Insulated Garage Slab – Vapor Barrier Too?
idahobuild
| Posted in General Questions on
Hello All,
I decided that we’ll be insulated the garage floor and installing radiant tubing for “later”.
Since we’ll have 2″ of XPS between the concrete and the 6″ of gravel; do we need to also have a vapor barrier under the XPS?
Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
idahobuild,
2" of XPS is 0.55 perms. That's a class two vapour-retarder, not a vapour-barrier. How much that matters in a garage I'm not sure.
Poly is cheap, I would include it. Remember it should be above the foam, not below.
https://www.greenbuildingadvisor.com/article/polyethylene-under-concrete-slabs
If you are considering heating the space at some point down the road, think through how you will limit the thermal bridging at the slab edge at the stem-walls and especially at the door. That's usually the big problem area.
Malcolm - I'm going to put vertical xps at edges, but since insulating the garage wasn't in the original plan, we are going to have thermal bridging at the doors. The stem walls are already poured so not much that I can change on them. I can't think how the the area where the garege floor's slab flows over the stem wall to the apron of the driveway can be isolated from the outside apron. Any thoughts on that?
We're stapling 1/2" pex on top of the insulation, would the poly still go on top of the xps and on top of the pex?
idahobuld,
I'd suggest tapering the tops of those pieces of slab-edge foam at the stem-walls so they aren't visible.
I don't have any experience with insulating at garage doors, but there has been a bit of discussion on GBA on the subject:
https://www.greenbuildingadvisor.com/question/slab-at-garage-door-how-to-insulate-hold-back-pex
https://www.greenbuildingadvisor.com/article/energy-edge-slab-insulation-system
Thinking about it again, I'm not sure the typical advice on where to put your poly is that important for a garage. You are probably fine with it under the foam (and definitely not over the pex). Just watch though - having the foam exposed makes it more likely it will rise during the pour, which can leave your pex too close to the surface.
YOu can run the foam right across the door. You can see it but the tires won't touch it. I am sure there are several ways to cover it, but I never got around to it.
If you think you may be heating the garage regularly in the future, then insulating under the slab and installing pex before the pour is appropriate. But if it seems that you likely never will heat the space regularly, then you might consider skipping the foam and pex, opting instead to insulating the stem walls down to footings. Ground heat then can come up slowly through the slab and moderate the temperature inside. This is what I did for our garage, in central NH (CZ6). The garage temperature, measured well away from the door and up perhaps 6" from the slab, typically bottoms out in the upper 30s, only once in an extremely cold spell getting down close to 32.
DickRussell - Thanks. The interior slab is insulated down the stem wall and under the entire slab. heating that has been in the plans from day 1. I just thought that for a few hundred bucks I could add insulation and pex in the garage - for those times when I am playing family mechanic.
At this point I guess that the only option for the thermal bridge reduction would be to carry the vertical XPS across the stem wall where the door openings are. Though, I don't like that idea, so I will likely reduce the pex spacing in the area and fight the cold when needed.
I would put a composite deck board or composite 2x6 on edge at the door. Anything is way better than the R0 of solid concrete.
My old garage had a PT 2x4 pocketed into the concrete at the door. It did rot away eventually (guessing 20-30 years or so), just popped it out and replaced it with a new one. Might also work here.
Akos - that is a good idea. I'll talk to the GC about doing that.