Exterior Insulation to 1976 home in Climate Zone 6
Hello,
Located in MN, my house is 2×4 wall construction with a poly barrier on the interior behind drywall. The sheathing is a 1″-1.125″ thick fiberboard, which I believe has an R-value of ~2.5. I have a contractor open to adding insulation and willing to do up to an additional inch on the exterior. I currently have no WRB and plan to add that as well. I have found that building guidelines recommend at least 35% or more of the total R-value on the exterior for Zone 6. The Dow Thermax sheathing cold weather performance and stability look impressive with an R 6.9 @ 1″, but I believe the low permeability of this insulation is a show stopper, so my plan is as follows.
For overall wall assembly, here is what I am thinking:
5/8″ Drywall | Poly – 2×4 with R13 1976 fiberglass | 1″ R2.5 fiberboard (existing) | 1″ Insulfoam GPS platinum R5 | Delta-Vent SA or Hydrogap-SA WRB | Rainscreen SlickerMax | fiber cement siding
The plan is to build the window frames out to place the windows as “outies” with the WRB sealed and flashed out to new window RO
Is the low permeability of Dow Thermax truly too risky?
Is it a problem attaching insulating and siding to the fiberboard?
I would appreciate any help or suggestions to improve this proposed construction plan.
Thank you,
Mike
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I've opened up a 35 year old install of your proposed wall before (drywall, poly, HD fiberglass batts, fiberboard, 1" unfaced XPS). I'm in a bit warmer climate here, north edge of Zone 5. The wall was pristine inside, as good as the day it was built. I can't see why that wall would not work just as well in your climate.
With any of this type of retrofit, the important detail that will make the biggest difference in energy efficiency is air sealing. In your case, this will have to be the exterior rigid. I would figure out how to best seal this to your foundation on the bottom and to your ceiling drywall on the top. Not the easiest in a retrofit but should be doable with a bit of elbow grease.
Hi AKOSI,
My plan is to drastically increase the air sealing by adding the Hydrogap-SA WRB and air sealing it to the foundation with either a fluid applied or a performance tape like Siga Fintrim that can seal to the concrete and WRB at the base of the wall.
Regarding your comment about not having problems with XPS installed on the exterior, are you suggesting that the 1" DOW Thermax sheathing would be acceptable to install? In many ways, my existing fiberboard with R2 is insufficient for preventing condensation on the interior face at low temp, and I will be curious to see if I have any water or moisture damage existing behind the wall after 46 years. With hydrogap-SA and Slickermax I will have a drainable plane on both sides of the new insulation. I like the idea of getting the almost 50% increase in R-value that polyiso provides over an EPS/GPS. After a bit more research, it seems the best place for the WRB is between the existing sheathing and the new exterior insulation and then placing the windows to that WRB plane (innie windows) prior to installing the continuous insulation. It also looks like any self-adhered membrane will require a primer to adhere to the existing fiberboard. So my new assembly is as follows:
5/8″ Drywall | Poly – 2×4 with R13 1976 fiberglass | 1″ R2 fiberboard (existing) | Primer + Hydrogap-SA WRB | DOW Thermax R6.9 OR 1″ Insulfoam GPS platinum R5 | Rainscreen SlickerMax | fiber cement siding
Who will be doing the work, how much if any is DIY? Are your windows new construction or retrofit?
Overall, you are better to select a foam that has some permeability to help with drying, your previous GPS option is a safer choice although the double vapor barrier walls do work as they have been built in my climate for many years. If you want polyiso EnerAir is a better option as it is permeable.
You may find this article to be of interest. They chose rock wool for the exterior insulation because there was poly in the walls.
https://www.greenbuildingadvisor.com/article/wrapping-an-older-house-with-rock-wool-insulation
Also, are you certain that what is inside the walls is R-13 fiberglass and not R-11 fiberglass?
My house (zone 6) built by others in 1978 has 4 mil warm side poly, R-11 fiberglass and 1" of Styrofoam exterior sheathing. Walls are like new inside. There is some permeability to 1" Styrofoam as I understand it.