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Community and Q&A

Bathroom over an unconditioned garage

jacobrobinson | Posted in General Questions on

I recently started reading/researching the topic of building science.  I have long term plans to build a house for my family one day or at least self perform as much as possible.  I have been around residential construction all my life, but in this part of the country (SW Ohio, Climate Zone 5) building science is seldom a primary thought on a new home site.  It’s, OSB sheathing, 2×4 walls, fiberglass batts, and brick veneer or vinyl siding.  In starting to do my homework for our own future build (5-7 yrs from now) I realized that I made mistake in the remodel of 2 bathrooms at my parents’ house.

The new bathrooms sit above an unconditioned garage, the garage is part of a walk-out basement.  The garage had a dropped textured drywall ceiling before we re-constructed all of the plumbing to sit inside of the 2×10 joist bays, instead of below them.  The garage has limited headroom so this 6″ of headroom gain was a major plus.

After the work was complete we stuffed the joist bays with 3 layers 4″ fiberglass batts (they were left over from other parts of a remodel) some of it had a foil face, but not all.  Then all of that was covered by 1/2″ OSB.  Other facts, this is a 12’x8′ area with exterior rim joist along one of the 12′ lengths.

My concern now, knowing what I have learned in the recent months about vapor, and the importance of air sealing, is that I have created a problem where cold air from the garage could travel through the poorly air sealed OSB ceiling and condense on the under side of the new OSB sub-floor that sits below concrete board and porcelain tile.  It has been in place about a year now.

I know now that what we did was incorrect, but my question to this group is should I be speeding over to start ripping all this out, air seal all floor penetrations, and remove any foil faced batts.  Or is it possible that while what I did was grossly wrong it would be permissible to leave in place.  The more I listen to the FHB Podcast and read on this site, the more sleep I am losing. 🙂  So I am asking here to get some confirmation, either way.

Thanks for reading my wordy post!

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  1. GBA Editor
    Martin Holladay | | #1

    You needn't worry about condensation against the underside of your subfloor -- after all, the subfloor will be relatively warm, so it's not a condensing surface. Your main worry should be frozen pipes due to a faulty air barrier and a casual insulation job.

    You can create an air barrier by taping the OSB seams with a high-quality tape like Siga Wigluv. If you want to improve the insulation value of the assembly -- and I think that would be a good idea -- you can install a continuous layer of rigid foam on the underside of the existing OSB ceiling, followed by 5/8-inch drywall (for fire safety).

    1. jacobrobinson | | #2

      Thank you for the quick reply! I appreciate all you do for the building science community and how active you stay in the "trenches".

      Good thought about the pipes, all the pressure pipes were ran in the stud walls of the bathroom. And I built small rigid foam boxes around the traps. I will order some high-quality tape, it will be a good excuse to place my first order with 475 Building Supply.

      Thanks again.

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