Details for Crawlspace Rat Slab
Code in my area (4C) calls for r 38 under the main floor. It’s a daylight basement and I was planning on a rat slab in the vented crawl area.
Do I need to insulate under the rat slab? Code calls for r10 under a slab, but wasn’t sure if they mean any slab or if rat slab not required since the area is vented anyway.(Crawlspace walls are icf)
Second question is I was thinking r38 batts under the portion of main floor and then attach 1″ rigid (taped seams) and some sort of sheathing (maybe 1/4″ luan) under that to keep rodents from chewing into foam and getting into the batts. (see pic) I’ve seen artcles linked from here describing almost exact setup but wasn’t sure if there was issue with having taller joists (14″) than batts (~12) so there’s a ~2″ air gap in there between the rigid foam and the batts. I could see doing all the work and have someone say that’s some sort of moisture trap and I rot all my joists in 20 years etc
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Replies
Jackie,
#1. Are you trying to warm a cold floor?
Are you saying that your daylight basement is connected to a crawl space?
#2. Do you want to insulate under the crawl space? If that's the plan, you might want to convert it to an unvented space. See this article for more info: https://www.greenbuildingadvisor.com/article/building-an-unvented-crawl-space
Sorry, a bit vague on my post I guess. I don't need to have a warm crawlspace floor, I was more concerned that since r10 is required under a "slab" that some inspector would say "well I know it's ridiculous but the code says r10 under a slab". I'm 99% sure it's not an issue as the crawlspace has to be vented due to high radon in the county. I really want an un-vented crawl, and argued with the local bldg department about having active venting but they still shot me down.
Jacky,
If your foundation walls are already insulated (ICF), and only part of the area is crawlspace, as Steve said, I'm not sure it makes any sense to vent it. That would require redundant insulation between the crawlspace and floor above, as well as insulating the wall between the crawlspace and the daylight basement. Moving the boundaries between conditioned and unconditioned spaces from the perimeter of the house to these areas complicates everything.
Similarly, the place to stop rodents intruding is the perimeter. There shouldn't be any reason to try and protect the floor above the crawlspace unless you are building that area on piers and it is completely open to the outside.
Oh trust me, I've been back and forth with the bldg department on having a sealed crawlspace and it's a no go because of radon. I even offered having it be conditioned. So yes, it sucks having to have nice insulated walls in the crawl and then have to put more insulation under the main floor. I've heard my state was working on revising code if there's active radon abatement but not sure when that will change. I assume with a rat slab, there shouldn't be a way to have rodents get in as long as I buy the nicest crawlspace vents I can find with heavy duty screens.
For an extra 200$ I can slap 1" rigid to the bottom of the floor truss. Somewhat redundant, but I liked the idea just to help for air sealing, and a little thermal break. I was just worried that the ~2" gap between the rigid and the fiberglass batts would be a spot where condensation could occur?
Jackie,
Sealed crawlspaces are conditioned. With the exception of the thickness of their slab they are constructed almost exactly like low basements, so I don't understand how radon can be an issue in the crawlspace and not in your daylight basement? I'm also puzzled by why if there is a radon problem in your area, abatement is not necessary? There is either a problem the needs dealing with, as they say there is in the crawlspace, or there isn't.
All that aside, to your original question: You can leave the gap directly under the floor, rather than between the two types of insulation.
I guess instead of saying conditioned, I should have said mechanically ventilated(I think that would be the nearest correct term, my mind is jelly trying to get ducks in a row). The basement portion is required to have a radon abatement pipe in it that goes up through the house and roof. I don't think it's required for under the rat slab, but I might tie it in anyway.
Lstiburek has argued in an older article (BSI-009 from 2010) that the air gap should be at the top of the cavity, near the floor. See Figure 7 from that article at BSC. If you do this, it's important that the perimeter (think rim joist) of your crawlspace have good insulation, otherwise you're short-circuiting the horizontal insulation.
What's funny is I was reading code the other day and saw it must be installed against sub floor. After you and Malcom responded, I did a double check, and viola' I found an exception. How I didn't see that, I have no idea, maybe I already read it and forgot it.
Exceptions:
1. The floor framing cavity insulation shall be permitted to be in contact with the topside of sheathing or continuous insulation installed on the bottom side of floor framing where combined with insulation that meets or exceeds the minimum Wood Frame R-value in Table R402.1.1 and extends from the bottom to the top of all perimeter floor framing members.
The ledger is bolted to icf wall, so perimeter of crawl insulated has a interior and exterior insulation.
Jackie,
It would be helpful to know your location.
Near Portland OR, climate 4c