Reality check on interior/exterior insulation Poly Iso, House Wrap and Roxul inside
Hi,
2×4 standard construction walls. Currently they have 40 year old builders grade wall insulation. I opened a wall to check out a situation and it’s in need of replacing across the house. I need to upgrade insulation.
Plan:
1) Remove internal drywall.
2) Replace insulation (which was poorly installed or has infestation problems, or water damage) with r15 Roxul. I don’t like spray foam inside.
3) Vapor barrier then new drywall.
4) Outside currently is t1-11 plywood.
5) Add House wrap.
6) 1.5″ poly iso.
7) Vinyl siding.
I plan on having 6 and 7 done professionally.
What am I missing? My napkin math has me at r15 inside with r9ish
outside .
Yes I know I’m also going to need to look at the other areas of the house such as the windows, attic and what not.
Thoughts?
Replies
Where are you located? My guess is you don't want the vapor barrier.
You will need to make sure you get the windows and doors flashed correctly to your drainage plane.
Hi - Upstate NY. We need it usually up here for drafts due to snow/water and wind. I'm not worried so much about that. I'm worried about the rest of the layering. Thoughts?
I would leave off the interior vapor barrier. Here is an article from Joe Lstiburek that details out how to best accomplish a vapor barrier. (https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers#:~:text=Vapor%20barriers%20were%20originally%20intended,in%20any%20air%2Dconditioned%20enclosure.)
As a side note, the exterior insulation with taped seams will accomplish the goal of eliminating drafts due to snow and wind. Make sure to use flashing tape on all the rough window openings. That flashing should go out over the exterior insulation.
My question is how is the "shelf" life of the fancy zip tapes etc. for the exterior insulation systems. I want to build for longevity 100+ year not 25 - 30 year. What is the best for longevity? I am willing to give a little on R and vapor values, if my son doesn't have to tear it out in 30 years.
The tapes don't have 100 years of use, so that's really an impossible question to answer. In my experience removing tapes that have been in place for a couple of years, the adhesive gets stronger with time.
Also, skip the interior vapor barrier, or use a smart vapor retarder. The PIC foam will act as an exterior vapor barrier. Add an interior one as well, and water that does get into the wall won't be able to get out. You'll guarantee your son is tearing something apart in 30 years if you have a double VB.