Rigid foam insulation and Roxul

We are finishing our basement and are just beginning the insulation phase. We had planned to use rigid foam insulation glued to the outside walls to serve as both vapour barrier and insulation, but are finding that the cost is quite high to achieve the minimum R12. So we were thinking that we could use one inch of rigid insulation topped by some Roxul and finished with a poly vapour barrier before drywalling. However, I have been reading some posts on the internet that suggest two things:
- a minimum of two inches of rigid foam insulation is required to have it act as a vapour barrier; and
- it is not advisable to have two forms of vapour barrier (the rigid foam and the poly) with any kind of cavity or other material in between because condensation will build up between them.

I don't know if these two things are true, first of all, but I also don't know whether we can use the Roxul without a vapour barrier on the inside.

Any advice would be most welcome!

First to answer your questions.

1-Incorrect, the panels themselves can act as vapor barriers ONLY if they are sealed all around the edges (such as w/ polyurethane expanding foam, but a single layer of cloased cell foam will work if you wanted to do that (even an inch or less). (but you don't want to)

2 - Absolutely correct. even though Roxol is impervious to mold and mildew and crap, your studs aren't, and you would probably have a cool swamp between there before two long however, a poly vapor retarder directly under your sheetrock should be adequate to prevent this problem...

The following is from the DOE
Avoiding Potential Moisture Problems

"In cold weather, warm inside air containing water vapor can get past the wall finish and insulation, condensing inside the colder wall cavity. In hot, humid climates the same thing can happen, just in the reverse direction. Humid outdoor air in the summer can condense inside cool, air conditioned wall cavities. If enough of this happens and the water cannot escape, wood rot, mold, and other moisture-related problems can occur. For this reason, building codes often require installing a vapor diffusion retarder on the warmest side of the wall cavity.

Foam board insulation is commonly placed between the exterior finish (i.e., siding, brick) and the studs of exterior walls. To prevent air infiltration, you should place rigid insulation boards tightly together and seal the seams with tape or caulk. However, this practice may worry some builders in cold climates since the foam board may act as a second vapor diffusion retarder. Studies have shown, however, that condensation rarely occurs in these areas unless something else is seriously wrong with the wall assembly (i.e., massive uncontrolled air leakage into the walls from the house). If the assembly is constructed correctly, the inside surface of the foam board stays warm enough to keep water vapor in its gaseous state long enough for it to escape."
-US dep of Energy

http://www.energysavers.gov/your_home/ insulation_airsealing/index.cfm/mytopic=11620

So, Navar, let me make sure I have what you're saying right: I should put another layer of poly between the Roxul and the drywall? So, one or more inches of rigid foam insulation against the concrete wall, sealed all around with tape and expanding foam, with studs on top, filled with Roxul in the cavity and topped off with poly vapor barrier before the drywall?

No. (my bad if I was ambiguous) I meant you could put the foam up but don't seal it. 2 vapor barrier = bad in my book. The vapor barrier always is more effective on the 'warm' side... and since you're doing the basement, I assume the warm side is the inside. So I would choose to use the poly over the roxol, and directly under the sheetrock. *instead* of making the foam a vapor barrier.

To be honest, I'm not sure I see an advantage to putting up the rigid foam panels in the first place. The r-value/inch is nearly identical to mineralwool batting (about 4.2 for roxol, and 3.8-4.4 for foam panels depending on type). so it's not 'better', and to form an effective vapor barrier out of the foam you have to go through a whole hullabolu, of tape or sealant/foam... it's a PITA for no advantage in performance.

Generally, the only advantage to rigid foam is when you are trying to get some r-value out of a narrow gap (like a garage door, or door for example) where batting wont fit, or can't be used.

Since your install is meeting an R-12 minimum code requirement, you HAVE to use 2x4 stud depth ( or 3 1/2" I mean), might as well use standard batting, and put a poly sheet over it, it'll be a helluva lot cheaper and easier than even messing with the rigid foam in the first place. poly is dirt cheap, and you don't need thick stuff).

bottom line is it'll save you money and time to not bother w/ the rigid foam at all, just slap up the batting, poly over it, and put up sheetrock. you can rest at ease knowing it's the greenest most energy efficient layout (unless you want to upgrade to 5 1/2" or 2x6 studs -- j/k), meets code, and is the cheapest method.

now that I think about it, there is *one* more thing you can do... isolate the studs from the concrete. they have this thin roll of ridged foam which is designed to seal the gap between foundation and base plates (for frame on block or panel construction). it acts as both a sealer, and an isolation membrane, decreasing the heat leech from the concrete. it's not a big effect (and probably not worth the effort (the cost is minimal) energy savings wise)

attached is a pic of the label and product just in case you care.