There is a simple test for whether your rim joist is the problem: does the house feel worse on windy days than on merely cold ones?
If it does, you are not losing heat through the insulation. You are losing it through gaps, and wind is doing the pushing.
Why wind targets this particular spot
Insulation is rated in still air. It slows heat conducting through a material and does nothing about air moving around it.
Wind creates a pressure difference across a building - higher on the windward face, lower on the leeward. That difference drives air straight through any unsealed path in the envelope. The rim joist is usually the largest such path in a house, because it is a junction, it is penetrated repeatedly, and it is typically stuffed with a batt that air passes through freely.
In one of the windiest cities in Canada, that adds up to something you can feel. It is also why the discomfort is intermittent rather than constant: on a calm cold day the house behaves reasonably, and on a windy one it does not. Insulation depth cannot explain that pattern. Air movement can.
The measurable consequence
A leak driven by wind pressure moves far more air than the same gap under still conditions. So the cost of an unsealed rim joist in Lethbridge is higher than the identical defect in a sheltered part of the province, and the payback on fixing it is correspondingly shorter.
If the house sits near a coulee edge or on open ground with no shelter belt, that effect is larger again.
What sealing it changes
- The draught at floor level on the main storey, because you have closed the intake.
- Cold floors, for the same reason. See cold floors.
- How the basement feels on a windy day, which in a finished basement is the difference between a usable room and a storage space.
Closed-cell foam is used here because it adheres to the surfaces, leaving no gap for air to travel in, and at adequate depth keeps the dew point inside the foam rather than on cold wood.
Do the top as well
Warm air escaping the attic is what creates the pressure pulling air in at the rim joist. Sealing only the bottom leaves the driver in place. The attic penetrations and the rim joist are two ends of one system, and the usual order is attic first, then rim joist.
See how to fix a draughty house.
What we do
We are a referral service. We do not install. We connect you with vetted installers - certification, insurance and WCB standing checked.
No price without a site visit. Get a quote, and tell us whether the draughts track the wind.
See also rim joist insulation province-wide and Lethbridge basements.