Insulate crawlspace floor?
Hello, I’m in Nova Scotia, Canada, climate zone 6b. My house (circa late 1930s) has a poured concrete basement with an addition over a dirt crawlspace. Crawlspace is just under 100 sq feet, just under 4′ high. None of the crawlspace is below grade, walls are just pt 1″ plywood with pt 2×4 framing. I have encapsulated the crawlspace with r20 rigid poly on the walls, joist headers also have rigid poly, all sealed with canned spray foam. Dimpled drain mat on the dirt floor with 2 layers of 6mm poly on top, all sealed with acoustical sealant, seams taped, etc. I’m pretty meticulous. The floor above the crawlspace is still cold but I want to keep the crawlspace within the thermal envelope of the house. There is a 8″ diameter hole in the concrete wall that separates the crawlspace from the basement; the basement has a professionally installed Sedona dehumidifier that also does the crawlspace via the 8″ hole. My question is, would it be a bad idea to insulate the crawlspace floor – rockwool comfortbatt on top of the 6 mm poly, with tyvek on top of the rockwool. All seams taped. The dirt floor is too uneven to use rockwool comfortboard. Not sure mold will grow between the poly and the rockwool, nothing for mold to eat. Moisture can still escape through the tyvek and the dehumidifier is set to 45% rh. I’d appreciate hearing peoples’ thoughts. Thank you.
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Replies
“I want to keep the crawlspace within the thermal envelope of the house.”
You need to connect the crawlspace to the house or install and operate the equipment necessary to keep the crawlspace at more or less the same temp and humidity as the rest of the house. Once you do that the floor can’t feel cold because the crawlspace will not be colder than the house.
Insulating the floor works against your goal of keeping the crawlspace dry and warm enough to prevent condensation and mold growth.
Walta
My read is that by "crawlspace floor" he means the dirt floor of the crawlspace, not the floor between the crawlspace and the room above.
He said there is an 8" duct connecting the crawlspace to the conditioned basement.
Kirstin357,
I don't see any downside to what you suggest. As Walta said, you may need some addition heat source to get things a bit warmer in there, but start with the insulation and see where that gets you.
There is always something for mold to eat. Dust in the air will provide enough.
I would use EPS here, since it's not air permeable. EPS is also much cheaper than Comfortboard. Since this small crawlspace is probably not something you ever really need to access, You could just put down the EPS sheets, ideally taping the seams, and call it done -- no need for a subfloor over the top.
I agree with the other posters about getting some warm air in there. Even with insulation, if you don't have any air flowing through from the conditioned spaces, the crawlspace will still get cold. Remember that insulation only SLOWS the rate of heat transfer between warm and cold areas, it does not STOP heat transfer. That means insulating the floor will make the crawlspace take longer to get cold, but it will still get cold eventually if you don't provide any heat to makeup for the losses.
Bill
Thanks for the responses. Bill: the dirt floor is a little even, which is why I thought comfortbatt might be good to get good contact on an uneven surface. With rigid eps, there would be some air gaps between the eps and the 6mm poly floor but since both are considered a moisture/vapor barrier, maybe that doesn't matter? As for getting her into the crawlspace, what about using an infrared heater on the basement side of the concrete wall between the two spaces. Leverage the thermal mass of the concrete wall for heat to move into the crawlspace? Relatively easy, low cost approach? Maybe I'm overthinking it, maybe just seal the crawlspace and get a little infrared heater for the main floor living space to warm the floor mire directly?
The usual concern with moisture is the risk of rotting out wood, notably wall or roof sheathing in the most commonly discussed assemblies. In your case, the moisture would sit on a polyethylene sheet under EPS foam board -- neither material cares if it gets wet, so the moisture won't cause any damage. The only potential issue would be mold, which should be minimal in this case, so I wouldn't worry about it. You can get poly sheet with an antimicrobial coating which would provide some extra insurance if you're concerned.
I would try to even out the dirt floor before moving forward with your project if at all possible. For the usual very dry dirt or sand material you find on "dirt" crawlspace floors, I find dethatching rakes are pretty good for evening out the surface.
Bill
Oops, autocorrect... *as for getting heat (not her) into the crawlspace..*
You can edit your post if you find a typo :) Just look below your post, after you post the replay.