EPS, XPS or Polyiso for exterior rigid foam?
Last week I got some great information from this board on my “Taping seams of roof sheathing” question. There was some thread drift there and we started talking about exterior foam and someone suggested I use Polyiso instead of XPS for my exterior foam.
I took that advice and started to look into Polyisocyanurate. I had previously written it off because I had “heard” it was twice the price of XPS and three times the price of EPS. However when I called up my insulation vendor I was shocked. It’s a little cheaper and provides a higher R value per dollar than XPS. Here is what my quote was:
2″ Polyiso 4×8 foil faced sheet (rmax thermasheath) 2.5lb density is R-31.1 – $33.64
2″ XPS 4×8 sheet 2.5lb density is R-10 – $34.99
2″ EPS 4×8 sheet 1.0lb density is R6.4 – $14.52
Cost per R for Polyisio, XPS and EPS is $2.53, $3.49, $2.26
I’m in a mountainous and snowy climate zone 6. My wall construction is interior latex paint vapor retarder, 2×6 stud cavity filled with BIBS, OSB, Tyvek, continuous insulation, then a mix of stone veneer, eifs stucco and siding on the outside. Obviously each exterior layer will have a drainage layer between the continuous insulation.
This leads me to a few questions
1) Why would anybody use XPS instead of Polyisio?
2) The R-31.1 is apparently an aged value. How much can I expect this to change due to the temperature effects and further aging?
3) How dimensionally stable is polyiso and are there any issues with installing siding over the top of furring strips?
4) In last week’s thread there was much discussion about vapor permeability of the continuous insulation. Foil faced polyiso is obviously impermeable but I think I might be able to get it mesh faced which will increase the permeability to 2ish. I think with R-13.1 I will exceed the code requirement of ratio of exterior continuous insulation to interior stud bay insulation. Therefore I should not have a condensation issue on my sheathing, right?
I really like the idea of using polyisocyanurate exterior foam it checks a lot of boxes: it’s better for the environment, it meets the code ratio for exterior continuous to interior insulation, it has better fire retardant properties. My installer says it is harder to work with than XPS or EPS because it doesn’t break like those do, so maybe I’ll expect slightly higher installation costs.
Am I missing anything in my considerations?