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Community and Q&A

Dirt crawlspace insulation

CtxGJ68dYY | Posted in GBA Pro Help on

I live in northern Minnesota. The house I purchased has a large bedroom, half of which is an addition to the house. It gets fairly cold in the room during the winter and I am looking at better insulating it. The addition portion has no slab or foundation under it. It was basically built as a fourseason porch with only footings. There is access underneath from outside with enough space for me to comfortably crawl underneath. It has dirt floor, batt insulation in the floor joists and a flexible/insulated heating tube going to the far end. No other pipes or wires. I thought about insulating it like an unconditioned crawlspace, only problem is there is no cinder block along the base of the walls. One side is a thinner wood frame covered with plywood. The other two sides are only tin sheets for looks. Wondering what I can do to seal and insulate underneath there. There is really no functional space underneath so sealing it up would be preferred.

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  1. GBA Editor
    Martin Holladay | | #1

    Here's a Fine Homebuilding article I wrote that should provide you with information to address your situation: How to Insulate a Cold Floor.

  2. CtxGJ68dYY | | #2

    Thanks, the article helped a ton. I do have a few follow up questions. My cantilevered floor essentially sticks out 16 feet. Will it still be effective? Also do I need to put in a vapor barrier, if so where? Finally, what should I do with the heating duct tube, leave it hanging where it is or try to seal it up inbetween one of the joists?

  3. GBA Editor
    Martin Holladay | | #3

    As long as you follow the advice in the article, the floor insulation will be effective. However, if the room is otherwise leaky -- for example, if there are air leaks between the wall bottom plates and the subfloor, balanced by air leaks in the ceiling -- the floor can still feel cold even after it is insulated well.

    You don't need a vapor barrier, because the plywood or OSB subfloor is a perfectly adequate vapor retarder. What you need is an air barrier.

    I'm not sure if you are describing hydronic heat tubing or ductwork. In either case, neither hydronic tubing nor ductwork should be exposed. Ideally, you will build a box to enclose all tubing and ductwork. The box should be airtight, and it should be roomy enough to allow for the installation of lots of insulation -- ideally, the same thickness of insulation that the building code requires for floors in your climate zone.

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