Upgrading to Continuous Insulation– Keep Interior Poly Barrier?
Z_Schuster
| Posted in Green Building Techniques on
My home, located in Minnesota, was built in the late 70s. The construction is 2×4 walls with batt insulation. There is NO solid sheathing. Instead of plywood there is 1″ rigid foam on the exterior along with horizontal furring strips. Some tar paper or something overlaps the furring strips and edge of the rigid foam, but it doesn’t seem to be a real air seal). On top of that is vertical cedar board siding. There may be diagonal components to prevent racking but I have not been able to get access to any portion of the wall to verify this. The inside of the exterior walls have a poly vapor barrier.
We will be re-siding in a few years. When we so that, I was thinking of taking down all of the rigid foam, addressing any sags in the batts, air seal the electrical boxes, etc… Then put up solid sheathing before putting the rigid foam back on (or put the rigid foam back up and layer sheathing over that like a DIY Zip system. If I do this, I could add additional rigid foam to further increase the R-Value.
While I’m interested in general feedback on this approach, my main question is if I did this, would it be good to remove the interior vapor barrier in the cavities to allow any possible moisture to dry inward? I wouldn’t be able to easily remove the poly between the drywall and the stud itself, but could clear out the portions that span the stud cavities.
Or am I overthinking this whole thing and going too far? Is there a simpler approach? I like the idea in that I’m re-using the existing rigid foam and in this case the lack of any solid sheathing makes getting to the stud cavities a lot easier than if there was plywood or OSB already.
I do have some other electrical and low-voltage work I want to do when I do this, so some portions of the rigid foam would be getting temporarily removed already.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Northern Minnesota is in climate zone 7. Most of southern Minnesota is in CZ 6, with the southeast corner in CZ 5.
Per the IRC, 1" of foam would not be enough to eliminate the poly vapor barrier in CZ 6 or 7. If you are in CZ 5 and the foam is XPS (R-5 per inch), then you could remove the poly and just rely on interior latex paint as a vapor retarder. If you added additional foam to bring it up to a total of R-7.5 in CZ 6 or R-10 in CZ 7, then you could also remove the poly in those climates.
Thanks Frank. I'm in the Twin Cities, so Zone 6 (I should have included that in my original post, apologies). The rigid foam appears to be "Dow" brand and is a light blue color. I had access to a small section of the foam during a window project and have the following information on it: Dow Styrofoam High Performance, FHA MR601, and BOCA 6934. I have not reached out to Dow but an initial search online didn't return much in the way of specs on R-value when searching for a combination of those terms. For simplicity, let's assume the R-value is around 4.
So let's say in the interest of maximizing energy efficiency while also trying to be as economical as possible, if I took down the rigid foam temporarily, air sealed outlets on the exterior walls and replaced/fixed sagging batts, (plus the electrical/low voltage objectives) then put the rigid foam back up, taped the seams, and put OSB over that and a weather barrier, I should be fine? Would it be better to do OSB to the studs, then the existing rigid foam over that to also act as weather barrier?
To be clear, I don't want to take the poly down if I don't have to. What I am concerned about is not taking it down and ending up with a situation where water gets in the wall and the wall can't dry to interior or exterior. It sounds like give the situation of my house that this wouldn't be necessary unless I added more rigid foam?
An initial inquiry to my city building inspector with this same question came back with not much in the way of an answer on if it was allowed, but a suggestion of removing the batt insulation and doing spray foam and then putting the rigid foam back up. That would be a lot more expensive for me. My main goal is air sealing to reduce drafts while re-using as much existing material as possible and avoiding potential rot issues from the aforementioned air sealing.
You can leave the poly up. Tens of thousands of houses with a poly vapor barrier and 1" of unfaced foam on the outside have been built across the northern US and Canada, so we know it works.
I'd put the OSB on the studs with foam exterior, unless you had a specific reason (such as cladding attachment) for doing it reversed. Air sealing should actually reduce the potential for rot. I suggest also taping the OSB seams with an acrylic tape like 3M's All Weather Flashing Tape 8067. For sealing the bottom edge of the OSB to the foundation wall, people here often recommend Siga Fentrim tape.
Thank you for your input!
Hello Z,
According to my, albeit limited understanding of building science, I would suggest you are on the right path with opening the walls from the exterior, removing enough insulation to check and then seal the existing electrical and plumbing penetrations, leaving the poly air barrier alone, adding enough exterior (soon to be mid wall) sheathing, leaving it breathable/vapor open rather than coated, so you have something to attach to, and back up against, adding back your blue polystyrene and or extra layers of insulation (probably best if it is a little breathable like mineral wool... for others to comment), then adding sufficient furring strips and or applying some sort of rainsceen type cladding.
In the process you likely will want to detail out your door and window openings with flashing solutions and or membranes to ensure mass water is kept out and air movemement is minimized to the best degree possible.
There is nothing wrong with building with no sheathing. My only reservaion about this approach is the lack of a solid critter barrier, but if this is not an issue, no need to change it.
Not sure I would strip and scrap the existing rigid, it might make more sense to add ontop. Maybe infill between the strapping with rigid and cover with a layer of R6 ZipR. That would get you close to something like this:
https://effectiver.ca/calculator/wall.php?id=11941
Which works pretty well in cold climates.
There are many permuations in between, my only recommendation is to stick to somewhat permeable exterior rigid. This would be unfaced or micro perforated rigid such as Halo. The unfaced XPS is pretty hard to tape as a lot of tapes don't stick all that well to it. The perforated faced rigid is much easier to detail.
Your existing blueboard is most likely unfaced XPS which is permeable, so it will work well even if you add more.
There actually has been a critter issue with woodpeckers. They're not looking for bugs, but keep making homes in the wall. This is the main reason we need to reside and looking at something that will be much more resilient to woodpeckers. But when we do that, I figured it would be a good time to do some insulation improvements and, if advisable, add sheathing.
I'm actually looking to keep the styrofoam but possible add to it. I'd rip some of the existing cedar boards to place between the WRB and the siding to provide a rain screen. I'd like to repurpose as much of the existing material as I can.
Thank you for the advice!