Critical details for basement walls against concrete

It's becoming more common to see people making the most of basement space - be it a new home with an entertainment area, basement bedrooms, or renovations to existing homes to create home offices.

There are several ways of doing this. Given that many older homes have uninsulated basements (that is, there is no foam on the exterior of the frost wall), we'll start with insulated interior wall systems and how to build them.

Interior insulated walls

Wood-framed, fibreglass insulated walls.

The most common is to use standard fibreglass with wood-frame walls. This approach has a few limitations:

• The vapour barrier that comes from the slab must be sealed to the vapour barrier of the interior wall.
• Studs cannot contact the concrete frost wall.
• Fibreglass insulation cannot contact the frost wall, and must be isolated with housewrap (or a similar material)

We have a simple one-page guide that will help illustrate this: basement wall details (10-01-24)

Wood-framed walls with foam

Another common way to finish out a basement is to apply 3" of extruded polystyrene foam (XPS) against the wall, then build a small wall out of 2x2s, or 2x4s turned sideways to provide anchorage for drywall.

The key consideration of this system is that the vapour barrier that comes from the slab (as required in modern Code) must be sealed to the foam. Illustration: foam basement wall details (10-01-24)

Site-applied foam is also acceptable, as long as the studs do not contact the wall, and the foam is covered with OSB, plywood, drywall or similar. In some cases, foam-application firms will suggest a spray-on fire-retardant instead: before this is done, ensure it's Code-compliant by contacting our office with product details.

Foam and fibreglass

This is a blend of the two systems above: the foam is applied to the frostwall, but a portion of the insulative requirement is served by fibreglass between studs. The most common approach is 1"of extruded foam against the concrete, with 2x4 walls and R12 fibreglass.

The requirements of this system are identical to the wood-frame wall with fibreglass:

• The vapour barrier that comes from the slab must be sealed to the vapour barrier of the interior wall.
• Interior vapour barrier is required. (This catches many by surprise)
• Because of the above, an air gap between the fibreglass and foam is recommended (but not required)

Illustrated guideline: combined wall details (10-01-24)

Walls for insulated frostwalls

Modern codes often encouraged the placement of foam insulation on the outside of the frostwall. In cases like this, where there is no need to insulate the interior, assemblies are much simpler. A simple stud wall with 2x2s or 2x4s laid sideways will do. The only considerations are:

• Studs cannot contact the concrete
• Vapour barrier must be continuous from the slab to the floor/ceiling assembly.