The joist header space is also called the rim joist space, foundation header space or simply the joist space. This is the area where the floor joists intersect and are supported by the foundation walls in both basements and crawl spaces. Put simply, it is the area where the house structure rests on the foundation. This area is prone to air leakage and is seldom properly insulated, resulting in unwanted drafts, dust and pollen entry and vermin access.

There are three major joint configurations, each with implications on how best to air seal and insulate:

• floor joists on a sill plate (Figure 6-18, Part A and Figure 6-19, Part A)
• floor joists partially embedded in the foundation (Figure 6-18, Part B and Figure 6-19, Part B)
• floor joists fully embedded in the foundation (Figure 6-18, Part C and Figure 6-19, Part C)

If it is not possible to extend the exterior foundation insulation to cover the full header joist area, the space will have to be insulated and sealed from the inside.

If the foundation walls are insulated from the interior, the air and vapour barrier must be continuous for the wall and header space.

Figure 6-19 shows how to air seal and insulate three common types of joist header spaces.

For fully-embedded joists, do not exceed a maximum of 25 mm (1 in.) of foam board insulation as the concrete below the floor may make the floor above uncomfortably cold and prone to damage.

For all the approaches illustrated in Figure 6-18 and Figure 6-19, building codes may mandate levels of insulation, so check with your local authorities as to the recommended levels and practices.

Polyurethane spray foam installed by a certified contractor offers excellent air sealing and insulation of this space. However, the foam must be covered with fire-resistant material if it is not covered by the basement ceiling.