Replies

1. Expert Member
   Akos | May 30, 2020 12:32pm | #1

   You can check here for details:
   https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies

   If you have less than the ratio of exterior rigid rigid shown in the table, you would need an interior vapor retarder.

1. 
   lhbasle | May 30, 2020 02:05pm | #2

   So as long as the exterior insulation is greater than 30% of the total I shouldn’t have an issue. My concern was trapping moisture behind the CI at the WRB (zip sheathing) and it not being able to dry outward through a foam board.

1. Expert Member
   Akos | May 30, 2020 06:17pm | #4

   Usually with exterior rigid you are worried about moisture from the interior of the house. With sufficient ratio, the sheathing is warm enough that the moisture won't condense, so the only source of water is from bulk leaks from windows and siding.

   For water to get there, it has to first get through your siding than the rigid foam. So not much can make it there.

   Having a textured WRB, or just anything that will keep a very small gap (1/8" is more than enough) between the foam and the WRB will allow for a fair bit of diffusion drying.

   People install foam over ZIP all the time, it works fine. You can also always go with one of the permeable polyiso products (IKO enerair) which does allow for a bit of drying towards the outside as well.

2. Expert Member
   MALCOLM TAYLOR | May 30, 2020 07:33pm | #5

   Lee,

   This helps explain Akos' point about water infiltration from the outside: https://www.greenbuildingadvisor.com/article/wrinkled-housewrap-behind-exterior-rigid-foam

2. 
   Jon_R | May 30, 2020 03:09pm | #3

   Research shows that building right at the code minimum % exterior R without an interior side Class II vapor retarder and without the ability to dry outwards will have some condensation/sorption and some elevated risk - I wouldn't do it.

1. 
   lhbasle | May 31, 2020 12:47am | #7

   Jon R, are you recommending the more open mineral wool then?

1. 
   Jon_R | May 31, 2020 07:42am | #8

   Various solutions will decrease risk, but I suspect that Class II (Table 3) paint on the interior would be the most cost effective and least disruptive to your plans.

   Consider using this calculator, but increasing the climate zone by 1 to be conservative.
   https://www.appliedbuildingtech.com/fsc/calculator

3. 
   lhbasle | May 31, 2020 12:45am | #6

   Since I already have the wrb I would hate to spend the money for the wrinkled wrap to put over it. Even if I do, most of the 2” boards give me about R8. That limits the total to R26.7 and the interior to R18.7. So that means I can only go R19 on my 2x6 wall while up to R23 is available. I had wanted to get to R30 but that doesn’t seem practical at this point.

1. 
   brianvarick | May 31, 2020 08:48am | #9

   Polyiso should get you way more than R8 on the exterior.

2. Expert Member
   BILL WICHERS | May 31, 2020 09:44am | #10

   A sheet of typical 2” polyiso is going to be about R13. 2” EPS would be a bit over R8.

   Bill

3. 
   Jon_R | May 31, 2020 09:48am | #11

   See here for why you want to significantly de-rate polyiso for this application.