As the title says, I'm getting ready to build a new basement in an unfinished portion of my bathroom. I'm seeing a ton of advice on various sites, but none that match my situation exactly. One wall of my bathroom will be against the concrete foundation wall. I'm in Connecticut, Zone 5, and IRC 2015 says I need R-15/19 (R-15 of continuous insulation or R-19 of interrupted stud bay insulation). The basics of my planned approach are this: 1" or 2" rigid foam against the concrete foundation wall, then 2x4 wall with R-15 Rockwool batts in the stud bays. My questions are mostly concerned with moisture, and not with getting the proper R-value. Here they are:
- EPS or XPS rigid insulation? EPS is more permeable than XPS, but is that good or bad?
The higher permeance of EPS is only "bad" if it's allowing excess ground moisture insufficient quantity to build up moisture in the studwall. But for R15 studwall insulation you'll need more than the
IRC prescriptive R5 for the above grade portion of the wall, since the prescriptive is predicated upon R13 in the cavity, not R15. To maintain the same ratio you'd be looking at a minimum of R5 x (R15/R13)= R5.8.
Also note, XPS is only warranteed to 90% of it's labeled R value. The higher R/inch labeled-R is an artifact of it's climate damaging HFC blowing agents, a soup of different HFCs, predominantly HFC134a, with a 100 year global warming potential about 1400x CO2. (If the US had signed on to the Kigali amendment to the Montreal Protocol this stuff would be contraband.) After 50 years it will eventually hit the same R/inch of EPS of similar density.
EPS is blown with hydrocarbons, usually a variant of pentane, with a 100 year global warming potential about 7x CO2. And most of the pentane leaves the foam and is recaptured at the factory, and plays no part of the long term performance of the EPS, which is very stable over time.
Polyisocynurate is also blown predominantly with low-impact hydrocarbons, and would be appropriate for insulating foundation walls, independent of facer type. (I have 3" of reclaimed fiber-faced 2lb density roofing polyiso on my basement walls, which runs about R17.)
2. 1" or 2" or rigid foam? EPS is R-4 per inch, and the Rockwool is R-15, so I don't need the extra inch for insulation purposes. But do I want 2" to reduce the permeance of the vapor barrier/retarder that the rigid foam is creating?
It takes a minimum of 1.5" of EPS for dew point control on an R15 studwall, as calculated above. At 1.5" Type II EPS (1.5lbs per cubic foot nominal density) runs a bit less than 1 perms, or about 2-3 x as vapor-tight as standard interior latex paint on wallboard, and about R6.3, which is plenty.
The 1.5" of EPS would also be enough to hit code-min performance using cheap R13s (kraft faced or unfaced) in the studwall instead of more expensive rock wool. Even R11s would make it, if going for compliance on a U-factor basis rather than R-value, but "contractor roll" R13s are cheap enough.
3. Do I leave an air gap, say 1", between the studs and rigid insulation, or butt them right against it?
Fiber insulation needs an air-barrier on all six sides of the prism to hit it's performance numbers, so snug it up tight to the foam. (Also, a hidden air gap would need fire blocking to meet code.)
4. Do I install a poly vapor barrier on the interior face of the stud wall before drywalling, which would be standard in an above-grade bathroom?
Absolutely NOT!! That would trap ground moisture in the studwall!
A kraft facer ona batt would be OK, since kraft facers are variable permeance, becoming more vapor open than standard interior latex paint when the moisture content reaches levels high enough to support mold.
And while poly vapor barriers might be "standard", in your area they are not required, and are not necessarily a good idea even in an above-grade bathroom, depending on the material stack up of the wall assembly. In zone-5A climates interior side polyethylene vapor barriers create more moisture problems than they solve, even on above-grade assemblies.
5. What about the rest of the walls that aren't against the foundation? I'm definitely going to insulate, but vapor barrier on the inside (bathroom side) of the wall or no?
Thanks in advance for any advice!
Don't insulate or install vapor barriers in the partition walls- spend the money on insulating the rest of the foundation walls instead.
If that's never going to happen and the rest of the basement is going to be dropping to some crazy-low temperatures, R11-R13 unfaced or kraft faced fiberglass would be fine.
There are multiple vendors of reclaimed and factory-seconds foam board in southern New England that can take the sting out of doing the whole basement.
Running these searches every week or so will uncover most of those operating near you. Or you could just figure out how much you need and have some shipped from
one of the
bigger operations.
