2015 Massachusetts R-value for walls?
This IECC compliance guide was published in 2015 and states Massachusetts minimum wall value of R-20. Is R-20 the current requirement for 2018?
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IRC 2018 code minimum walls for zone 5 is still R20 or R13 + R5 continuous insulation for framed walls.
(It's about 3/4 of the way down the page.)
We're retrofitting our home with Rockwool Comfortboard 80 and are trying to select the most appropriate thickness. Our 2x4 wall cavities are filled with Rockwool Comfortbat R-15. Comfortboard 80 is available in 1.5" (R6), 2" (R8) and 3" (R12) thickness.
1.5" (R6) will meet the standard and is available at our nearest Lowes Improvement Center. I don't know if we should consider 2", 3" or 2 layers of 1.5" Comfortboard 80 for added R value. Any thoughts or suggestions?
What constitutes "...most appropriate thickness...." depends a lot on your goals. R6 is sufficient for dew point control for R15, and the high vapor permeability of rock wool gives a drying path to the exterior at just about any thickness.
But it's expensive- pretty pricey per square foot per R compared to rigid foam, and a bit fussier to install. If going any thicker than 1.5" consider using RECLAIMED 2" roofing polyiso, which would be better than R10 @ 2", and easier to install, and less than 1/3 the cost of virgin stock foam, available from multiple vendors in MA. Even derated to R5/inch (most will do at least 10% better than that) at about 4-6 cents per R per square foot it's really quite cheap.
If one of the goals is to be foam-free used foam doesn't cut it, but if looking to minimize the total environmental impact re-using insulation is quite a bit greener than any virgin-stock material.
The two biggest vendors of reclaimed & factory seconds foam in MA are Nationwide Foam in Framingham, and Green Insulation Group in Worcester, but there are others.