Replies

1. Expert Member
   Dana Dorsett | Dec 04, 2018 02:14pm | #1

   A simple air gap created by the furring isn't much of a thermal break- about R1, reducing the heat transfer through the framing faction by about 20-23%.

   At the same wall thickness installing a continuous layer of 3/4" polyiso (R5-ish) would more than double the R-value of the framing fraction, reducing it's heat transfer by more than 50%, and reducing the heat flow at center cavity by about 25-30%. Long-nailing drywall to the studs through 3/4" polyiso isn't a problem. This solution would bring the wall performance fully up to IRC 2018 "R13 +5" code minimum for zone 4.

   If a full layer of 3/4" polyiso breaks the budget, cutting 4" wide strips of polyiso centered on the studs will thermally break the framing fraction, without boosting the performance at center cavity, at only 1/4 the amount of polyiso.

1. Expert Member
   MALCOLM TAYLOR | Dec 04, 2018 02:58pm | #2

   Dana,

   I was going to suggest foam strips as an alternative, but had no idea how wide they should be. The 4" rather than say 2" is because?

2. 
   MrDIY | Dec 05, 2018 09:47am | #6

   Thanks for your response! The mysteries off heat flow are slowly being revealed to me as I embark on this project - never to late to learn though. I'll cost out the polyiso and select one of the options you suggest.

2. Expert Member
   Dana Dorsett | Dec 04, 2018 03:19pm | #3

   At 2" if misaligned you may not get the full R-5 at the corner of the stud edge. A 3D THERM model would also easily show the improved benefit that comes from having that ~1.25" nominal overhang onto the open cell foam. The cold stud is making the open cell foam an inch away from the stud colder than it is mid-way between the studs. Another R5 over the cooler fringes of open cell foam is still "worth it" , with a much higher bang/buck than adding another R5 at center-cavity.

   The cold-fringing of the cavity fill near the studs is pretty obvious when looking closely with a thermal imaging camera, if you've ever had occasion to play with IR cameras. Those fringes aren't just 1.5" or 2" wide:

   https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQ0Q0EEQ7zWOfwmnfmzy_7bNEmeWE9bU-vAmo_i8lOoQ0A8h4tJ

1. Expert Member
   MALCOLM TAYLOR | Dec 04, 2018 04:10pm | #4

   Thanks. That's exactly what I needed to know.

1. Expert Member
   Dana Dorsett | Dec 04, 2018 04:57pm | #5

   Malcolm: Here's a slice of a simple studwall modeled in 2D THERM:

   https://www.buildingconsulting.ca/images/22x6wallwoodinsulation.jpg

   The same model in pseudocolor (IR camera style) rather than isotherm lines:

   https://www.buildingconsulting.ca/images/22x6wallwoodinsulation3.jpg

   (clipped from this page: https://www.buildingconsulting.ca/therm-examples )

   The first inch or so either side of the stud is clearly takes the lion's share of the additional performance gain. Even 6" wide strips might be reasonable, but if you're going that far with it you might as well just do a full continuous layer.

3. 
   MrDIY | Dec 05, 2018 09:55am | #7

   Another problem I created for myself is on the ceiling / roof (the ceiling is being raised to the rafters except in the center of the space). I had a misunderstanding with the foam guy and he sprayed open cell to fill the 2 x 6 cavity. (I was expecting all closed cell but he only used it in the crawl space.) I am now planning to attach 2x4's perpendicular to the rafters and spray that cavity with closed cell. Any issues I should be aware of there - or better ideas?
   THANKS AGAIN!

4. 
   seabornman | Dec 05, 2018 10:52am | #8

   Keep in mind that adding foam to interior will require added wood nailers at corners and possibly ceiling and makes you think about electrical outlet installation.

5. 
   MrDIY | Dec 05, 2018 12:54pm | #9

   Yes I've thought of that - a PITA but not a deal breaker. I'll likely add extensions. I could just yank them and re-install but then I'd have to re-foam those spots - so probably not worth it the extra effort