Replies

1. GBA Editor
   Kiley Jacques | Apr 06, 2021 03:17pm | #1

   Hi Roger,

   I’m going to give your question a bump in hopes that experts will weigh in. I know they will have questions to help clarify your conditions in order to give an informed answer. While you wait, take a look at this article by Martin Holladay. It lays out some of the things you need to take into account when insulating floors with spray foam: Thermal Barriers and Ignition Barriers for Spray Foam.

2. Expert Member
   CARL SEVILLE | Apr 07, 2021 01:52pm | #2

   I would go with closed cell foam, as it will be much more resistant to vapor transfer than open cell. When the AC is on the floor will be cold, and open cell foam may allow vapor to condensate on the underside of the floor between it and the foam. Closed cell will tend to avoid that situation.

3. 
   Nola_Sweats | Apr 07, 2021 02:06pm | #3

   I'm in New Orleans and I have closed cell under a raised wood-framed house. I was told that open cell under a house is not allowed, at least in New Orleans, for reasons having to do with the ability of water and/or water vapor to pass through it. I think it was a flood insurance requirement, but I'm not certain. Carl's also right that open-cell can allow water to condense on a cold subfloor in summertime.

4. Expert Member
   KOHTA UENO | Apr 07, 2021 04:39pm | #4

   Joe Lstiburek wrote a column about these assemblies--takeaway is that closed cell works quite nicely to keep the wood warm (avoiding condensation) and controlling vapor flow.

   BSI-009: New Light In Crawlspaces
   https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces