Sandwich batt insulation with rigid foam in mini crawl space
jeuteneier
| Posted in General Discussion on
Hey all. Been a while since I last posted.
I just built a 30″ bump out from my garage and the joists and rims are 2×6 PT. I have lots of 3″ Polyiso and considering the following two insulation details and wondering if there is risk with trapping moisture.
Clearance from bottom of joists to soil is about 4 inches
Option 1
3/4 Flooring
3/4 T&G Subfloor
3/4″ Rigid foam over top of all the joists for thermal breaks
3″ Polyiso foamed in
unfaced rockwool R15
3/4″ rigid foam board to protect and add extra insulation (could use other wood based sheet good
Option 2
3/4 Flooring
3/4 T&G Subfloor
3/4″ Rigid foam over top of all the joists for thermal breaks
Kraft faced R21 pink insulation
3/4″ rigid foam board to protect and add extra insulation
thanks!
Replies
I am having a hard time imagining what your structure and the options look like.
Here's a drawing
You do not want a sandwich with foam on the outside.
and where are you located? Heating zone?
Will you heat or cool the garage?
it seems close enough to the ground, you will want to use treated cdx under the floor. foam would likely become a refuge for pests.
30 inches is not that far. Why not use two layers of the 3 inch you have lots of, and top up the treated 2x6 with ripped half inch plywood or OSB, then your flooring system of choice?
A last question is how you will access the underside of those treated joists with 4 inches of space to the ground.
Good questions.
Pacific Northwest - Portland, OR
Yes the bump out will be heated - not sure if I'll get a window A/C.
I have lots, but also have to save enough for the roof
Treated CDX on the bottom sounds wise. I have a bunch of regular CDX on hand though so I will take that chance.
I won't be able to access from underneath. I plan to install the bottom CDX with angle brackets or toenailed screws from above.
Thanks!
That 4" crawl space is gonna be a haven for critters. You might want to figure out how to make sure it's impenetrable to all sorts of critters.
there will be soil right up to the PT plate so they would have to dig down in the soil to get in. I was thinking of taking some sheet metal and going even deeper.
Critters do dig. Given my decade+ doing foundation inspections I've seen a lot of what you would not expect to see. I'd recommend 1/8" mesh hardware cloth from top of plate to about 12" below grade.
thanks ct_yankee. good advice.
Update:
I was able to install 1/2" CDX from above by using a simple L bracket and toe screwing. Was pretty sturdy actually. I left some gaps for a little air flow.
The plan is to go with option 1 but instead of using foam on the bottom I'm using the CDX. So it's:
Top
-- 3/4" rigid foam (over top of all the joists)
-- 3" Polyiso
-- 3 1/2" rockwool (compressed to 2")
-- 1/2" plywood (secured to bottom side of joists)
Bottom
My question is if moisture will get trapped by the CDX? I was thinking of drilling some holes in it to allow for some air into the rockwool.
thanks!
As I was waiting out the rain I did a bit more digging. I read a Fine Homebuilding article that suggested continuous foam can be used on the bottom with batts on top for floors over a crawl space. Pic attached is a detail from that article.
So why not have all the air sealed foam on the underside and between joists, then fill with both foam and batt? See attached detail
Your second drawing is labeled foam bottom sheathing. That isn’t the case, right? You are sheathing with treated ply, is that correct?
Oh shoot I forgot to add the ply beneath that but yes there would be foam for thermal bridging
Here is the fixed detail pic
The only suggestion I have is to eliminate any surface of foam to be open to the exterior.
Ants love to go through / behind foam.
So do termites.
Mice will fit into some pretty small openings.
was planning to wrap the rim board foam with cedar.