Replies

1. Expert Member
   Dana Dorsett | Oct 22, 2018 03:22pm | #1

   IRC 2018 code minimum walls for zone 5 is still R20 or R13 + R5 continuous insulation for framed walls.

   https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency?site_type=public

   (It's about 3/4 of the way down the page.)

2. 
   Justin_DeSilva | Oct 22, 2018 03:49pm | #2

   Thanks Dana,
   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?

   
3. Expert Member
   Dana Dorsett | Oct 22, 2018 04:22pm | #3

   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.