Replies

1. GBA Editor
   Martin Holladay | Nov 05, 2018 06:24am | #1

   Andy,
   It looks like Grand Rapids is on the border of Climate Zone 5 and Zone 6. If you go ahead with your plan, it's OK to use 2-inch mineral wool (call it about R-8, depending on what product you use). But if you switch to rigid foam -- and if you determine that your house is in Zone 6 -- be aware that the minimum R-value of the continuous exterior insulation would be R-11.25.

   For more information on minimum R-value requirements for continuous exterior insulation, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

   With semi-rigid mineral wool, you can get away with a lower R-value, because mineral wool is vapor-permeable. But the tradeoff is that mineral wool is a fussier material to install, since it is squishier than rigid foam. For more information on installing exterior mineral wool, see "Installing Roxul Mineral Wool on Exterior Walls."

   For an overview of different wall design options, see "How to Design a Wall."

2. GBA Editor
   Martin Holladay | Nov 05, 2018 06:29am | #2

   Andy,
   Q. "I saw some designs having a smart vapor barrier like Certainteed's MemBrain between drywall and studs. Is that necessary?"

   A. In your climate zone (either Zone 5 or Zone 6), building codes require the installation of an interior vapor retarder. Options include vapor retarder paint or a product like MemBrain. In general, though, attention to airtightness matters more than your choice of vapor retarder.

   For more information on vapor retarder requirements, see these articles:

   "Vapor Retarders and Vapor Barriers"

   "Do I Need a Vapor Retarder?"

3. 
   AndyZb | Nov 06, 2018 12:55am | #3

   Great, thanks very much Martin!