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Insulating rim joists without sealing the sides?

moistycanadian | Posted in General Questions on

Hey –

I have ~1950s house in Quebec (zone 6A I guess) with an unfinished basement that I am trying to insulate in order to make the winters a bit warmer indoors. After some research, I believe that the way the header space in the basement is made is a bit unconventional. Or perhaps standard for the 1950s but not anymore. 

Anyway – the setting is that there doesn’t seem to be a sill plate. The floor joists sit directly on the concrete foundation and the sill plate is not inset/recessed, it sits flush with the interior of the concrete foundations.

The standard procedure would be to insulate the header area with rigid insulation and spray in the edges in order to seal the area and prevent any air drafts. I am very apprehensive to do this due to moisture issues. The header space is about 1’10” above grade so I am not terribly worried about moisture traveling up the concrete into the wooden foundation, even if I live in a generally moist area. What I am more apprehensive about is that the house has had, on two occasions in the past five or so years, streaks of water running from the header space onto the wall of the basement. In the first case I think it was due to the roof being very old and leaking rain water inside the house – we fixed that and we had no more leaks. Last year it happened again, I believe this time due to one of our gutters having been blocked by debris and consequently we had water run down our whole facade, so I imagine the water traveled inside cracks of the brick masonry.

I am a bit worried about air sealing the header space because I am a relatively new homeowner and I cannot foresee all the ways by which moisture can travel towards the basement header space and rot the foundations. I wouldn’t have guessed that a leaky roof or a clogged gutter would do that – but here I am.

So I am planning to install rigid insulation in the header space using construction adhesive directly on the rim joist and NOT air seal it in any way for now. I would still have cold-ish floors due to the draft and energy loss in the basement, but hopefully putting in the rigid insulation would help at least a bit, and not air sealing the area would mean that water can pass through if there ever is any. Does that plan make any sort of sense whatsoever? I’m afraid I’m not a pro and I don’t know all the ways in which this can backfire on me. For instance, wouldn’t glueing the rigid insulation still create the potential for pockets of moisture, thereby making the whole thing moot?

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