Island Basement Systems Case Studies: Rim joist replacement due to condensation

Thursday, July 15th, 2021 by Chris Schmidt


The rim joist of this crawlspace was rotted due to condensation forming, which was a major structural issue. Condensation forms on rim joists through a combination of factors; when the crawlspace is passively vented to the exterior, poor insulation/air/vapour barrier on the rim joist, and humidity comes up through the skim coat. Passively vented crawlspaces rarely work as expected, due to stack effect in a home. And in the winter, all the moisture in the crawlspace will develop as condensation on the cold rim joists. This condensation then rots out the wood, causes mould, musty smells, and attracts carpenter ants.


Three steps are needed to prevent rim joist condensation:

1. Stop the earth moisture from wicking up through the crawlspace floor. A new groundseal is usually the most effective method.

2. Stop using passive vents in the crawlspace walls and switch to mechanical ventilation.

3. Install some form of insulation on the rim joist that has air & vapour barrier componet. Batt insulation sucks at this, even with 6mil poly and accoustical sealing. Closed cell spray foam is the best solution for retrofit crawlspaces.


In this crawlspace, we replaced the rim joist and to prevent the problem from re-occurring, we installed our patented CleanSpace two-layer ground seal liner, R20 closed cell spray foam, and mechanical ventilation.

This crawlspace is now clean & dry, the structure of the home is sound, and the owners love their home!

Project Summary

rim joist repair: condensation damage

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Island Basement Systems
467 John St
Victoria, BC V8T5H1
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