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Community and Q&A

Effective air barrier from uninsulated basement?

QSqqYgeczQ | Posted in Energy Efficiency and Durability on

We are in the process of retrofitting a 1949 house with a full basement in Cottage Grove, OR. We insulated the ceiling of the basement (floor of main dwelling space) with R-30 fiberglass batts. We are planning to install a hardwood floor on top of the existing doug fir subfloor, and are wondering about the best way to proceed with sealing the unheated basement space from the heated main floor. We will likely never finish the basement to be heated, as it will be used for root cellar/cheese cave. Should we put a plastic barrier under the batts? Under the new floor? Both? Or another material? It is definitely drafty from below due the chimney effect. We are also air sealing the attic, but plan on finishing that space eventually to be part of the thermal envelope. At this point we have R-30 batts in the attic floor as well. Anyways, thanks for taking the time.

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  1. GBA Editor
    Martin Holladay | | #1

    Unfortunately, you put the cart before the horse. If your intended insulation is fiberglass batts, you have to do the air-sealing work before you insulate, not after.

    If you really want to do a good job of air sealing, you should remove the fiberglass batts so that you can perform the necessary air sealing work. Typical air leakage locations include the entire rim-joist area around the perimeter of the basement (an area best sealed with closed-cell spray foam), plumbing penetrations (especially but not limited to traps under bathtubs and showers), electrical penetrations, duct penetrations, and chases around chimneys.

    Once your air sealing work is done, you can replace the fiberglass batts. Don't forget to install weatherstripping on the door at the top of your basement stairs.

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