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Detail of conditioned room floor above unheated concrete walled garage Zone 5

maine854 | Posted in GBA Pro Help on

I’m working on finishing off an office above a concrete walled garage built into hillside. Humidity is high in summer and water table is too, so lots of moisture around the concrete on garage level. Planning to condition (heat+cool) the office level with a mini split, and want to be sure I follow good building science in the garage ceiling to properly keep garage-level moisture at bay. Would it be wise/unwise to use Zip Sheathing on garage ceiling to be a moisture barrier? And for insulation in garage ceiling are there any hygroscopically preferred choices–rockwool, closed cell, fiberglass Batts, cellulose? Thank you for any comments or ideas.

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  1. user-2310254 | | #1


    I'll ask a question to give your post a bump. What type of ceiling (cathedral or flat) will you have in the office?

  2. maine854 | | #2

    Thanks, Steve. Cathedral ceiling in office made from 14" TJI so that they can rise above a steel frame carrying hip roof--a different subject, but posts here are very clear I should insulate under my zip roof sheathing with closed cell for vapor barrier. Still thinking of filling the rest of office roof cavity with open cell, dense-pack cellulose, or rockwool. Walls are r-6 insulated zip over 2x6 frame.

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