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Community and Q&A

Stone foundation floor insulation

T8z4B7zSRb | Posted in GBA Pro Help on

We have purchased a 100yr old home in central Massachusetts with a stone foundation. The basement is used solely for the purpose of holding the heating and hot water systems. We are in the process of addressing significant water problems. To date we have installed sump pumps, air exchange ventilation system, dehumidifer and we have begun parging the stone walls with hydraulic cement. Long term plans are to regrade the exterior and divert roof water away from the building. That being said we believe the basement will never be entirely dry and the cost to do so is prohibitive.

We would like to insulate the floor between the basement and first floor. I had intended to use standard fiberglass unfaced insulation between the joist. Often moisture is absorbed by this insulation and can cause mold issues. Can we alleviate this problem by installing a breathable house wrap? Do you have any other recommendations?

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  1. GBA Editor
    Martin Holladay | | #1

    Fiberglass batts are not a good choice. Better choices would be closed-cell spray polyurethane foam or a layer of rigid polyisocyanurate insulation attached to the underside of the floor joists. If you decide to use the rigid polyiso, be sure to seal the seams with foil tape.

  2. jklingel | | #2

    "Long term plans are to regrade the exterior and divert roof water away from the building." If possible, I'd sure move that project to the short term list.

  3. user-659915 | | #3

    John K has it exactly right. Surface water management on the exterior is key. Without that your other efforts to dry out the basement are likely to be a waste of time, money and energy.

  4. T8z4B7zSRb | | #4

    So exterior house wrap stapled over the fiberglass is not an option, correct?
    What about firesafe rockwool covered with house wrap?

    Money and impending weather constraints will dictate the subsurface work be done next spring.

  5. GBA Editor
    Martin Holladay | | #5

    I don't think it's ever advisable to install fiberglass batts or mineral wool batts in a basement or crawl space.

  6. stuccofirst | | #6

    just keep the airflow moving and the dehumidifiers working until you can alleviate excess moisture entering the subspace.

  7. T8z4B7zSRb | | #7

    Thank you all for your input


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