Correcting a Failing Wall Assembly Mid-Construction
Disastrous wall assembly
I currently have a wall assembly consisting of poly mid wall. Actually R12 is outside of poly and R20 inboard. Which is probably opposite of what I’d want. I’m in climate zone 7a, Canadian prairies.
It’s a double 2×4 wall but instead of poly being on the inside wall it is on the interior of the outside wall.
In short there is insufficient insulation on the outside of the poly to keep it warm enough so some condensation is occurring on the inside of poly in winter. Wall assembly from outside is as follows: tyvek, osb, 2×4 wall with r12 batts, poly, 2×4 with r20 batts, drywall.
To fix I’m thinking adding more outboard insulation. I’d prefer rockwool or something vapour open as there already is poly in the wall. However the cost is more than twice that of EPS foam. Would EPS be advised against? I was considering 2″ thick. Final wall would then be R20 both inboard and outboard of the poly which I realize probably still isn’t enough outboard so maybe 3″ foam… the vapour sandwich thing might be an issue but lots of builders do a similar thing by putting a poly vapour barrier next to the drywall and then a sheathed 2×6 wall with R22 batts and then 1″-2″ EPS (XPS is also commonly done) on the outside of sheathing. It’s been done for years and I’ve never heard about any problems. It’s a pretty dry climate here.
Option 2 is removing the poly from the outside by cutting out a 4′ strip of osb all around the house. Then resheathing with plywood. I’d use vapour retardent paint on the drywall. Re-establishing a good air seal without the poly is a bit of a challenge but probably doable. I don’t like option 2 but at least I know it’ll dry out as again there is visible moisture accumulation on the interior side of the poly when I cut out pieces of osb to inspect from outside.
There is currently no siding on the house so any fixes need to come from the outside. Any recommendations would be appreciated.
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Replies
Before going too crazy, are you having moisture issues as is? Is there significant condensation on the poly?
A bit of learning curve, but it might help to build a model of your wall here:
https://www.ubakus.de/en-ca/r-value-calculator/
Their pre-built examples (ie passive house wooden wall) might be a good start to modify.
Outdoor temperature used would be the mean outside temperature during the coldest 3 months. Make sure to set the indoor to something reasonable like 20c 30% RH. You want to look at the moisture analysis report to see if you'll get a lot of condensation.
In your case, just vapor barrier paint might do the trick.
In case you do need more exterior insulation, you can use any unfaced rigid insulation or permeable polyiso. Since you'll only need r8 to r12, these are still somewhat permeable and will allow for some drying to the outside.
Thanks for the link. I input my assembly and it said the condensate accumulation for the assembly was on the higher end but still within limits... It would take a month to dry in summer. There is definitely more moisture than I am comfortable with present on the poly right now, the back side of the insulation touching the poly is damp. Adding 1.5" of rigid EPS seemed to suffice according to their calculations to stop moisture forming on the poly.
Thinking more about your assembly, I don't think there is a way of saving it without exterior insulation.
The problem is your first condensing surface, the poly, has no moisture storage ability. Neither does your insulation. Usually you want something like OSB or plywood here which could hold a bit of moisture and prevent the liquid water buildup you are seeing.
Your best bet really is the exterior rigid insulation. This will bring up the temperature of the poly above dewpoint and prevent condensation. I would err for the side of more exterior rigid as you never want condensation forming on the poly.
Pick unfaced or fiber faced rigid insulation so you still have a bit of drying capacity to the outside.
It sounds like this is a new house that is under construction, right?
Are the insides of the walls entirely finished (electrical outlets installed, mudded and taped, painted)?
Is the interior of the house being used as it will in the long run (typical heating schedule, ventilation, and moisture loads)?
Is there any construction activity ongoing right now or recently that is generating a lot of moisture?
Yes house is under construction. The moisture on poly was noticed just after the house was boarded with drywall. There was no excessive moisture loads at that time. Since then it was mudded and taped which would be more moisture than normal but I had a dehumidifier running for about a week during mudding and taping. The poly mid-wall was added to appease building inspector (in the end he would have been fine with original plan). Original plan was supposed to be a tyvek layer as an air barrier mid wall (in place of the poly) and then use drywall as another air barrier on the inside with a vapour retardant primer.
I was reading more on GBA and the ratio I've read recommended of outboard foam to inboard fluffy insulation for climate zone 7 is a minimum of 44% of total R value needs to come from the outboard foam layer. In this case any insulation outboard of poly would be similar to that application... I would think? So R12 fiberglass combined with R6-8 foam would suffice (Total of R18-20 outboard making it close to around the 50% of total R-value). If I'm thinking through that wrong let me know... To me it seems like the best solution at this point. Again open to other options.
I'm just a hobbyist (and a Canadian.) You've got an interesting predicament between your two proposed options (external insulation or tearing out the poly.)
I don't know how to best think about the impact of external insulation. You said you were in Canada. Here is how I think about it. If you have the poly at the 50% mark, then that means when it is 20 degrees inside and -30 degrees outside, that the temperature would be -5 degrees at that point in the wall. If you have effectively no vapor barrier between your interior space and the wall then it feels to me like you would be getting condensation on that surface based on the amount of humidity that you will have at that point in the wall.
I do think you should consider asking a professional for a plan. I'm a little apprehensive about the idea of only adding external insulation. Once you get the siding on, there are no easy fixes.
The idea of cutting an access strip in your sheathing to be able to pull out the poly would probably not be too bad.
One consideration will be air sealing. You can achieve an air seal at the external wall (tape the sheathing seams, etc,) It's a little harder now than if you had planned on achieving an airseal from the start but it still may be practical. You can also take a few steps at the internal wall if you are at the stage of having the walls mudded but they aren't yet painted and the baseboards aren't yet on.
So your strategy could be:
- Use vapor barrier primer on the interior walls
- I assume you did not use vapor barrier electrical boxes but you may be able to improve the seal in those retroactively with duct seal (check if this is approved and recommended)
- Rip out the internal poly
- You might still choose to add a layer of external insulation - as that will only help your building and may reduce condensation on the back of the sheathing. A highly permeable option like Rockwool would probably be best.
Where is the poly in your ceiling assembly?
Poly is located next to drywall in the ceiling. It's all tied in with the wall poly for a pretty good air seal. There is no moisture problems in ceiling.
If I remove poly and batts from the outside I can access back side of electrical boxes to spray foam and seal that way... thanks for sharing your thoughts. I really don't want to tear out poly but it is the safest option probably...
ashleighsell,
This is a tricky one. The two methods of installing exterior foam that work are:
- To use a sufficiently thick layer to keep the sheathing warm enough to avoid condensation. Because that severely limits drying to the outside, the advice is to avoid having interior poly, so there is some drying path.
- To use a thinner layer of foam, which allows some drying, and install an interior vapour-barrier to limit the amount of moisture that gets into the wall.
I agree with Akos, if you go that route: Concentrate on adding enough exterior insulation to keep the poly from attracting condensation, but not enough to completely preclude some drying to the outside, as the outer part of the wall can't dry inwards. You aren't trying to stop condensation on the sheathing, just the poly. The ratios of insulation outside the poly to that inside need to include both the foam, and the batts in both walls. That's tricky to get right.
The sure-fire solution, which eliminates all the risk, is permeable exterior insulation like mineral wool board. Unfortunately it's also very expensive.
I also agree with canada_deck that removing the poly might be easier. If you decide on that solution, don't add any exterior insulation. The outside of double-stud walls relies on very good drying capacity to stay safe. The less that impedes that the better.
Thanks for the reply, my question would be what is to much foam where it no longer allows drying? The eps foam I have available is 3.5 perm per inch. Does 2" of foam cut that in half to 1.75 perms? I can't find any info on how much perms are affected by thickness. Is it linear or like a hockey stick?
ashleighsell,
From Martin's article: https://www.greenbuildingadvisor.com/article/all-about-vapor-diffusion
"Multiple layers
How do you calculate the permeance of two layers of material? The formula is:
Total permeance = 1/ [1/(permeance of layer 1) + 1/(permeance of layer 2)]
A simple example is when a builder installs two layers of the same material — say, asphalt felt. In this simple example, the permeance of two layers of asphalt felt is half of the permeance of one layer of asphalt felt."