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I'm planning to finish my basement. Most of it will be finished but there will be some areas that are unfinished for storage. I'm planning to use rigid foam insulation and seal it up.

What should I do in unfinished spaces? If I run the rigid foam into those areas, it needs a fire block --> so I have to put drywall in front of it.

What are people doing for the unfinished space? Can imagine putting up additional walls just to cover the foam?

Should I stop the foam and seal it off vertically? Do I then insulate the interior wall that will border the unfinished space? Since it's an open area should the moisture have air movement to dry up?

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When I partially finished my basement, I insulated the exterior walls of the finished part, and left the interior walls and the unfinished without any insulation. My AHJ (in Ohio) approved this, stating that there was no need by code to insulate the unfinished part, nor insulate between the finished and unfinished parts. He did insist on insulating the rim joist in the finished part, which is very sound advice - that is the bulk of heat loss in a basement (unless your frost depth covers most of the basement height).

If your AHJ allows for insulating only the finished area, I would do just that, and leave the walls to the unfinished area without insulation.

In my basement I have around 420 sqft finished area, an adjacent 320 sqft unfinished area and another walled off unfinished area of 220 sqft. The finished area has a single heating vent at the ceiling. The finished area is slightly cooler than the rest of the house, but is noticeably warmer than the adjacent unfinished area. The walled off unfinished area is again colder than the unfinished area that is next to the finished area, so the heat gradient is fairly obvious. However, even during the coldest weather the coldest part of the basement is still fairly warm, and with a frost depth of only 32 inches my experience might not apply to colder climates.

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  • Many below-grade basements even here in Minnesota don't have insulated walls. The ground stays at approximately 50 degrees F year-round. so you can get away with just insulating the floor rim, which is usually above grade. If you really want to insulate the whole thing, consider foil-faced panels for those areas.
    – isherwood
    Jan 29, 2019 at 19:43
  • @isherwood I agree. However, the AHJ insisted on insulation, so I used XPS to gain water resistance (over fiberglass, as suggested by the AHJ...). I would hazard that the heating vent contributes a lot more to the climate of the finished area than the insulation, and that without insulation the overall feel would have been about the same.
    – Eli Iser
    Jan 29, 2019 at 19:52

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