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Community and Q&A

Converting a garage into an office?

GBA Editor | Posted in Energy Efficiency and Durability on

Existing attached garage: Slab on grade, uninsulated slab, uninsulated walls. Live in northern California, in Marin County – close to the coast. No moisture problems on slab – seems pretty dry.

I was going to put in a sub floor as close as possible to existing slab – frame directly on top of slab.

QUESTION: Should I fill in between the floor framing (2 x 4’s) with rigid insulation and put in directly on the floor or leave a gap (airspace) and install it tight to the subfloor? Or, simply fill the void completely with batt insulation? Money is an issue here as well, so blown insulation is not an option. It’s a DIY project. Also, I was thinking that I would not need a vapor barrier on the floor as it seems dry.


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  1. Riversong | | #1

    "Seems dry" is not good enough. Tape a piece of clear plastic for 24 hours on the concrete and check for condensation. If there's none, it's ok to build on. If there is condensation, then I would paint the floor with two coats of UGL Drylock latex masonry sealer.

    Do not use batt insulation on concrete. If you frame directly on the concrete, you'll have to use pressure-treated wood (there's a number of non-toxic alternatives available). Infill with rigid foam board. Better yet, set an inch of XPS foam board directly on the concrete and install framing over that to stop the thermal bridging through the framing. Install additional foam board as infil between the framing. If you use foil-face isocyanurate for this, you can benefit from leaving an air space between it and the subfloor as a radiant barrier.

  2. Daniel | | #2

    Thanks Robert - I like your idea regrading thermal bridging. There are currently two coats of sealer on the floor. Have a New Year. I really appreciate your input!

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