Replies

1. GBA Editor
   Brian Pontolilo | Jul 25, 2019 09:15am | #1

   Hey....

   First, it would be great to know your name.

   Second, I unfortunately am having a hard time following your description of the assembly. Are you talking about site-made SIPs for walls? Walls and roof? Do you have a section drawing? Or could you please simply list the layers of the assemblies from the inside out? That will be helpful to get some good answers to your questions.

2. 
   Jonathan_C | Jul 25, 2019 11:02am | #2

   Hi Brian!

   This is a link to my first Q&A, which has discussions on what the build is:

   https://www.greenbuildingadvisor.com/question/thermal-bridging-4

   Zone 5A (SE Michigan)

   The floor build is as follows: trailer frame | treated 2x4s | treated 3/4" plywood | 2" of EPS or polyiso| 3/4" T&G plywood. I will be taping the seams in between the treated plywood sheets with a caulk and 3m 8067. Seams between the rigid foam board sheets will also be taped with 3m 8067. I had not planned on taping the T&G seams.

   The walls build is as follows: 2x4 stud | 1/2" plywood | Tyvek Housewrap as WRB | 1x4 furring for rainscreen | 2" of EPS or polyiso | metal siding. Plywood sheet seams to be filled with caulk and 3m 8067. Housewrap seams and seams in between rigid foam board sheets to be taped with 3m 8067. Bug barrier along bottom of walls, in space created by 1x4 furring strips.

   Roof build is as follows: 3/8" un-treated plywood | 3.5" of R-15 Rockwool | 2x4 trusses | 1/2" or 3/4" un-treated plywood | 3, 2" layers of EPS or polyiso | 2x4 studs for venting | metal roofing. The interior and exterior plywood seams will be taped with 3m 8067. The 3 layers of foam will be staggered and taped. I am still scratching my head on whether to mechanically fasten the foam boards or glue them or both. The final layer of foam board sheets will have their seams taped with 3m 8067.

   My questions on taping are what I call "exterior" edges, in my head. I don't know if all these exterior edges need to be taped or should be left open. The housewrap taping will already include all of these "exterior" edges being taped. The question I am asking is if I need to then tape the rigid foam board to itself along wall to wall exterior edges, the rigid foam board to the soffit along the roof line perimeter, and the rigid foam board to the trailer.

   Thank you for your help!

   Jonathan

1. Expert Member
   Akos | Jul 25, 2019 11:53am | #4

   That is a bit too much taping, but it never hurts.

   You always tape across transitions from floor to wall, wall to wall and wall to roof. The air barrier needs to be continuous whichever cross section you draw the line along.

   For the walls your EPS goes under the rain screen:
   -2x4 stud | 1/2" plywood | Tyvek Housewrap as WRB | 2" of EPS or polyiso | 1x4 furring for rainscreen | metal siding.

   Despite what the manufacturers say, detailing house wrap as an air barrier is futile. Doesn't hurt, taping the sheathing underneath is what you need to get right.

3. 
   Jon_R | Jul 25, 2019 11:21am | #3

   Consider using fully adhered WRB. It should air and water seal better and reduce the amount of tape needed.

   > 1x4 furring for rainscreen | 2" of EPS or polyiso | metal siding

   I wouldn't use EPS (or tape) outboard of a rain screen.

4. GBA Editor
   Brian Pontolilo | Jul 25, 2019 12:44pm | #5

   Hey Jonathan,

   I think Akos and Jon gave you good advice. And I think you're overdoing with the tape, but that's an issue of expense and consumption; you're not going to cause any problems.

   Make sure your air barrier is continuous. That's the key. And as Jon noted, the exterior insulation on your walls should be against the sheathing, not on the furring strips.

   Also, I'd use some sort of underlayment with lapped seams on top of the roof sheathing to shed water in the even of leaks. Felt would be fine.

   Finally, to your original question about a membrane between the 2x4s and plywood in the flooring system. A lot of deck builders do this between joists and decking and think it might be a smart idea for your project. Even treated lumber rots when water is captured and left to sit.

1. 
   Jonathan_C | Jul 25, 2019 03:02pm | #6

   Brian/Jon/Akos,

   Thank you for your comments and setting me right on the rainscreen.

   If I skip the housewrap, what is considered a fully adhered WRB, per Jon's comment?

   My plan for making a continuous air barrier is to tape all plywood seams. And then add the EPS on top, without additional taping at exterior EPS interfaces.

   Thanks,

   Jonathan

1. Expert Member
   Akos | Jul 25, 2019 04:08pm | #8

   Fully adhered WRB is mostly needed if you have board sheathing with lots of gaps or questionable architectural details such as a pop in window.

   Stick to standard housewrap, under foam getting a crinkly one adds extra drying capacity for a small extra cost.

5. 
   Stockwell | Jul 25, 2019 03:13pm | #7

   Fully adhered examples---Henry Blueskin or Solitex Adhero

   Check with the tape manufacturers on using their tapes on pressure treated wood. Some require steps to be taken before use.