BSC crawl space insulation clarification
We are adding an addition to our house in Denver (adjacent). The house is built over a 9’ deep basement but the addition will be built over a 4’ deep crawl space for budget purposes.
The crawl space will be sealed and conditioned, and I plan to use the BSC recommendations for an encapsulated crawl space, but am somewhat confused by their recommendation.
BSC calls for the perimeter walls to be insulated, using rigid foam, but calls for the floor of the crawl space (i.e. the dirt floor) to have a vapor barrier, but no insulation.
This is somewhat contrary to recomndations for a basement, which is to have rigid insulation underneath the slab. There is a distinction between a crawl space and a basement in that the basement is built to be an occupied space, but aside from that, is there a building science reason why insulation should be omitted from the (dirt) floor of a crawl space.
The insulation in this instance would be rigid foam (likely XPS) and installed below a vapor barrier. And also to clarify I understand that the subfloor (above i.e. between the crawl space and living space) should be uninsulated.
Thanks for your insight!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Chris,
Like Building Science Corporation, I advise readers that most crawlspaces don't need insulation on the dirt floor -- just polyethylene. The main reason is that the energy savings associated with adding floor insulation are too low to justify the investment.
If you are creating an unvented conditioned crawlspace, you want to insulate the crawlspace walls. However, you don't want any insulation in the crawlspace ceiling (between the floor joists).
Further details can be found in this article:"Building an Unvented Crawl Space."
If you want to upgrade your crawlspace to make it usable for storage, it's certainly possible to include continuous rigid foam insulation between the dirt and the polyethylene. If you do that, you'll probably also want a concrete slab. There is nothing wrong with these details other than the expense.
The main reason that continuous horizontal rigid foam is suggested for basement slabs is that the rigid foam reduces the chance of summertime condensation on the slab, thereby making the basement smell better, and reducing the chance that cardboard boxes placed on the slab will get damp on the bottom.
Martin,
This is over 2 years later, but my situation is similar to the one described here. I have a crawlspace that will be conditioned, meaning the foundation stemwall is insulated and there is a plan to move air and condition it from the crawlspace and into the building envelope. I'll install the plastic seal the floor, as required by code.
My question is regarding the insulation between floor joists, that is, between the crawlspace and the main floor. Obviously if both zones are sharing air, then insulating the floor isn't going to help keep the dwelling warmer.
My building official suggested that it could be beneficial to insulate the floor anyway, because it would dull the sound of water moving in the pipes below the floor.
Any thoughts on this? I wanted to insulate the floor before water lines went in, so I wouldn't have to work around them, but then again, I guess I could wait to see if the sound of water moving down there is actually worth addressing.
I would appreciate any input on this.
Thanks!
Paula,
If you want to reduce plumbing noise the best strategy is to insulate the pipes, not the floor. I would wait until the house was complete then insulate any specific pipe you thought was annoying.
Any noise issues I've heard of with waste pipes have been in walls, or the ceiling above. I've never heard of problems with crawlspaces.
Oh - and if you insulate before you r0ugh-in your waterlines and drains, your plumber will kill you.
haha!!! Good advice, Thank you so much Malcolm! I agree it may be far more cost-effective to simply put insulation around individual pipes.
For particularly noisy pipes, use MLV (Mass Loaded Vinyl) sheet as a pipe wrap. This is a heavy materail specifically intended to reduce sound. It's not a cheap material, but it's very good at what it does. Note that it doesn't really provide much, if any, thermal insulation, so you might need to put regular pipe insulation over it.
Bill
I’m currently working on a similar project. They are putting a plastic mat down first. Then a flexible foam insulation, then the vapor barrier. I did not want a vapor barrier without insulation. Sure-Dry has a website that can help clarify. Hope this helps.