Replies

1. 
   Peter Yost | Apr 03, 2019 05:43am | #1

   Hi John -

   To answer this question, need to know what climate zone you are in. For a vented roof assembly in Climate Zone 6, 7, or 8, a Class I or II vapor retarder on the "warm-in-winter" side of the assembly is required (and either of the two smart vapor retarders you cite complies).

   Peter

2. 
   user-231615 | Apr 03, 2019 06:53am | #2

   Lexington, Virginia climate zone 4a

1. GBA Editor
   Martin Holladay | Apr 03, 2019 08:32am | #3

   John,
   In your climate zone, an interior vapor retarder is not required, so you can skip the MemBrain. However, if the MemBrain is already installed, it won't cause problems.

   Remember that the drywall needs to be installed in an airtight manner (which means attention to airtightness at all penetrations).

1. Expert Member
   Dana Dorsett | Apr 03, 2019 08:38am | #5

   The path to drying around 1" foam AND the flange of of the I-joist is pretty long. I'm not comfortable counting on that unless the foam is also fairly vapor permeable, such as unfaced Type-II EPS.

3. Expert Member
   Dana Dorsett | Apr 03, 2019 08:33am | #4

   In zone 4A a minimum of 30% of the total R has to be on the exterior of the fiber insulation to have adequate dew point control at the foam/fiber boundary.

   Rather than being a problem, the Membrain will mitigate the risk, but it isn't going to be sufficient to eliminate that risk.

   Instead of 1" foam, 3" of foil faced polyiso (R19-ish) with 10" of low density fiberglass (not dense-packed blown or "cathedral ceiling" batt, for R37-ish performance @ 10") AND the Membrain would have a tiny bit of margin.

4. 
   user-231615 | Apr 04, 2019 05:53pm | #6

   That makes sense. I will eliminate the 1”foam. I think this detail will work well and provide R52.