Greenest rigid foam board for interior use
We are remodeling our kitchen/dining/living room in our ranch home in Massachusetts. We have put a tray ceiling in one side and a cathedral ceiling in the other side of the room.
The inspector has said we need to get to R-60 for insulation in the attic or roofline because of new energy code. With 2×8 rafters furred out to 9″ he told us our only option was spray foam. I’d prefer not to use spray foam due to the cost, the fact that it’s not a very environmentally friendly option, and if an install goes bad it’s tough to remediate.
After working with a local HERS rater they told me all I’d need in the attic is R-38 if I insulate the 2×4 walls with an R-15 batt, then put a 1/2″ layer of rigid foam affixed to the studs, then put sheetrock over that.
My question for the experts here is which type of foam should I use? EPS, XPS, PolyIso? We are concerned about indoor air quality and potential off gassing, but feel like this is still a better option than using closed cell spray foam on the entire roof.
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EPS and polyiso have roughly the same carbon impact on an R-value basis. The new generation of XPS has about twice their impact. Legacy XPS has many times more climate impact.
EPS is vapor-open, it's about R-4/in. Polyiso is vapor-closed if it has a foil facer, and can also act as a Faraday cage, interfering with radio and cell phone signals. Polyiso ages to about R-5.5/in. You can get it with facers made out of paper (or glass-reinforced paper) which is more vapor-open and does not create a Faraday cage.
At this point I am leaning toward EPS for many of the reasons you state and also from what I've read it is relatively inert and won't off gas as much as the others.
With EPS installed on the interior, is there a need for a vapor barrier?
It depends how well your wall can dry to the exterior. If you have a vapor-open WRB and a vented rain screen or vinyl siding, you probably don't need a vapor control layer. If you have less drying ability on the exterior, I would use a variable permeance membrane. EPS does little to slow vapor flow. In MA, vapor flow is usually from interior to exterior, so you want to either stop the moisture before it gets into your walls or allow it to travel through unimpeded.
The exterior sheathing is 80s OSB wrapped with tyvek and hardie siding. For insulation in the wall cavity we were planning on using mineral wool batts.
With that in mind, would we need a variable permanence membrane? Do you have a recommendation on a preferred product for that?
Thanks for your help!
Hardie directly against Tyvek with OSB will allow some drying but not much. You might be ok but it would be safer to use a variable permeance membrane. I usually spec Siga Majrex or Pro Clima Intello. Rothoblaas has a similar product that you might be able to get through your lumberyard. Or you can use Certainteed Membrain; its performance numbers aren't as good as the others and it's more prone to tearing, but it costs roughly half as much and would probably be fine in your case.
One note: while you're at it, this is a good time to make sure your air control system is robust. It's not very important where in your assembly is the air control layer but it should be somewhere. OSB is airtight if you can seal all of the seams. The vapor control membranes can be made airtight, though it's most challenging with the Certainteed product. Drywall can be made airtight but it's not the most reliable location due to penetrations.
Thanks. When using one of these variable permanence membranes., should they be installed between the studs and the layer of EPS, or do you affix the EPS directly to the studs then put the membrane between the EPS and the drywall?
When I asked the HERS rater about this, they said said I don't need any kind of membrane and that the paint will act as a vapor barrier which seems a little suspect to me, but I'm certainly out of my knowledge here.