Foil faced foam over concrete in walkout basement ok?

[Mjhar]

Going to finish my basement and here is the plan according to what I have read here in other threads.
1. 6mil Poly
2. 1" Foil Faced Polystyrene
3. 3/4 OSB
4. Laminate

Question is, is the foil faced foam ok? It is NOT polyiso and the foil is perforated. It is called Perma "R". I found it cheap ~$8 an inch.

Thanks for all the info.

 

[Dana]

What is your location/climate zone?

To be clear, your intended stackup is:

indoor air | laminate | OSB | 1" Perma-R | 6 mil poly | concrete | outdoor air or soil

????

 

[Sponsor]

 

[Mjhar]

East Tennessee right in the middle of zone 4. You have the stack correct.
Thanks

 

[Mjhar]

Also forgot to mention that it is a walkout if that matters.

 

[Dana]

At 1" the Perma-R is about R4, which is well below IRC 2012 code minimum for zone 4, for either above or below grade applications. For below-grade basement walls it needs to be at least R10 continuous insulation, and for above grade it needs to be R15-ish.

The 6mil poly isn't really necessary for above grade walls, but might be needed below grade depending on the waterproofing, backfill type , & drainage of the soil on the outside of the foundation, and the vapor permeance of the laminate. If the laminate is 3 perms or higher you'll be just fine without the 6 mil poly unless the concrete is pretty damp.

To hit R10-ish you could install 1.5" polyiso instead. If using polyiso, raise the bottom edge of the polyiso 1/4 off the slab to avoid moisture wicking into it, and fill the void with can-foam.

Alternatively you can keep the 1" Perma-R and add a 2x3 stud wall with R8 batts or compressed R11s (unfaced or kraft faced) between the Perma R and the OSB. The vapor permeance of the laminate (or rather, lack thereof) may be a problem in this stackup. Did you have a particular product in mind?

To hit R15-ish could leave the Perma-R and ad at 2 x 4 stud wall 16" on center, with unfaced R15 rock wool or HD fiberglass fiber insulation. Since it's not a structural stud wall (it's not holding up the house) it's fine to use 24" spacing, in which case R13 batts would still hit code-min.

With any stud wall it's worth putting a half-inch to an inch of rigid foam (anything but polyiso) between the bottom plate of the studs and the slab as a thermal & capillary break. This is far superior to using pressure-treated bottom plates on bare concrete.

 

[Mjhar]

Sorry I was unclear. This is for the floor over the concrete slab. My concern is that moisture might condense on the foil under the osb.

I will be using the foam on the walls also so your answer was helpful though.

 

[Dana]

DOH!

Of COURSE it's a floor! (What was I thinking?) I was fooled by the bottom-up ordering of your list, I guess.

The only thing I'd change in the stackup is (working top down, the way I can visualize it best):

Indoor air
_______
laminate flooring
_______
OSB
_______
6 mil poly sheeting
_______
Perma-R
_______
Concrete
_______
gravel (hopefully)
_______
dirt

The EPS foam is tolerant of moisture (they use it for dock floats, eh?) so putting it on the dirt & concrete side of the poly vapor barrier is fine.

If the vapor barrier is under the foam as in your original stack there is likely to be more space for moisture to pool than if sandwiched between the foam & OSB, and even minor leakage could end up affecting the finish flooring. With the vapor barrier above the foam any liquids that spill through the floor evaporate more quickly since they're warmer than they would be under the foam, and any leakage of liquid up through the concrete has another inch to go before it gets to the subfloor.

Perma-R is a low density (1.0lbs per cubic foot nominal) Type-I EPS , with a specified compressive strength between 12-17 psi. That will still work in this application, but you have to take care to lap the seams of the foam and the OSB by at least a foot. If the edges of the foam coincide with the edges of the OSB in a high traffic area there could be some rockering & permanent compression over time. If it's lapped by at least a foot, even when walking at the OSB seam the weight is distributed by the EPS. If the edges are aligned since there's no mechanical load transferred from one sheet of EPS to the adjacent sheet it's more likely to compress along that edge. Using ship-lap or t & g OSB helps this a bit too, but it's absolutely necessary.

 

[Mjhar]

Thanks again. You have been a huge help to me directly in this thread and in all the other threads you contributed to that I have been poring over for ideas.