How to insulate basement bathroom ceiling - or not?

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Old 03-18-18, 10:52 AM
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Exclamation How to insulate basement bathroom ceiling - or not?

We're finishing off half the basement and adding a bathroom. All rough work is done. The walls are getting built with a styrofoam/drywall system (no studs - 5/4 furring strips) using these panels: https://www.homedepot.com/p/Owens-Co...43WD/100320293 The exception is the bathroom - only 44 inches (shower wall) touches the cinder block wall, so most of the bathroom is studs and will receive kraft faced fiberglass batts.

The quandary is the ceiling and the wall that touches the masonry. There's a bedroom with hardwood floor above the bathroom. We want to insulate the ceiling to keep warmth in the bathroom and traditionally it would get kraft faced batts too, but what about passing moisture into that hardwood floor? It seems like a 6mil poly vapor barrier would be better, but trapping moisture in the bathroom is a concern (vent fan is installed). There's also trying to get the barrier sealed with fan/lights intruding.

Secondly, there are three joists bays over the masonry wall from the joists running above the bathroom. The foam panels act as a vapor barrier ceiling to floor, but in the bays, there are different approaches. If the ceiling is fiberglass, it seems like the bays need to be filled to the rim joist the same way. Years ago a Canadian DIY TV guy (forgot which one) told me to add styrofoam between the joists to extend the vapor barrier, with fiberglass out to the rim joist.

I do not want to trap moisture anywhere or allow condensation from warm and cool surfaces to meet.

Looking for thoughts on what to do.

Thanks guys!
 
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Old 03-18-18, 02:22 PM
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You dont need to do anything special except have a good ventilation fan in the bathroom to remove warm moist air!
 
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Old 03-18-18, 05:58 PM
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What about the joist bay cavities above the masonry wall?
 
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Old 03-19-18, 09:24 AM
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The rim joists are insulated to the inside of the basement wall. You insulate your walls up to the floor but leave every 5th or 6th open to the inside of the basement, that way you have opening to the back side of the insulated wall.

Yes it leaves a small area open but you have 99% of the wall insulated and cold air falls.

This way no water vapor can be trapped!
 
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Old 03-20-18, 06:26 PM
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@Marq1 - I have no idea what this means:

The rim joists are insulated to the inside of the basement wall. You insulate your walls up to the floor
Can you explain further?

FYI - about half of the basement will be unfinished.

Thanks!
 
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Old 03-29-18, 05:14 PM
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Any help on this anyone?

Also - faced or unfaced fiberglass in the ceiling of the basement bathroom under the first floor plywood?
 
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Old 03-30-18, 05:39 AM
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Here is a quick sketch, the area behind the wall is open at the top.

I insulate the area between the floor joists, above my walls, but leave every 3rd of 4th open to allow air flow.

With this, no vapor is trapped, no water will form and as long as there are no foundation leaks nothing will get wet.

Faced or un-faced is your preference!

https://www.doityourself.com/forum/a...1&d=1522413570
 
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