To protect the insulation, framing and wall finish from possible water damage, cover the basement walls with house wrap. Plastic was once the material of choice, but in some cases where air and moisture entered the insulated wall, condensation formed on the plastic and caused wetting and mould issues within the wall.

By using house wrap, moisture (not excessive and not leakage) that penetrates the new wall will dry out either into the interior of the house or to the top of the foundations that are above grade. The building paper should start at or just above the grade line and extend down to the basement floor and under the bottom plate of the frame wall. Mechanically support it with strapping such as 1 x 3 lumber.

SAFETY WARNING:

Do not use asphalt building paper (tar paper) inside of the house because it can release toxic vapours.

Framing

The next step is to install a frame wall. You can use one of two approaches:

• Install the wall close to but not touching the house wrap (12-mm [½-in.] spacing) using 2 x 4 or 2 x 6 lumber.
• Position the framing further away from the foundation wall to allow for a layer of batt insulation between the framing and the wall.

Allow enough room for a firm fit without compressing the insulation.

The second approach takes up more interior room but provides more insulation, less thermal bridging through the studs and better moisture protection. The bottom plate should sit on top of the extension of the house wrap and on a continuous impermeable membrane such as a sill gasket (see Figure 6-14).

Next, fasten the top plate to the bottom of the joists. Where the wall runs parallel to the joists you will have to build in a nailing support for the top plate (the approach you use will depend upon your particular house). Now is the time to square up the walls.

If your home is in an area of expansive clay soils, you may have to include a 25 mm (1 in.) gap between the top plate and the bottom of the floor joists to allow for vertical movement of the basement slab/floor. Talk with your local building authorities about how to best handle this situation.

Next, install the studs 610 mm (24 in.) on centre (i.e. measured from the centre of one stud to the centre of the next). Confirm that this spacing will offer structural support for your finishing needs. Make sure that the studs are perfectly vertical and accurately spaced so that the insulation will fit snugly and the finish can be installed without problems. Measure each stud separately. Extra framing is needed around doors in the foundation, but window openings only need a single stud, as the wall is non-load bearing.

If all alignments are perfectly level and square and there are no obstructions, you may be able to build the wall on the floor, tilt it into place, shim the bottom plate and secure the wall frame in place. Then install any required wiring and plumbing rough-ins.

Insulating

If you left a space behind the frame for batt insulation, you can now add the insulation between the studs and the wall in a horizontal layer. The insulation must be tight against the foundation wall. Next, fill the frame wall with a vertical layer of batt insulation fitting it snugly between the studs, allowing no gaps, air spaces or over-compression. Alternatively, if using fibreglass blown-in insulation, fill all cavities to the manufacturer's recommended density and RSI level.

Finishing

Install a vapour barrier over the studs and insulation. In basements that have proven to be dry, a polyethylene vapour barrier is suitable. However, if you have any doubts or there is a risk of dampness in the basement, there are two alternate methods that may be better suited.

The first alternate method is to use polyamide sheeting, a breathable membrane also known as Nylon-6 vapour retarder film or smart barrier. When installed on the warm side of an exterior wall, smart barrier has a water vapour permeance that changes with the conditions within the wall. If the relative humidity in the wall cavity increases, the smart barrier will allow the wall to dry out toward the interior unlike other sheet-type vapour barriers. If you are using smart barrier, carefully follow manufacturer's directions and installation requirements, though its application is similar to polyethylene sheet with a few important exceptions.

Leave enough of either the polyethylene or smart barrier at the top to connect to the air barrier in the joist header space. Seal all edges, seams and penetrations with acoustical sealant or other approved materials. All joints should overlap over a stud and be sealed with a continuous bead of sealant that is run between the layers of material at the lapped joints. Staple the barrier to the stud through the bead of sealant (see detail in Figure 4-3).

The second alternate method is to use an air and vapour barrier system known as the airtight drywall approach (ADA). The ADA method uses rigid materials, typically drywall, very carefully and thoroughly sealed to the framing and all other component connections by using adhesive-backed foam tape and flexible caulking.

Attention to detail is critical. The ADA method works only if it is completely sealed and tied into the air and vapour barrier system in the rest of the house. The following is a non-inclusive list of ADA sealing considerations:

• Frame airtight boxes for plumbing, water and drain line penetrations with gasket on the face of the box and caulk the pipe penetrations (see Figure 3-6 and Figure 6-16).
• Use special airtight electrical switch and plug boxes that include a gasket on the face of the box and caulk all wire penetrations.
• Do not run wiring or plumbing from exterior walls into interior walls unless all holes are caulked (see Figure 6-16).
• Air seal all window and door frames using expanding foam and appropriate caulking (see Figure 7-7).
• Install foam gaskets and caulk the upper edge of the top plate and all other framing members that are in direct contact with floors, slab, interior walls and ceiling.
• Air seal all edges and perforations of obstructions such as stairs that are against the exterior wall.
• Frame in and air seal separately any electrical panels that are not surface mounted on the finish and then air seal all penetrations.
• Remember to insulate and seal the joist header space (explained later in Joist header space) before attaching the finishing surface. This area is particularly prone to air leakage and must be properly sealed and insulated as part of any basement retrofit.
• Finishing must include a vapour barrier primer or paint.