance of 2 to 5.8 perms). The idea behind this approach is that if the stud bays get a little damp during the winter, the moisture will be able to dry—albeit slowly—to the exterior.

Seal air leaks at the perimeter of the rigid foam
Whatever type of rigid foam you choose—EPS, XPS, or polyiso—you need to pay attention to airtightness. Tape the foam seams, and seal the perimeter of each piece of foam with high-quality tape, caulk, or canned spray foam. Once this rigid-foam air barrier is sealed, the stud bays can be insulated normally. The studs also can be padded out with vertical wooden spacers or horizontal strapping to accommodate more insulation.

In most cases, this type of wall is designed to dry to the interior, so don’t install any polyethylene on the interior side of the wall. If your local code official insists on an interior vapor retarder, install a “smart” vapor retarder (one with variable permeance) such as CertainTeed’s MemBrain.

OPTION 2:
SPRAY-FOAM INSULATION:

Using spray-foam insulation
A second option for insulating a wall with no sheathing is to use spray foam. Polyurethane foam has the added benefit of sealing air leaks, but you shouldn’t consider installing it directly against the back of the siding. Spray foam, especially closed-cell spray foam, will cling tenaciously to siding, gluing everything together. This will make future siding repairs—for example, replacing a cupped or split piece of siding—next to impossible.

In addition, 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 mess that’s difficult to fix.

Finally, the spray foam will limit the ability of the back 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.

Making spray foam work
If you want to use closed-cell spray foam, install felt paper in each stud bay, up against the back of the siding, before spraying. (Open-cell foam is also an option, but it must be at least 3 in. thick.) Cut the felt as long as the stud bays, but about 2 in. wider. Fold over the extra inch on each side to form a stapling flange. Use an ordinary staple gun to secure these flanges to the sides of the studs, making sure to leave a 1⁄2-in. to 3⁄4-in. air gap between the back of the siding and the felt.

As the spray foam expands, it tends to push against the felt, reducing the depth of the air gap. But the felt has a rough texture and will be wrinkly enough to keep air between the felt and siding, even when compressed by the spray foam.

A variation on this method uses drainable housewrap instead of felt paper. Drainable housewraps are available from several manufacturers and have bumpy, crinkled, stamped, or channeled textures that maintain an air gap of at least 1 mm.