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Insulation question: Wondering what additional insulation to add to basement/crawlspace.

Keith D | Posted in General Questions on

Insulation question: We have 32’x20′ basement/crawlspace. Floor is 3/4″ stone over plastic. Inside perimeter walls were insulated with rigid foam when hse was built 1984. Used to have fiberglass batt insulation between floor joists, but squirrels destroyed it. We removed it and installed spray foam where joists meet sill plate. Basement has boiler and hot water heater. Boiler is used for heat 3 dozen times in winter for a few hours each time. (Main heat is wood stove.) Help! We have cold feet on the first floor. We live in Maine. What do you recommend for additional insulation? Many thanks.

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Replies

  1. user-1072251 | | #1

    Have you had a blower door test done? You may have significant air infiltration into your crawl space, which will cool off your floors. And if squirrels can get in, you obviously have some large holes. For the last three years since I changed to electric heat (mini splits) in my old farmhouse and sealed and insulated the basement floors and walls, my basement has been unheated, but draft-free, & stays about 55. Our floor are mostly uninsulated, and are cool, but not uncomfortable. Roxul might work for under floor insulation as it isn't attractive to rodents, but you need to seal up the space.

  2. GBA Editor
    Martin Holladay | | #2

    Dear Maine homeowner,
    Bob is right. If a basement or crawl space has insulated walls, but still feels cold, the most likely cause is infiltration. Due to the stack effect, basements and crawl spaces are depressurized with respect to the oudoors. The indoor air is always sucking at the cracks, pulling in outdoor air.

    Here is a link to an article on addressing those cracks: Air-Sealing a Basement.

    In an older house, it can be hard to track down all of the air leaks. Persistence and a blower door can help. Remember, also, that sealing air leaks in your attic is important. If you reduce exfiltration through attic cracks, you will also reduce infiltration through basement cracks. For more information on this aspect of the work, see Air Sealing an Attic.

    For an overview of the problem you are describing, see Cold Floors and Warm Ceilings.

    Finally, your situation is worsened by two facts:

    (1) You are heating with a wood stove. Wood stoves are strong drivers of convection: they cause a plume of hot air to rise from the stove, which contributes to the stack effect and the cold floor problem.

    (2) Most of the time, there is no operating heat appliance in your basement to keep your basement warm.

    I live in a house like yours. I have a cold cellar. I heat with a wood stove. For most of the winter, our floors are cold, so we wear socks.

    I know the solution -- tracking down the air leaks. The work is tricky. Some day I'll get around to it.

    -- Martin Holladay

  3. Keith D | | #3

    Bob, Martin. Thank you for your responses. Heavy socks would be a lot less expensive and easier than crouching around the basement, looking for leaks. Thanks for the links to the articles. I will do my homework, sitting by the woodstove.
    Regards,
    Keith

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