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Finished basement joist bay insulation. How to do it?

syakoban | Posted in Energy Efficiency and Durability on

We’re refinishing half of our 1960’s basement. It’s reasonably dry, but some walls weep in the spring and occasionally in the summer. We’re using Foamular polystyrene panels to insulate the hollow cinder block walls and attaching them with 1 x 3 furring strips which will mount the drywall. The dividing wall unfinished to finished is all wood 2 x 4 construction as is the bathroom which has one wall that is cinder block. The wood stud walls will be insulated with fiberglass batts.

We just passed all rough inspections and are ready to close. The issue is insulating the joist bays from the foam panels out to the rim joist throughout the finished basement and especially in the bathroom. The photos show the cinder block bathroom wall covered in foam and the joist bays.

I’ve been told not to insulate the bathroom ceiling at all for moisture concerns. I’ve seen otherwise too. It’s below a bedroom with hardwood flooring and we don’t want to affect it. That would mean the joist bays are open to the bathroom ceiling letting cold air in above a warm/moist bathroom. We could just insulate the bays from the foam panels to the rim joist, but I’m not convinced that is sufficient.

My thought is that all of the joist bays should be filled (foam panel to rim joist) with unfaced fiberglass to protect them from moisture accumulating inside due to a temperature differential, especially if the cinder block walls weep behind the foam and moisture rises to the bays. I think the bathroom ceiling should be filled with fiberglass batts too.

What is the right way to insulate to avoid moisture/mold issues? What should be in the bathroom ceiling faced fiberglass, unfaced, or nothing?

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  1. user-2310254 | | #1

    Where are you located? When bulk water enters the basement, where does it go? Do you have a perimeter drain?

  2. Jon_R | | #2

    Fiberglass is so air permeable than it doesn't do much for moisture. Consider closed cell spray foam for the rim joist and no insulation above the bathroom.

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