Conditioned Crawl Space Insulation
Hello GBA Community and Pros.
I am building a new house in central Idaho. Cold winters and hot summers. It is a single level 2,000 sf with a crawl space which will be about 3 feet tall.
I have searched on how to insulate the crawl since it will not be vented and will be “conditioned” space.
A recent new build in our valley did spray foam on the interior of the crawl walls. Our county requires that the foam be covered by fiberglass batts because the foam is flammable. So this is all fine I suppose.
But then on GBA I saw a few people comment about the benefit of NOT insulating the inside of the crawl so the mass of concrete can “modulate” the temperatures. Would you then insulate the outside of the crawl? I am very curious about this. Are there any advocates for this method?
Thanks group!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Blackfeet, this and the "related content" are worth reading: https://www.greenbuildingadvisor.com/article/details-for-a-closed-crawlspace.
If the crawlspace is not going to be accessed regularly, many places allow an intumescent coating over the foam as enough fire protection. Or they will likely allow Thermax brand polyiso rigid foam, which can be purchased with a white facing.
I have tried various methods and in most cases prefer to insulate the interior; concrete does well as an exterior material, and the benefits of thermal mass are greatly overstated in most cases. Thermal mass is most helpful when you have large outdoor temperature swings between night and day. You can elect to put insulation on the exterior instead; it's standard practice in many places. You just need to find a way to protect the portion of insulation that's above grade. Some builders cut the insulation off at grade, which leaves the part of the foundation that sees the coldest temperatures uninsulated--not very smart.
Thank you Michael. Good point about the insulation being exposed on the outside. I had not thought about that. I appreciate the comment on the Thermax and will talk to my building department bout the intumescent coating. Can you tell me how many layers of the Thermax you would use? Or tell me what r-value you would go for with spray foam. Much appreciated.