is 1″ xps styrofoam behind plywood sheathing in wall asking for trouble?
My existing exterior walls (going from inside to out) have spray foam in a 2 x6 wall cavity (no sheathing yet),1″ of xps foam, plywood sheathing, tyvek, ceder clapboards.
We have had some moisture damage to plywood which I think is from exfiltration of moist air at some imperfections in the wall. I am addressing the source with an ERV (dropping rh level of winter inside air significantly) and sealing leaks discovered with a blower window :>
I may add a rain screen when rebuilding this wall outside the sheathing and WRB, before the clapboards.
Should I also be putting a drainable WRB or something between the styrofoam and the inside face of the sheathing to help protect this face from getting wet in the winter?
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I don't see any benefit to a drainable WRB or anything else between the interior foam and the sheathing. Moisture from the interior will be in the form of water vapor, not liquid water, and you want that to pass through (i.e. dry) to the exterior.
You didn't mention a separate vapor retarder. The IRC requires either a class I or class II vapor retarder on the inside of the wall in NH. A 1" thickness of XPS is usually right at the borderline between class II and class III. If the spray foam is closed cell, the two foam layers combined will be class II. If the spray foam is open cell, you might be a touch short on the vapor retarder requirement. In that case, adding a rain screen will reduce the risk substantially by making it easier for the sheathing to dry out.
Most wall damage is caused by water leaking in from the exterior. Correctly flashed openings integrated with the WRB should handle that. A rain screen gap will make it harder for water to even reach the WRB.
Thanks for you comments FrankD
It is closed cell spray foam in the bays, about 2.5" sprayed directly onto the 1" xps board.
If you consider the spray foam bonded to the XPS, Is the first place it might condense where the outer face of the XPS meets the inner face of the plywood sheathing?
We are going to add ventilation at the outer face of plywood so it can dry outward.
I was trying to think through if it would be worth making a little drainage space between those 2 layers - maybe a rain screen there would be more useful but not sure if we have the space to give up.
But maybe the rain screen outside the plywood is good enough. Kind of stuck with the 2X6 wall/1" XPS now but may be replacing a lot or all of the plywood so trying not to miss anything easy at that point.
With that much closed cell foam, you've got a good vapor retarder, so there won't be much water vapor getting to the sheathing. Although people often talk about condensation, water vapor doesn't actually condense on wood. What really happens is that water vapor gets absorbed directly into the wood, so there won't be any liquid water to drain out.
What might help is to tape all the seams and edges of the sheathing to create a really effective air barrier. The spray foam helps with air sealing but it doesn't address leaks under the bottom plate, between the double top plates, between king and jack studs, etc.
This is exactly how I did my wall assembly in my house ...
2 X 4 wall, closed cell spray foam (2-3") & 2" EPS on the outside (so profile at windows was a 2X6)
The difference (besides 2X4 v 2X6 & 2" EPS v 1") was that I used metal let-in bracing, rather than plywood/CDX on the exterior (so skipped sheathing) & strapped the wall vertical for a rainscreen before attaching cladding.
begreener,
It seems to me like you got it right -
I see three things that should make your assembly better than my original one:
1. Leaving out the plywood so it can't get wet and rot,
2. Adding exterior insulation to further reduce chances of condensation.
3. Adding a rain screen in case there is any condensation.
Seems like each point works toward reducing/eliminating condensation.
The closed cell foam enhances the walls structural capabilities ...
Are you convinced the moisture is in the form of vapor from inside?
Are you sure it's not bulk moisture from the outside?
Since the spray foam is adhered to the XPS, it essentially acts as one solid layer. The two combined are plenty for condensation control and permeability is pretty low, I can't see moisture make it through that much to get to the sheathing.
If you have rotted plywood, the issue is with flashing or siding install.
Agreed!
I didn't really mention but I think moist air was getting through some imperfections in the wall system, getting by the foam layers.
It is a post and beam house, with these 2x6 based site built "panels" where some of the crucial points, like where the wall "panel" meets the roof panel is hidden behind posts at the top of the wall. Corners are hidden, etc.
From the pattern of the worst damage I believe some air was escaping from house there into the wall panel at the top, across a large part of this wall.
I have sealed the air leaks I could find with a blower door. I have installed an erv which is reducing winter humidity a lot. So I've improved the situation but I still need to rebuild the wall to repair the damaged plywood, and some framing.
So, when I rebuild, I want to do what I can to add insurance that if air does leak and condense, it will be handled by the rain screen.
Now, more in line with what you are saying, the 30 year old tyvek is in rough shape and I think it is pretty much doing nothing now. So, water may be entering through the siding, siding to trim connections, etc.
That will be addressed also with a new, high quality WRB, properly taped, flashed and otherwise detailed.
Now, begreener - you gave me another idea. Maybe in the rebuilt wall, I too can ditch the plywood! It is a 2x6 wall, 24" on center. If you remove the plywood, you have the 2x6 studs with spray foam and the 1" of styrofoam. What if we could figure out a way to brace it without plywood, then add some more thickness of styrofoam, then strap for rain screen, the siding and trim. I will have to learn more about the bracing! But you got me thinking - thanks!
tighterhouseNH,
From a moisture management perspective walls without sheathing, or with very permeable sheathing, can make a lot of sense, and there are alternatives to resist shear forces, but I think it's also important to acknowledge the other attributes sheathing brings to structures.
- Sheathing provides a solid continuous, robust boundary layer between the inside and outside against air, lateral and impact loads, pests, and even against intruders.
- Sheathing also makes framed walls into box beams, meaning they are much more resistant to deformation or failure if the load-bearing parts below fail or move over time.
- In some ways it can also be seen as a sacrificial layer which takes the brunt of enclosure failures and moisture damage so they are less likely to affect the structure inside.
I'm not saying you shouldn't explore eliminating it, just that a house that relies on bracing also loses some other attributes sheathing brings.
Edit: A second anonymous copy of my post has appeared below and can not be edited or deleted. The lack of tech support for GBA is becoming ridiculous.
tighterhouseNH,
From a moisture management perspective walls without sheathing, or with very permeable sheathing, can make a lot of sense, and there are alternatives to resist shear forces, but I think it's also important to acknowledge the other attributes sheathing brings to structures.
- Sheathing provides a solid continuous boundary layer between the inside and outside against air, lateral and impact loads, pests, and even against intruders.
- Sheathing also makes walls into box beams, meaning they are much more resistant to deformation or failure if the load-bearing parts below fail or move over time.
- In some ways it can also be seen as a sacrificial layer which takes the brunt of enclosure failures and moisture damage so they are less likely to affect the structure inside.
I'm not saying you shouldn't explore eliminating it, just that a house that relies on bracing also loses some other attribute sheathing brings.
Thanks for that Malcolm, I do understand that there are reasons why houses are built this way, and it may be impractical, as a retrofit with little access to the interior of the studs to make that change now.
One additional point to keep in mind however,
this is a post and beam house and the general concept of a strong frame inside the house, then lighter panels of insulation outside the house hanging off the frame, might come in to play here.
So it does have some advantages over a conventional house in that these panels are not supposed to provide much structure.
I will say that they do stop the frame from racking, there would not be much doing that without the panels.
Are there water resistant sheet products that could replace plywood?
Other options than plywood for the needed bracing?
As I said prior, the 2-3” of closed cell foam increases the structural strength a good amount !
tighterhouseNH,
"Are there water resistant sheet products that could replace plywood?”
We don’t typically select sheathing by its water resistance because like framing the assumption is that it is not a part of the wall that is there to resist bulk water intrusion or become wet. Walls should be designed so that the first and second layers of protection (the cladding, flashing, rain-screen gap, and WRB) do that work. If you sheathing is suffering water damage, the wall has failed.
The two main strategies used to avoid moisture damage from the interior are to a) Use a permeable material that allows that moisture to dry to the outside - preferably one like plywood that opens up and becomes more permeable when damp. Or b) Remove the first condensing surface (the sheathing) and substitute strapping or a sheet WRB instead.
Thanks Malcolm, what would be an example of a sheet WRB?
tighterhouseNH,
Tyvek Commercial would be my choice.
https://www.greenbuildingadvisor.com/article/building-walls-without-sheathing
Ok I was wondering if it was something rigid. Yes definitely will install a quality WRB.
Realistically, a new wall assembly with plywood would probably work fine if I made the improvements that I am planning on making — High-quality WRB properly detailed and rain screen. Maybe exterior insulation too. I appreciate all your useful comments!
Yes, the closed cell foam is helping, so that’s a plus.
Looking at the options, it might be that one by four diagonal bracing is the most practical alternative.
Even though we already have 1 inch of Styrofoam outside of the 2 x 6 wall, I can see some quick 1 3/4” cuts with a Skillsaw could facilitate adding bracing and then be filled in with matching strips of foam. Then new layers of foam could be added outside of this, then rain screen strapping, then siding. Hmm
Or could the 1x4 be exterior to the 2 x 6 studs instead of cut into them? In that case we would only cut out a strip of the 1 inch foam insulation to install the diagonal 1x4s.
In my house I used an EPS product from Dow called “Wallmate”
https://gcsupply.ca/product/dow-styrofoam-wallmate/
I installed horizontally across studs
Comes in 2’ x 8’ sheets - where the sheets butt together was grooved to accept 1”x 4” strapping (screwed to studs) so it all laid flat in same plane
I then covered with a wrb & strapped vertically to create a rain screen
BTW - this house was built on Lake Sunapee
Ha, I’m not far from you closer to Keene
So installing those horizontally would accept a one by four every 2 feet horizontally across the stud wall. I wonder if that would satisfy bracing requirements.
Another interesting possibility. If you wanted to install two layers of this, you could probably install the second layer with every other piece flip-flopped to interlock.
tighterhouseNH,
Unfortunately horizontal bracing does very little to resist shear. It needs to be diagonal.
Thanks, Malcolm, a lot of thinking out loud here.
Next thought would be you could install those insulation panels on the diagonal which would put the one by four braces on the diagonal too.
My house has a 12/12 pitch roof, so if you started installing at the top you’d be on a 45° diagonal.
That would result in a one by four every 2 feet on a 45° diagonal.
Maybe we’re getting somewhere with this idea?