Should I put 2" DOW insulation board on the outside or inside of my semi-conditioned crawl space?


Should I put 2" DOW insulation board on the outside or inside of my semi-conditioned crawl space?

Asked by Doug Keaty

I am looking for your thoughts on placing 2" DOW Blue Board, over water proofing, on the exterior of the cmu stems. I would like to continue the 2" blue board up the exterior osb of the framing to prevent thermal bridging. I can build 2x4 walls with cellulose insulation because of the added R-11 blue board. I like the idea of having one thermal line on the exterior wall all the way down to the footing. Some want me to jog the insulation to the inside of the stem. The floor joists are hung inside the cmu stems on a sill plate. I will vent the crawl space with a HRV. The price should not be considered in your comments -- I'm with Greater Albuquerque Habitat for Humanity, and we get the Dow board and other products at no cost.

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Danny Kelly's picture

Hi Doug,

Not exactly what the code says out your way, but here in NC we would be required to have a break in the insulation for a termite inspection gap. Even if it is not required by code, it is a good idea. Foam board is a perfect nesting area for termites.

  • Since you have the gap anyway, putting the foam on the interior of the crawl space should be fine.
  • In theory, having a continuous thermal line is ideal, but you are not really losing much by moving it into the interior -- be sure you insulate the rim joist well.

Using an HRV is an interesting idea -- most closed crawl space designs use a supply-only or an exhaust-only ventilation system -- using a balanced system seems a little unnecessary, but I do not really see anything wrong with it.

  • Assuming you are bringing hot dry air into the crawl through your HRV should be OK from a moisture and condensation problem, although there could be an energy penalty associated with that.
  • Is your HVAC equipment in the crawl space?

Advanced Energy actually did a study of closed crawl spaces in Flagstaff. I would review that prior to making any final decisions.

Good luck -- love Habitat -- great organization. Here in Charlotte, we just celebrated the 25-year anniversary of our local chapter.

For more information:

Check our home foam insulation Q&A to see what other homeowners and contractors are saying about foam board insulation, etc."