Replies

1. 
   Chris_King | Dec 26, 2017 12:27pm | #1

   Also thinking of throwing some spare Roxul into the cavity after the EPS. And where the ground meets the cement outside I would switch to XPS I got from a construction site from polyiso due to moisture issues with poly.

2. Expert Member
   Dana Dorsett | Dec 26, 2017 02:33pm | #2

   Yes, it's fine to install 2.5" EPS on the band joist & foundation sill. It's better to use un-faced EPS if you're planning to use foil faced foam on the exterior later. At 2.5" Type-II (1.5lbs per cubic foot nominal density) EPS runs about 1 perm, the boundary between Class-II and Class-III vapor redardency. In zone 4A you could also just caulk the seams and install carefully trimmed R15 rock wool batts. It's also possible to use 2" fiber-faced reclaimed roofing polyiso, which is usually cheaper than high density batts or virgin-stock EPS:

   https://cnj.craigslist.org/search/sss?query=rigid+insulation

   In zone 4C (the coastal Pacific Northwest ) you'd still want at least 3/4"-1" of EPS between the batts and band joist for dew point control, but in 4A it's not really necessary. At 1" Type-II EPS is between 2.5-3 perms, which is quite a bit more drying capacity, almost as vapor open as interior latex paint. See:

   https://up.codes/viewer/utah/irc-2015/chapter/7/wall-covering#R702.7.1

   On the foundation walls themselves stick with just the foam, no fiber. The foundation is usually 2-3x as thermally conductive than a band joist, so it's at least as important to insulate the above-grade portion foundation walls as it is to insulate the band joists. If using polyiso, don't let the cut bottom edge of the foam contact the soil, since polyiso is somewhat hygroscopic and will wick up moisture.

3. 
   Chris_King | Dec 26, 2017 06:00pm | #3

   Thanks Dana for the detailed answer.

4. GBA Editor
   Martin Holladay | Dec 27, 2017 05:34am | #4

   Chris,
   Here are links to two articles that explain what you need to do:

   Insulating rim joists

   Building an Unvented Crawl Space

5. 
   Chris_King | Jan 16, 2018 09:36pm | #5

   Hey guys was wondering about swapping the interior EPS for open cell foam in the joist bays. Would cost more but would save a lot of time. Would that still be vapor open enough if I were to add polyiso on the exterior of the rim eventually? Everything I’ve read including the links suggests closed cell but then no exterior foam.

6. GBA Editor
   Martin Holladay | Jan 17, 2018 05:55am | #6

   Chris,
   First of all, it sounds as if you are worried that the installation of closed-cell spray foam on the interior of your rim joist will preclude the installation of exterior polyiso. Your worries are unfounded. As long as your foundation wall includes some type of sill-seal between the top of the foundation wall and the mudsill, there is no need to worry about foam on both sides of the rim joist.

   Second: The reason that most experts advise builders to use closed-cell spray foam rather than open-cell spray foam at the rim joist is that it is advantageous to stop outward vapor drive. You don't want moisture from the warm, humid interior to diffuse through the insulation layer and reach the cold rim joist. So closed-cell spray foam on the interior is better than open-cell spray foam.

   The sooner you can get your exterior polyiso installed, the better (and the less this issue matters). Once the exterior polyiso is installed, the rim joist will be warm, and the open-cell spray foam would become safe. Before the exterior polyiso is installed, however, the use of open-cell spray foam is somewhat risky.

   Adding it all up is complicated. The final decision depends in part on whether the installation of exterior polyiso is a certainty, or just a hoped-for dream.