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Are unfaced fiberglass insulation and rigid insulation board a good choice to insulate floor joists?

tc3fngrs | Posted in Energy Efficiency and Durability on

I live in Memphis, TN. Original part of house is 70 years old (1100sf). The addition in rear of original house is 45 years old (1000sf). No insulation in crawlspace of either section. Floors are cold, mostly in addition, and uninsulated. Crawl space is vented. At best there is 32″ of head space and at worst 18″ in addition. Limited to “belly space” in original section. No outside gutters yet.

Everyone I read seem to say that I should encapsulate the crawl space. But due to the short wall space around most of the house, I’m thinking I would have better results by doing the following under the addition (since it is most accessible):
1. Add fiberglass insulation batts between floor joists. Min R-19
2. Add rigid foam board with vapor barrier and seal seams. Min R-6
3. Check rim joints for needed insulation. Add spray foam if necessary.
4. Add 6mil plastic for groundcover
5. Add gutters around house

I steer away from spray foam for the crawl space due to having to hire it out and the likely mess if I had to replace anything.

What do you think?

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  1. GBA Editor
    Martin Holladay | | #1

    1. The best approach is to lower the floor level in the crawl space (by excavating with a shovel and bucket) and creating a sealed crawl space. More information here: Building an Unvented Crawl Space.

    2. The second-best approach is to insulate the floor joists by following the advice in this article: How to Insulate a Cold Floor.

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