Can I use R-15 rock wool batts in a REMOTE wall with R-20 exterior foam?
I have read many articles on this site (thank you) and am currently using some of what I have learned in my house construction in Ridgway CO. For reasons I need not go into, the house has two wall systems – the north and west walls are built from ICFs. The south and east walls will be 2×4 studs, plywood sheathing, Tyvek and two layers of taped exterior foam with an R value of 20.
I have read that the rule of thumb is insulation of 1/3 of that exterior R value can go in between the stud bays while still allowing enough interior heat to get to the sheathing and prevent condensation (the sheathing will obviously only dry to the inside of the house).
I have read the advantages of Roxul mineral wool batts and like the idea of using them (compressed fit, no sagging or convection currents in the walls, water and mold resistant etc). The problem is they come in R-15 and not R-10 (R-10 would be my 1/3 insulation ratio). I read somewhere that in some climates you can get away with a higher ratio but I don’t know how to go about figuring out whether I can or not. Ridgway, CO is in climate zone 6 with an annual mean of 8,000 heating degree days and max of 9,000. Average minimum temps in December and January are around 5ºF although we have seen temps as low as minus 22ºF. We are in a fairly arid climate with an average rainfall of around 17”
So my question – given these are south and east facing walls which will see some sun and given the climate information, can anyone advise me whether the Roxul batts are a good choice – or should I punt and go to a standard R-13 fiberglass batt to avoid condensation on the sheathing?
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