I live in Louisville, Kentucky, so we have relatively cold winters (nighttime lows consistently below freezing) and relatively hot, humid summers (daytime highs consistently in the muggy 90s). My house was built in 1905 and seems to have originally had a cellar/crawlspace. During a renovation in the 1990s, they poured a concrete slab in the basement and concrete walls halfway up the existing brick foundation. But they only did that in the northwest corner. The rest of the basement is a crawlspace with about 2 feet of clearance between the dirt floor and the joists of the first floor. The created basement is where the water heater and furnace live, and duct work runs through the crawlspace.

The only separation between the crawlspace and the basement are some planks that are nailed into studs that run from the new concrete half-walls to the floor joists.

I would like to insulate the crawlspace and basement. Here's what I think I'm supposed to do:

  1. Cover the dirt of the crawlspace with 6 mil (or thicker) plastic sheeting that runs up the foundation wall and attaches below the sill plate.

  2. Attach 2-inch extruded polystyrene foam insulation to the walls over the plastic. Here's where I have questions.

    • Should this insulation go from the dirt floor all the way up to the floor above, hugging the wall as I go up, or should I stop just below the top of the brick foundation so that the sill plate and joists are left visible for termite inspections?
    • If I'm supposed to leave a gap for inspection, should I place foam over the area and attach it loosely (maybe with just tape, so it can be folded down during an inspection) so there's some insulation there, or is the fact that it's not securely attached mean it's not worth doing?
    • It seems that I need a 15-minute fire barrier over the foam, and 1/2" drywall is standard. Do I really want to put drywall on top of plastic in my crawlspace? If it gets damp, that could be awful. Could I use cement board instead? Apparently 3 inches of mineral wool would also work, but that seems like an open invitation to dust. Apparently, you can just use an ignition barrier if the space isn't living area, but I would like to use the area for storage. Anybody have good ideas of what to use here?
  3. What the heck do I do on the exterior basement walls? Should I run plastic down the outside of the wall so moisture can drain to the floor? (There is a sump pump.) Should I only insulate down a couple of feet below grade?

Any help would be greatly appreciated. I've been searching the internet for the past several weeks, and the more I search, the more contradictory information I get.

  • You should update your question with answers to the following if possible: What (contradictory) information have you found? Subgrade temperatures are fairly constant, so what are you trying to provide thermal insulation against? E.g., the first floor joists and subfloor from the crawlspace temperature and humidity? Or just the crawlspace from exterior temperature. Do the ducts sit in the joists or run underneath them? Why the termite concern: Is your sill near ground contact anywhere along the perimeter? What is your sub-sill perimeter construction that you think you need a fire barrier?
    – feetwet
    Jul 30 '14 at 2:46
  • Sorry I haven't responded to this in a while. I started on something else that has taken all my time, and a realtor said the basement/crawlspace is pretty standard for the area and won't cause concerns for buyers when I sell. The main issue I was confused about was whether I needed fireblock over rigid foam. I discovered that if a crawlspace/basement is not used for storage or anything other than mechanicals, no; if you store stuff or people visit, yes. Since my crawlspace is not separated from the basement, I think I'd need firebreak. Aug 6 '14 at 12:36
  • 3
    @TOB If you've found an answer to your own question, could you post it as an answer and accept it? That way, it won't keep coming back to haunt us. Thanks!
    – Niall C.
    Aug 7 '14 at 0:04

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