Replies

1. GBA Editor
   Martin Holladay | Jan 29, 2013 08:39am | #1

   David,
   If the crawl space walls are insulated, and there are no open vents, then there is no need for any insulation in the crawl space ceiling. For more information, see Building an Unvented Crawl Space.

   If you prefer to have a vented crawl space, or a crawl space with uninuslated walls, you can insulate the crawl space ceiling. As you guessed, fiberglass batts won't work. Remove the fiberglass batts and put them in a dumpster.

   Then install at least one continuous layer of 2-inch polyisocyanurate under the bottom of the floor joists. (Two layers of two inch foam, installed with staggered seams, is even better.) Foil-faced polyiso is the easiest type of rigid foam to tape.

   Tape the polyiso seams carefully with a high-quality tape, and seal the perimeter of the ceiling with canned spray foam or European tape.

   (The rim joist areas should have been insulated and air sealed before the ceiling was installed.)

2. 
   redeft | Jan 29, 2013 09:20am | #2

   Martin,
   The crawlspace has four Anderson® windows, which I can seal with tape or caulk.
   The walls have 2" SilverGlo™, (which is an expanded polystyrene foam insulation that's been infused with graphite to improve its insular ability by 24%.) Screwed and glued to the poured foundation wall. All seams are taped and covered with vapor barrier up to the rim joist.
   The rim joist areas are insulated with fiberglass and it is in need of replacing.
   We have radiant heating in the ceiling of the crawlspace(floor of the living area).

3. GBA Editor
   Martin Holladay | Jan 29, 2013 09:27am | #3

   David,
   You'll need to air seal and insulate your rim joists with spray foam or rigid foam insulation.

   Because of the hydronic heating pipes in your crawl space ceiling, you should install insulation at the ceiling level as I recommended.

   Here's a blog that contains a warning -- why you want to do a good job at the rim joists: Lessons Learned the Hard Way.