Replies

1. GBA Editor
   Martin Holladay | Jul 01, 2013 05:17am | #1

   Mark,
   I'm not aware to any articles that directly address that question. It would be helpful if you could describe the situation you want to address. Are you insulating a wall? An attic floor? Something else?

2. 
   user-481260 | Jul 01, 2013 07:22pm | #2

   Hi Martin,
   I am conceptualizing an adaptation to the traditional double stud wall by replacing the 6mm poly vapour barrier with 1.5inchs of close cell spray foam.

   traditional double stud wall:
   2x4(batt) - 3.5" gap (batt) - 1/2ply wood - 6mm poly vapour barrier - 2x4(batt)

   adapted double stud wall concept:
   2x4(roxul batt) - 4.5" gap (3" roxul Comfortboard* + 1.5 close cell spray foam) - 2x4(roxul batt)

   I am interested in knowing if spray foam can be effectively sprayed on roxul Comfortboard and stand up to the rigourous pressure exerted on a vapour barrier. I have selected roxul because it is more rigid then a fiberglass batt and as a result will provide better support for the spray foam. I have not include the construction method and would like to focus on my question.
   Thanks for your help.

   *http://www.roxul.com/products/residential/roxul+comfortboard+is

3. GBA Editor
   Martin Holladay | Jul 02, 2013 05:27am | #3

   Mark,
   I think that it should be possible to install spray foam against the mineral wool -- assuming, of course, that you can find a spray foam installer who is willing to do it.

   However, I think you may be putting the spray foam on the wrong side of the wall assembly, due to a mistaken (and old-fashioned) idea about the need for an interior vapor retarder. Most building scientists would advise you to install the foam on the exterior side of the wall, following the "flash and batt" method. If you search the GBA website for "flash and batt," you will find lots of information on the topic.

   To learn why you don't need to worry about an interior vapor retarder, I suggest that you read the following three articles:

   Do I Need a Vapor Retarder?

   Vapor Retarders and Vapor Barriers

   Forget Vapor Diffusion — Stop the Air Leaks!

4. 
   user-481260 | Jul 02, 2013 06:11am | #4

   interesting articles - it cleared some misnomers I had.
   I did notice in "Do I Need A Vapor Retarder?" that one of the exception to the rule is in extreme cold climates. I live in the Canadian sub-arctic were we get long cold winters. With this information, does a vapor barrier make more sense?

5. GBA Editor
   Martin Holladay | Jul 02, 2013 07:08am | #5

   Mark,
   Yes. In your climate, there is more of a justification for an interior vapor barrier than in warmer climates.

   So your proposed assembly might be appropriate for your climate.