Replies

1. Expert Member
   Dana Dorsett | May 13, 2019 04:04pm | #1

   is it 2x4 or 2x6? I'm guessing 2x4, but in case it's 2x6:

   At 2" you have about R8.4 at full depletion of it's (climate damaging) HFC blowing agents, which is still more than adequate for dew point control on 3.5" of fiber insulation using standard latex interior paint on gyprock as the interior side vapor retarder, in US climate zones 5 & 6, the green and blue part of this map:

   https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2014/05/15071255/Climate-Zone-map-including-Canada-700x417.jpg

   In zone 7 (purple) it doesn't quite cut it for 3.5" of fiber insuation, but using a "smart" vapor retarder or "vapor barrier latex" on the interior side.

   If the framing is 2x4 and you're not putting more than 1.5" of fiber insulation it has plenty of dew point margin in any Ontario location.

   See: https://up.codes/viewer/wyoming/irc-2015/chapter/7/wall-covering#R702.7.1

2. 
   Le_Marquis | May 13, 2019 06:16pm | #2

   The walls are 2X4. Based on your answer, I will not add a second barrier then.

   Thanks for your answer, Dana.

3. 
   Jon_R | May 13, 2019 09:14pm | #3

   Being picky: I don't know your code, but in the 2015 IRC R702.7 (which in theory, should describe generally good practice), you will find "continuous ... over 2x? wall". Your XPS is discontinuous and not "over", so the IRC would require a separate Class I or II vapor retarder on the inside. The possibly relevant differences with foam location - studs are colder and air leaks are more likely. Lstiburek (an expert from Ontario) also recommends Class II:

   https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

   Summary: there are some arguments for adding a smart-retarder (like MemBrain) under your drywall, even though you have a good XPS to fiber ratio. It won't hurt.

   More important is interior side air sealing. I'd detail the drywall to be a second air barrier. Redundancy is good.

4. 
   Le_Marquis | May 13, 2019 09:28pm | #4

   Jon, I will be honest: I'm not smart enough to understand what the hell you are talking about. What is "2015 IRC", "2x wall", "over the wall", "Class 1 or Class 2", and "redudancy"?

   I did understand about the importance of air sealing though :)

   Thanks for enlightening me here.

1. GBA Editor
   Martin Holladay | May 14, 2019 05:35am | #5

   Marc,
   When Jon wrote "2015 IRC," he was referring to the 2015 version of the International Residential Code -- a building code that is used in many U.S. states, but not in Canada.

   When he referred to installing rigid foam "over the wall," he was referring to a continuous layer of rigid foam on the exterior side of the wall sheathing. You're not using that method. You are using a method called "cut-and-cobble." For more information on the method you chose, see this article: "Cut-and-Cobble Insulation."

   When he wrote about Class I and Class II vapor retarders, he was referring to a system of vapor retarder classification in the U.S. building code. If you are curious -- most Canadians aren't -- here is a link to an article that explains more: "Vapor Retarders and Vapor Barriers."

   When Jon wrote about "redundancy," he was talking about preventing a possible moisture problem by using two preventive measures at once -- an approach that is more commonly called the "belt and suspenders" approach.

5. 
   Le_Marquis | May 14, 2019 08:02am | #6

   Martin, I've read your blogs, posts, articles with great interest over the years. I want to thank you for being such an incredible resource for home owners.

   Kuddos to you, Sir.

6. 
   Peter Yost | May 14, 2019 06:30pm | #7

   Perhaps a final note:

   Another important moisture management element in a bathroom is as really effective spot exhaust ventilation system. The vapor pressure in a bathroom can be intense BUT it is also wildly intermittent. If you manage it intensely during those periods of use, that takes, literally, a lot of pressure off of this space.

   Peter