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Community and Q&A

Insulating below addition

MaineMatt | Posted in Energy Efficiency and Durability on

I have a ~1100 Sq foot ranch with a ~200 sq. foot addition on the back. The house proper has a full basement below it, but the addition has a dirt floor crawlspace below it. Access to the crawlspace is through a small plywood “door” accessed from outside.

Right now the crawlspace has fiberglass batts between the floor joists and some thin poly sheeting layered on the floor (not sealed in any way). Cinder block walls on 3 sides, butting up against the foundation on the 4th. There’s no water in the addition, and it has its own heat source (small pellet stove) so there are no utilities to worry about down there. I’m in southern Maine, so figure climate zone 6A.

An issue that may be related is that we’ve had a problem with really high humidity in the house – possibly coming up through the unsealed dirt floor? We’re still running a dehumidifier now to keep the humidity under 50% and the outside temp is well below freezing.

In any case, I’d like to do more to insulate this space, but I’m unsure what the best way to go is. From reading other posts, it sounds like 8mm poly sealed to the walls and insulation on the walls is the way to go, but where I don’t have anything down there that needs to be in the house envelope I’m wondering if that’s right for me?

Any suggestions/thoughts would be appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    Here's an article that should answer your questions: Building an Unvented Crawl Space.

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