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Community and Q&A

Insulating Floor in Apartment Over Garage

ddrake | Posted in Green Building Techniques on

Hello all, and thanks again for all the helpful advice on my other recent questions.

I’m preparing to insulate attic, stud bays, and joist bays between first and second floors on my ADU project (PDF wall section and floor plans attached). Two story, garage and office below, apartment above. Climate zone 5.

I’ve made a few changes since the permit was issued and construction started last summer, mostly due to materials costs and materials and contractor availability. At this point, I’m doing nearly all the work myself. The project has been dried in since last fall, with 4″ exterior foam and WRB in place. We’re wrapping up siding, and electrical and plumbing rough-in, and have started drywalling the apartment ceiling.

Originally, I planned dense pack cellulose for stud and joist bays, and loose blown in cellulose for the attic. When it became clear I wasn’t going to find an insulation contractor to do the dense pack, or at least not for what I can afford, I started looking at fiberglass batts, specifically the new Dow Corning batts. Mineral wool is no longer in the budget.

Then I read a couple article and comments on GBA suggesting dense pack was doable using rental equipment. Since I’m doing blown in cellulose in the attic, I ordered a little extra and some insulweb, figuring I’ll try a stud bay or two while I have the machine for the attic. If it works, great–if not, back to fiberglass.

However, reading recent comments on this blog post: https://www.greenbuildingadvisor.com/article/insulating-a-wood-framed-floor-assembly#comment-231902
suggests blown in or dense pack in the joist bays between the first and second floor may be especially challenging.

Additionally, because I’m locating the water heater and plumbing manifold in the garage, and some DWV and pex in the joist bay above the wet wall dividing office and garage, the inspector asked that I provide a heat source for the garage, with a 1000 W Cadet wall heater being an acceptable option.

I’ll be carefully airsealing the garage ceiling (5/8″ Type X drywall) and doing blower door testing.

Apologies for the long-winded intro, but the question is: if the garage and office are maintained at say, 45-50° minimum, what level of insulation in the floor below the apartment is necessary or desirable? And is using fiberglass batts likely to be less labor and more forgiving than attempting to blow in cellulose?

And should the interior wall (the wet wall) between office and garage get as much insulation as it will hold (which is not much, considering the space occupied by plumbing)?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    I don't think you'll be happy with the results of your DIY dense pack efforts with box store rental blowers. Box store blowers are not the same level of perfomance as the pro units the insulating contractors use. I think you'll be much less frustrated just using batts. Use batts, use some nylon twin stretched across the ceiling perpindicular to the joists to support the batts while you hang drywall, and you'll have a pretty simple project to complete. I would carefully air seal things, being especially careful around the perimeter on the exterior similar to what you'd do with a rim joist in a basement.

    You need to hit at least code minimum for your climate zone, more is better. I would probably fill the joist bays with fiberglass batts and put 1/2" or 1" polyiso under the joists behind the drywall. The polyiso isn't required, but it would add R value, help with thermal bridging, and with taped seams and a caulked perimeter, it would also act as a secondary air barrier.

    Bill

  2. plumb_bob | | #2

    Air barrier and CO alarm are critical when the garage is attached to the living space. If the upstairs is at negative pressure it can draw polluted air from the garage, with safety and air quality implications.

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