Exterior Insulation: XPS vs. Polyiso Foam
Building in Zone 6 White Mts. I am a paying member, so I have read a pile of articles for last few months , Like Mr. Holladay’s, minimum foam. I also read the article about choosing xps over ISO in cold climates and many many others.
So here is where I am at. We are framing now so I need to order foam asap. I was set on 3in of XPS, but looking at prices, I could do 2in of poly-ISO cheaper. Is the long term pay off of longer screws and more foam worth it? My original goal was R 40 walls and R 60 celling,. Not sure if all that extra R will ever pay off. Heat will be wood stove and heat pumps. The walls will be dry wall with vapor retarder paint, 2×6 with R23 Rock wool or similar brand and zip sheathing. Windows will be “innie” installed at the sheathing plane. . The roof will be a other post as half the house is cathedral and racking my head about that one haha.
So what do you recommend, 3in of xps or 2in of Iso? I would layer both in 2 layers with the outside layer taped. Furring strips and LP smart side clapboard siding. I ruled out Rockwool comfort board due to price.
Thank you-Chris.
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Replies
Unfaced graphite EPS. R5/in or slightly higher at cold temps. No thermal drift over time like XPS. And WAY less global warming impact than XPS if you care. Unfaced EPS is also more breathable than poly iso. I’m planning to use 2.5” in the same climate zone. According to the literature, that gets you R12.3 at 40 degrees.
R-Shield is in the Bozeman Montana area. Maybe you can find a mfg closer to you?
https://rshieldinsulation.com/max-library/
You could also do 3” of regular EPS. Depending on the density you pick, that’ll get you over R13 at 25 degrees.
https://rshieldinsulation.com/wp-content/uploads/2020/12/BSI02-R-Shield-Insulation-TechData.pdf
I assume you know that there is at least one brand of Polyiso (Thermax) which seems
not to drop in R-value at low temperatures- no doubt you will pay more for it than for other brands. I'd probably give them a try rather than mess around with the
longer screws and the extra fiddle-faddle associated with thicker foam.
Yes, I was just having a hard time finding it. Any suggestions? Thank you
With wood heat, you just need to put another piece of wood in the stove. I think in practical terms you would have a hard time telling the difference between the 2 proposed insulation strategies in terms of comfort or payback. Put your energy and bandwidth into air sealing.
I have a hard time with deciding how much exterior rigid foam to install. It will take forever to pay off any amount that you install. Air sealing first is 100% the correct answer.
Using the advice given. I already planned on very tight air sealing. Air sealing the foundation to sill etc. and all other openings. Can I just put on 2in of iso one layer and just tape the seams and not worry about it. Yes it will shrink some, but that would be the cheapest option.