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Community and Q&A

Crawlspace Insulation: Interesting result thanks to mother nature: I had to put insulation directly on top of vapor barrier

user-1031346 | Posted in Energy Efficiency and Durability on

We recently experienced light ‘flooding’ of my unfinished basement that I was in the middle of framing. In order to get items away from potential water damage, I moved 10+ rolls of unfaced batt insulation into my crawlspace (which is 3 feet above the height of the floor of my unfinished basement). Well to get the rolls in the crawlspace, I had to open the rolls and layout it out directly on the dry vapor barrier. I proceeded to take care of the water on the unfinished basement and left the insulation on the floor of the crawlspace.

A week later, I returned to pull out the insulation from the crawl space – and I was AMAZED! The crawlspace was noticeability warmer – to the point where I am wondering, why don’t I just leave it there on the floor?

Let me describe my crawlspace: I have a sealed conditioned crawled space with the follow layers from ground up to the subfloor:
-thick Vapor Barrier
-2 to 3′ crawl/air space
-unfaced R-19 batt insulation
-radiant barrier, radiant floor pipes

So if I am confident that I can keep water and pests out of the crawlspace, why wouldn’t I keep unfaced insulation on the ‘floor’ of the conditioned crawlspace? Again, it was an amazing improvement. Thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    Obviously, insulation works. If your crawl space is warmer than the soil temperature, then installing insulation on your crawl space floor will keep more heat in your crawl space and prevent heat loss from the warm air to the cold soil.

    In the long run, however, fiberglass batts are not a good choice for this location. You would be better off using rigid foam (EPS or XPS). And if you buy EPS or XPS -- or better yet, polyiso -- the best place to install it would be the underside of your joists, not the floor of your crawl space.

  2. stuccofirst | | #2

    insulation on the floor of the crawlspace isn't necessary if you are separating the crawlspace from the conditioned living area. It is better to insulate the underside of the floor, and keep the unused crawlspace out of the heated space. Only if you have ducts running through the crawlspace would you want to consider bringing the crawlspace into the conditioned space.

  3. user-1031346 | | #3

    Martin: Thank you. Not only did you answer my immediate question but you answered my follow-up question with your single reply. In addition, thanks for also mentioning polyiso as my gut has been telling me to go with that in this case but is seems like polyiso is rarely mentioned compared to EPS or XPS on the site.

    Also your comment on ‘Insulations works’ made me chuckle as I realized that I have been jaded as I haven’t seen good results in the past. I attribute this to 1) not treating my house as a complete system – or not putting enough ‘work’ to hit the tipping point to see/feel the impact of proper insulating techniques 2) I have been burned by so called insulation experts in the past that were just interested in selling me incorrect products at high margins. Live and learn. Kudos to GBA for helping to provide great insights and direction!

    Other suggestion: If it doesn’t exist already, perhaps someone could draft a CAD drawing for conditioned crawlspaces with radiant floor heating.

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