Insulate Conditioned Crawlspace Floor/Ground? (RE: BuildingScience.com InfoSheet-512)
I was reading the Crawlspace Insulation Info sheet on Buildingschicence.com Info Sheet 512 (https://buildingscience.com/documents/information-sheets/crawlspace-insulation).
The first paragraph says:
“Crawlspaces should be designed and constructed as mini-basements. Crawlspaces should not be vented to the exterior (see FAQ on Crawlspace Venting). They should have their floors uninsulated, the ground vapor sealed, their walls insulated and air sealed, and their air conditioned with indoor air.”
Why shouldn’t the floors be insulated? I am assuming they mean the floor of the crawlspace.
I was thinking of doing 6-8″ of 3/4 clean gravel. 2″ of XPS and then 15″mil vapor barrier for my crawlspace in Marine 4C. The Walls are 8″ ICF. Would this cause an issue?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It's confusing because floor is used for the dirt and for the crawlspace ceiling, which people call the floor.
Given no perfect one-size-fits-all, I'd personally recommend you temperature check your dirt floor. Is YOUR floor crazy cold? If so, then insulating it makes total sense.
HOWEVER, EPS direct on the dirt...you need a FIRE ignition barrier or a thermal barrier. EPS smoke is toxic. Smoke is usually the killer in a fire? Make that smoke like real life actual toxic.
1 inch dirt/concrete, 1.5" Rockwool Comfortboard 80, drywall, it all has to be installed as an assembly. You can't just drape Rockwool Safe 'n Sound onto the floor tight. Safe n' Sound is approved in an assembly (with studs).
So it's complicated. No one really addresses rigid foam HAS to have a fire barrier over it, even in a crawl space. Any other advice is both against code and manufactures Spec Sheet.
Thanks for the in-depth reply. Seems like the wording was just unclear.
I'm curious about the fire barrier info you bring up. What is the fire barrier in concrete free slabs? Is plywood a fire barrier?
https://www.finehomebuilding.com/project-guides/insulation/concrete-free-slab
EDIT: I'm definitely confused by the fire barrier code. It seems my ICF crawlspace walls can be left exposed.
Usually the fire barrier in walls is drywall. Plywood can be a fire barrier if it's 3/4". There are certain polyiso foams that are fire-rated, other than that they all require a fire barrier. I know for spray foam it's common just to spray on a layer of fire suppressing paint, I don't know if that's OK with board foam.
They are referring to the first floor, not the floor of the crawlspace. Later on they say: “ In cold climates or in very low-energy buildings, installing insulation below the ground cover is recommended. ”
I agree it was ambiguous.
ok thank you. that makes sense.
I think the theory is that the amount of energy saved by insulating the ground under a crawlspace isn't worth the cost of doing so.
Two thoughts on that. First, in most of the US the ground a few inches under the surface is below room temperature, so year-round heat is lost downward. In the winter it means more heating, but in the summer it contributes to the cooling of the house. So the net energy flow isn't that great.
Second, think of what the earth is. At the center there is a ball of molten metal at something like 5000 degrees, the surface is basically at room temperature. The only reason the surface temperature of soil fluctuates is because it's in contact with the air above, if you go down about 10' the soil will stay at about the year-round average temperature above, go deeper and it starts getting warmer. There's good reason to believe that the soil under a house will stabilize close to the temperature of the house, especially if there is good insulation to the sides.
cedarknoll,
I think Matthew is right - they mean the floor above. They refer to the floor and the ground separately. Joe Lstiburek recently shared a series of building assemblies on GBA for various climate zones, and in the colder ones there was insulation on the ground. The ones that didn't were in warmer regions, where as DC says there is a benefit to leaving them exposed to the soil underneath.
ok thanks. I just wanted to make sure I wasn't missing something. That makes sense to me.