Subfloor replaced. Now what? Insulation?
Skillzzz9
| Posted in Energy Efficiency and Durability on
I live on the coast of south Texas and have recently replace the subfloor, will plates and some joists, all due to rot. The contractor added more vents around the house to improve moisture removal, but I’m not convinced that is all that needs to be done. The crawl space is dirt floor with numerous obstacles in place that cannot be removed (remnants of an old chiller system). I was wondering if putting up foam board under the joists would keep out moisture or should we try to encapsulate with closed cell insulation. The house was built in 1937.
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Replies
On the south coast of Texas crawlspace vents brings more moisture into the house than it takes out.
But if you can't put down a ground vapor retarder and make it a conditioned crawlspace, the next best thing would be to install rigid foil-faced polyiso under the joists, taping the seams with a high quality aluminium tape. It would take R13 to meet current code performance if installed between the joists, but you'd be fine at R10-R12 of continuous foam.
Seal the band joists to the foundation with can-foam or closed cell spray foam, at which point you can use high-density R15 batts (rock wool or high density batts) cut to fit snugly to insulate the band joist.
2" of closed cell polyurethane spray would get you there too, but it's more expensive, had a much bigger environmental hit, and won't be as vapor retardent as foil-faced polyiso.
Foil faced EPS could work too, but unlike polyiso it melts when burning, and is thus not quite as safe to use without thermal barriers against ignition.
Cap nails or cap screws can be used to hold up the foam, but for the longer term I'm personally more comfortable using 1x strapping perpendicular to the joists, nailed to the joists with ring shank nails. YMMV.
William,
The crawl space vents are the problem. You need to create a sealed (unvented) crawl space. Here is a link to an article that explains the necessary steps: Building an Unvented Crawl Space.
If you have abandoned equipment in your crawl space, you can still drape that equipment in a vapor barrier. You may need to use something tougher than 6 mil poly to do this -- perhaps TuTuf or 20-mil pool liner. Once you have draped all the bumpy things with your vapor barrier, install more of the same material on the level sections of your dirt floor, and seal the seams with a high-quality tape. You may need to weigh down the seams and edges of the vapor barrier material with a few bricks.