Tall crawlspace encapsulation
StevenNC8b
| Posted in Green Building Techniques on
I am building a new home and plan on encapsulation. My lot is sloped so I have a fairly tall crawl space. It is 4′ tall at the masonry side where it is below grade and steps down to the low side with 8′ wood framed walls on the back. Is there any reason I can’t use batt insulation in the stud framed walls? It would provide a higher R Value than foam boards and cost much less.
I am near Charlotte NC in zone 8b if that makes a difference
Thank you in advance
Steven
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Where the walls are exposed on the outside (i.e. above grade), they are basically just regular exterior walls, and can be insulated the same way as any other exterior walls. You only need to insulate the "basement way" with foam board when you're dealing with below-grade foundation walls.
Bill
And it won't cause any problems to cover the insulation with the reinforced poly? I was worried about condensation
Steven,
The poly, which runs over your floor, should stop somewhere in your concrete stem-walls. Above that the wall should, as Bill said, be exactly like the exterior walls on the rest of your house.
So there is no type of vapor barrier? How do I get it sealed up air tight? Also the rest of my exterior walls have Sheetrock on them. What keeps the insulation in place? I could put house wrap, I am sure my inspector will want something.
Thanks
Steven,
Sorry, I had forgotten that in zone #3 you don't need a vapour-retarder on exterior walls, just drywall. The usual advice would be a variable-perm membrane, which unlike poly allows some moisture through, which keeps your walls safer. However they can be pricey. I'd ask your inspector what is usually done. If they will allow house-wrap, I'd use that. Make sure to seal it well as an air-barrier.
My inspector said day light wall needs vapor barrier.
I asked if the Stego instructions to take their vapor barrier (per RC 2015) behind or in front of rigid to the top of the stem wall was okay. No issue.
Then seal the rim joist and mineral wool without vapor barrier for fire thermal protection. Inspector was good as long as the mineral wool was perm attached, not just friction fit.
Ryan_SLC,
Stephen is in Climate Zone #3.
Unless something has recently changed:
1. The IRC doesn't not require a vapour-retarder on walls in Climate Zones 1, 2 or 3.
2. In Climate Zones 4 and up, a class 1 or 2 vapor-retarder is required on framed walls.
3. No framed walls are required to have vapor-barriers.
Note that "air barriers" and "vapor barriers" are NOT the same thing, although sometimes a vapor barrier can do double duty here. Vapor "retarders" are yet another thing...
An "air barrier" can be lots of things, and is often just the interior drywall, especially if you detail it air tight. I usually do that myself. Drywall doesn't block moisture though.
A "vapor barrier" is something like sheet of polyethylene, which will block the movement of moisture. Poly can also act as an air barrier, but since it's often easy to punch holes in it or slice it, it's not usually considered a long-term reliable air barrier. It's easy enough to put up a poly layer as a vapor barrier, and then use drywall over that as an air barrier though.
Vapor "retarders" are selective membranes that allow moisture to move under certain conditions. Usually this means a vapor retarder will let a wall dry out, but will limit how much moisture gets into the wall in the first place. These membranes are things like Intello, MemBrain, sometimes even plain ol' kraft paper on fiberglass batts.
I personally like to use MemBrain over the studs under the drywall, and I detail both layers air tight (and the exterior sheathing too if it's going up new). A little redundancy in your air barriers is usually a Good Thing for long term air tightness of the assembly as a whole.
Bill
Thanks guys. This is helping a lot. I will ask the inspector about house wrap. I'm not sure if it is much cheaper than some of the products I have seen online but it is much easier to get.
I took a look at that map. I would have thought the white line indicating hot humid would be higher. We get a good amount of humidity here and we are well above the line.
Is their a preference between fiberglass or Rockwool? Both are readily available and the price difference isn't huge.
Thanks
FOR YOUR INFO: There is a difference between plant hardiness climate
zones and climate zones based on temperature in North America. In the
Charlotte area it will be 8B for plants but climate zone 3A for temperature.
Not very important but does sometimes cause confusion when talking to
florists and architects.
Oh thanks. I did not realize that. I thought they were using the plant map for reference. I have a lot of experience with building but very little with green building. This is going to be my personal home so I wanted to try a few things
Steven, I've attached the IEEC map with climate zones for your reference.