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Old 03-30-2018, 08:28 PM   #1
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finished basement joist bay insulation???


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?

Thanks guys!
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Old 03-30-2018, 08:54 PM   #2
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Re: finished basement joist bay insulation???


No pics. Need one more post on here to get a pic pass.
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Old 04-02-2018, 02:36 PM   #3
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Re: finished basement joist bay insulation???


As long as both floors have conditioned air (meaning mostly that both are heated), I do not insulate the lower floor's ceiling.
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Old 04-03-2018, 12:55 PM   #4
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Re: finished basement joist bay insulation???


Fiberglass is lousy for joist bays because it needs a vapour barrier. Vapour barriers are almost impossible to do right in that area.

Rigid foam or roxul board works better.

You don't need fiberglass batts in the batrhoom ceiling.

You do need a good exhaust fan vented outside and run while showering and 15 minutes more after finishing.

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Any water penetration issues must be resolved before insulating and finishing a basement wall.
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Old 04-03-2018, 12:57 PM   #5
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Re: finished basement joist bay insulation???


Quote:
Originally Posted by smarsh View Post
...some walls weep in the spring and occasionally in the summer.
What is the right way to insulate to avoid moisture/mold issues?
Where will that weeping water go if/when it shows up?

Quote:
What should be in the ceiling(s)?
nothing.
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Old 04-03-2018, 03:01 PM   #6
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Re: finished basement joist bay insulation???


In most cases the weeping water should be eliminated before project proceeds. If you decide to move ahead with a known water issue there are extreme measures that can drain that water behind the wall to a sun-slab drainage system, but I didn't hear anything like that.

I don't know how to suggest proceeding from whare you are.

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