The new basement bathroom is framed and passed inspection. Ready to insulate and close. My first question is about the ceiling. We plan to use fiberglass batts between the joists in the ceiling. Above is a finished hardwood floor bedroom. In the framed walls we'll be using paper faced fiberglass, paper to the bathroom side.

I see different info on the ceiling. Looking for thoughts on whether there should be faced or unfaced fiberglass. Rockwool?

Thanks guys!

  • What's on the other side of the walls? What USDA climate zone are you in? Can you post a photo?
    – Bryce
    Mar 30, 2018 at 1:51

1 Answer 1


Consider if you need ceiling insulation at all. In a bathroom there's lot of extra moisture, and you don't want some to enter the space between floor and the fiberglass moisture barrier, and have trouble drying out.

Floor insulation helps with sound transmission a bit (though an STC rated ceiling is better). But in your case it might have more downsides than upsides.

This IS a good chance to insulated the joist bays from the outside, usually a weak place in standard home construction. Post photos. Research and remember some golden rules of insulation: seal air leaks first, never trap moisture between two vapor barriers, use the right insulation for your climate zone. Get a local expert, as climate matters a lot.

  • I've wondered about trapped moisture but also want to keep the heat in the bathroom. The other issue is the joist bays open to the exterior wall and cold coming in from there. One short wall of the bathroom has solid foam insulation panels up to the bottom of the 1st floor joists. So moisture between the foam and cinder block exterior wall can seep up into the joist bays where the ends are capped with a rim joist above grade. So there is the added concern of cold, moisture, and mold if the ceiling is not insulated all the way to the rim joist. All thoughts are appreciated!
    – Steve Y
    Mar 30, 2018 at 2:07
  • You'd be better off to seal those joist bays against the rim. Rigid foam cut to size and pushed against the rim, and foam around the edges to seal the joist bay. It would do more than just insulating the joist bays with fiberglass.
    – freshop
    Mar 30, 2018 at 17:16
  • You could continue the rigid foam to the floor, and then take a right angle to the exterior wall. Then, add venting to the outside. Kinda weird, depends on your climate.
    – Bryce
    Mar 31, 2018 at 6:16

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