Exterior wall and roof insulation
hello,
have a question about my wall insulation package. hoping im not too far gone as my framers have already installed my 1 5/8″ polyiso insulation and vapor wrap on the exterior of my 1/2″ plywood sheathing.
my home is in middle Tennessee, so very hot and humid summers, and wet mild to cold winters (maybe a snow or two).
here is the layering plan I was presented…keeping in mind the polyiso and vapor wrap are already installed to the outside of the sheathing.
thx in advance for your input!
Your thoughts?
Walls from outside in:
real wood sliding
½” slats
Perforated radiant foil barrier
vapor wrap
1 5/8” polyiso foam board
½” plywood
2×6 studs = 16” O.C.
Spray foam all corners inside studs
1 5/8″ polyiso foam board between the studs
Batt insulation
Leave 1” gap behind sheetrock
Roof from outside in:
Shingles
perforated radiant foil barrier
ice and water shield underlay
1 1/2″ polyiso foam board
1/2″ plywood
2 x 8 studs
Spray foam all corners inside studs
1 1/2″ polyiso foam board between studs
batt insulation
Leave 1” gap behind sheetrock
thx again!
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Replies
Hi Tyler,
I think you are on the right track but will want to make some modifications to your wall and roof stack up. (See https://www.greenbuildingadvisor.com/article/five-rules-for-wall-design for more info.) For example, you should avoid using the cut-and-cobble method since it is likely to create problems over the long-term. (See https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation for why.)
Hi Tyler.
With low-perm exterior details, you want your walls and roof assemblies to be able to dry towards the interior. And since the sheathing is the most likely part of the wall to get wet, the assembly should be vapor open from there inward. I would agree with Steve and also suggest skipping the polyiso inside the walls and roof and just use vapor open insulation.
Also, you should have a plan for air sealing.
Are you air sealing the sheathing or the rigid foam on the exterior? If not, you can air seal the stud bays and the drywall, but don't do all this work and not air seal. You can test it with a blower door and use that process to make sure you have done an adequate job. This may be helpful: Blower-Door-Directed Air Sealing.
You can use these two articles to make sure that you have the right ratio if exterior to interior R-values for your location: Walls that Work and How to Build an Insulated Cathedral Ceiling.
Finally, keep in mind that radiant barriers only perform if they are facing an air space: Radiant Barriers: A Solution in Search of a Problem.
steve/brian,
you are speaking above my pay grade!
I follow you on the cut and cobble, I will just spray foam the corners between the studs and use batte, but not sure I follow you on the location of the air seal?
also, am I ok laying my metal roof directly on ice & water shield and polyiso foam with plywood sheathing below polyiso?
or do I need a sheathing/ice & water/polyiso/sheathing?
thx so much!
Hi Tyler,
Here's an article that will help you to understand why air sealing is important https://www.greenbuildingadvisor.com/article/whats-more-important-air-sealing-or-insulation).
You can improve your home's performance by taping the seams on the plywood sheathing. Several tapes are recommended in this article: https://www.greenbuildingadvisor.com/article/air-sealing-tapes-and-gaskets.
It also is a good idea to hang your drywall in an airtight manner. See these videos for details: https://www.greenbuildingadvisor.com/green-basics/video-how-to-hang-airtight-drywall-1-of-3.
If you plan to have cathedral ceilings, note that you need the right insulation technique to prevent moisture issues. See https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling for more information.