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My house consists of two units, the main and basement apartment to which the main is not connected indoors.

I was planning to put fiberglass insulation between the joists and use ultra light 1/2" sheetrock but then a friend told me to use 5/8 for fire resistance reasons. Assuming this is not subject to code inspection, would it be acceptable to go with 1/2"? I've seen sagging between joists as a reason but I'm not seriously concerned about that. In addition to being lighter and easier to work with, I prefer 1/2 to allow that 1/8" extra ceiling space, which is scarce.

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Yes, 1/2” thick gypsum board can be installed on 16” o.c. Joists for ceilings with batt insulation. (24” o.c. spacing would require 5/8” thick gypsum board without sagging between supports.)

The Code does not require any special fire separation between units, because there are only 2 units. I also doubt a Building Permit is required because it’s not structural, fire code, energy, or exit related, etc. (That’s why we don’t get a Permit for replacing carpet, painting, etc.)

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  • Keep in mind that sound will make its way through via the joists regardless of your insulation between joists. How much vertical height is there to the underside of the joists?
    – user68386
    Sep 24 '18 at 18:16

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