Remove the existing wall board or plaster from a wood frame wall, add or upgrade the existing wall cavity insulation and then install the vapour barrier, new drywall and finish. Better yet, reduce thermal bridging by upgrading the stud cavity and adding 50 mm x 50 mm (2 in. x 2 in.), 50 mm x 76 mm (2 in. x 3 in.) or even 50 mm x 89 mm (2 in.x 4 in.) horizontal strapping, and fill the space with insulation.

Add a vapour barrier on the warm side of the wall within the first third of the thermal resistance value of the overall wall insulation. (See the 1/3 - 2/3 rule in Control of moisture flow.)

Alternatively, after filling the cavity with insulation, screw or nail the rigid foam board directly to the exposed studs. Foam board materials, with their high RSI (R) value, use less interior space and provide a thermal break. Depending on the type and thickness of the foam board used, either seal the foam board to act as a vapour barrier or install an approved vapour barrier (see Chapter 3, "Materials").

If you are considering having spray-foam professionally installed, there are two basic techniques - one that uses low-density spray foam and the other, high-density spray foam.

For low-density applications, you can choose to remove any existing wall cavity insulation and fill the cavity or better yet, strap the wall and then fill the full, deeper space. The low-density foam will act as an air barrier, but you will have to install an additional vapour barrier before covering the wall with drywall and finishing.

For high-density spray foam, first strap the wall to create a deeper cavity. Then spray a continuous layer of foam (minimum 50 mm [2 in.]) against the exterior sheathing and also cover the framing members. Fill the rest of the cavity with your choice of insulation (e.g. low-density spray foam, glass, mineral or cellulose fibre). Since the high-density spray foam acts as a vapour barrier, the wall does not need a polyethylene vapour barrier. Instead, before finishing the wall, install a smart barrier or use the ADA method as described in Frame wall with single or double layer of batt insulation.