Avoiding spray foam in basement sills
I’ve been working with an energy auditor and insulation contractor to seal up our home in climate zone 5b. I’ve been told the only option for sealing the basement sills in this climate is spray foam. I am really trying to avoid this product entirely, and want to find alternatives. I have access to attractively priced reclaimed foil-face polyiso rigid foam, butI I’ve heard lots of negatives about cutting this stuff in to rafter and stud bays, in terms of blockage of moisture. However, can I block it in with the foil facing inward? Are there methods of using poly film or some other air sealing material? What’s the poor, green man’s approach to sealing up this area?
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Replies
A. Bradford,
It's fine to use rectangles of rigid foam to insulate the interior of your rim joists. You can use XPS, EPS, or polyiso. If there is a foil facing, the foil can face either direction (although it makes more sense for it to face the interior).
Of these three types of rigid foam, polyiso is considered the most environmentally friendly, with EPS a close second and XPS a distant third.
Make sure that you pay attention to airtightness when installing these rectangles of rigid foam. You can seal the perimeter of each piece with caulk, high-quality tape, or canned spray foam.
For more information, see Insulating rim joists.
Thanks Martin.
That was simple. I guess this is not as critical a zone as, say, the roof. I had read some posts where somebody had used this method in the sills and found them dripping with moisture from poor air sealing. I have also read how even the smallest air sealing failure causes disastrous consequences in this method of roof insulation.
A. Bradford,
If you have an air leak in this location, you are more likely to get infiltration than exfiltration. In winter, infiltrating air is dry.
If you have an air leak in an insulated ceiling, you are more likely to get exfiltration than infiltration. In winter, when condensation is an issue, exfiltrating air is damp.
That is just the clarification I needed. Thank you.
Note that Martin's suggestion of using canned spray foam for sealing the perimeter probably does not violate your criteria on avoiding the impacts of spray foam because not only is the quantity tiny in comparison, but also, the "one part" stuff you buy in small cans has much lower environmental impact than the professional "two part" stuff.