How to Improve This Wall Assembly’s Drying Potential
The house we bought 10 years ago in Canadian zone 6 was state of the art when it was built in 1990 with double wall assembly: siding/horizontal furring strip-/sheeting membrane (Tyvek)/1’’ XPS foam panel-2×6 wall braced diagonally and filled with fiberglass bats /polyethelene vapor barrier/2×3 wall filled with fiberglass bats/gyprock.
We have a double vapor barrier and the exterior insulation is not thick enough to prevent the foam panels sheeting being cold in winter.
The inside vapor barrier was installed with great care to seal all gaps and joist bays (acoustic sealant etc..). The few areas of the walls where I was able to inspect the foam panel from the inside did not show mold, but there are some signs of rust on nails and discoloration on studs.
My worry is not so much water vapor ingress from the inside, but water infiltration in the wall from other sources such as failures in the rainscreen and exterior flashings.
I tried to inspect all sill plates as water tends to gravitate there. But half of them are not accessible (where floor joists are parallel to the foundations). I did not see obvious rot , but in one instance where the end of a sill plate was visible edge-on I did notice signs of early water damage on the underside ( the plates are not treated).
I am planning to replace the engineered wood siding (rot)and all windows, as some are beginning to warp and we had leaks from defective waterproofing between window frame and Tyvek. I will also add a new sheeting membrane on top of the old one.
My question is this: Can I do anything to improve the wall’s drying ability short of replacing all the XPS foam with, for example, rockwool panels or fibreboard panels ? I was thinking of maybe removing 2 feet of the XPS foam at the wall bottom to allow me to better inspect all sill plates and then replace it with 1’’ EPS foam panels which have somewhat higher perm rating ( 5 instead of 1). I could also leave a gap between the new panels and the old one to improve drying. I am not sure however if this will really be worth doing. Any thoughts from the experts would be greatly appreciated.
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Replies
romarin,
That 90's wall is now probably the most common one being built in Quebec and Ontario, and they appear to be performing fine. There has been a lot of conjecture about why they work when they don't adhere to the rules around exterior foam thickness. The answer seems to be they are safe as long as they just have thin enough foam to provide some drying, and a good interior vapour-barrier. So I wouldn't worry about the wall assembly and would concentrate on your concerns over bulk water infiltration from the outside.
A new carefully detailed rain-screen will do that. Properly flash and air-seal the new windows, install vertical furring vented at the bottom, and the siding of your choice.
If you want more insulation go with the recommended thickness for your climate. The danger comes from going beyond the 1" but not all the way.
Hello Malcom
Thank you for the fast come back. You are quite right that the XPS being only 1'' is in fact a Cat 2 vapor retarder and not quite a barrier. Also the extra care put in the inside vapor barrier probably helps. I agree with you that the real problem is the overall rain screen integrity and this is why I wanted to redo the exterior shell using the best techniques, of course obtained from this site.
At 1" XPS isn't a vapor barrier- it's usually over 1 US perm, and it's facing an exterior air gap created by the furring. That's still a reasonable drying path, and a well detailed interior side polyethylene air & vapor barrier does a great job of quelling interior moisture drives, which is why 99.9% of the homes in Canada built with that stack-up aren't failing. Swapping out the XPS for EPS isn't going to be a big improvement.
Fixing the bulk water management details ( flashing etc) is EVERYTHING. Get those right and the vapor permeance of the exterior insulation doesn't matter very much at all. Get them wrong and it'll still fail, probably even with rigid rock wool.
Romarin,
This article should answer your questions: "Rethinking the Rules on Minimum Foam Thickness."
>"Mar 11, 2021 04:45am"
Damn you're getting up awfully early (or staying up awfully late) these days Martin! :-)
Thank you Martin for the link. I had not seen this very interesting article. More than 10 years ago, I had researched wall assembly designs for canadian zone 6 as I wanted to build my own house (ended up buying an existing one) . I have a science background and I was surprised by: 1- that despite the importance of the subject for people’s lives and pocketbooks and despite all the work done on developing construction codes and standards, there was still much controversy and unknowns 2- the relative paucity of empirical testing 3- the actual technical complexity of the subject.
I see that good progress was made since, but some controversy still exists. Nevertheless, relative to my particular case, advice received so far converge and also corresponds to common sense. Yes, rainscreen integrity is key, whatever the wall. But my concern was the fact that nothing can be perfect in a construction setting. So ideally, you would like to have some failsafe, ie drying to the exterior. I understand that I have some drying potential presently and that it might be enough. The important opinion however is that simply replacing the bottom 2 feet of the XPS with EPS would probably not be worth it.
I also found useful to be reminded that you can mitigate some problems by managing air conditioning and inside humidity and air pressure (which I will have to check) . Although, in the case of air conditioning, we have been experiencing longer summer hot spells to the point I wanted to increase the cooling capacity !
According to local contractors, there are two aspects of my wall which are not so common, namely: the absence of structural sheeting and the fact that it is a double wall with the vapor barrier between them. The double wall yields almost R30 insulation behind the 1’’ XPS. The vapor barrier is roughly 62 % inside the R30 bat insulation and I believe this is more or less correct.
I was wondering how these particular aspects of my wall impacted the conclusions drawn from the experience with single 2x6 walls covered with thin XPS
I suppose that not having structural sheeting is no disadvantage, on the contrary.
Due to the higher insulation, the inside surface of my foam is probably colder than in a single wall assembly, but I guess that if these walls do not generally show signs of moisture, this would also apply to me. In fact the double wall should in principle provide me the luxury of less penetrations in the vapor barrier (inside raceways for wires). But in practice, I still get air drafts at some plug receptacles ( failures of the vapor barrier?). Finally I was wondering if having the vapor barrier 2/3 of the way in the insulation reduces any potential problems from air conditioning ? Sorry for the long texts and thank you again all commentators for your help.
"I did notice signs of early water damage on the underside ( the plates are not treated"
Do they have a capillary break at least?
"I was wondering if having the vapor barrier 2/3 of the way in the insulation reduces any potential problems from air conditioning ?"
I think in theory, yes, but so too does the exterior XPS and the rainscreen gap. Which of these offers the greatest benefit, I don't know. I almost would say the rainscreen gap depending on the siding. I imagine AC induced condensation is a non-issue for you in any case, unless direct air leaks circumvent everything (same goes for the opposite [winter] drives really).