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Should batts go to the floor in basement?

mloop | Posted in General Questions on

I’m insulating my basement with 2″ EPS secured to foundation wall (installed to the concrete floor and gaps filled with spray foam where the floor was uneven),  2×4 studs 24″ OC filled with Rockwool and Membrain. I’m Ontario Zone 6.

I’ve heard that some recommend that the batts should NOT go all the way down and a 6″ gap from the floor should remain in the event of a basement flood.

Question 1: Should the batts go all the way down to the base plate or should there be 6″ gap from the concrete floor?

Question 2: Is there value in leaving a gap to minimize the batts potentially getting damps from potential capillary moisture from the base plate? My floor is uneven and the base plate was shimmed in many places. As such, no sill gasket was installed and there are a few places where the base plate rests on the concrete floor.

Question 3: Should I foam the gaps between the base plate and the concrete floor?

Many thanks!

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  1. Expert Member
    Michael Maines | | #1

    How likely is your basement to flood? If the risk is low, as it should be with new construction, I just leave a 1/2" gap to avoid capillary water movement. If the basement is at somewhat high risk of flood, I would not use fluffy insulation at all. (EPS will drain after it gets wet.)

  2. DC_Contrarian | | #2

    Six inches seems excessive. Put a pressure treated bottom plate on the all and run the batts to it. If the water goes over the bottom plate you're tearing the drywall out anyway.

  3. Expert Member
    BILL WICHERS | | #3

    Rockwool shouldn’t wick water, so you shouldn’t have an issue with moisture soaking up from the floor. A pressure treated bottom plate is a good idea, but it’s not a capillary break. For a capillary break, you really need a plastic material (sill seal, but EPDM or HDPE is better). It sounds like you’ve already framed out your wall though, so you’re stuck. I wouldn’t leave a gap at the bottom of a mineral batt, but if you don’t seal the small gaps under that bottom plate, you leave a sort of drainage gap to help limit the chances of liquid water building up to a point where it would reach the batts.


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