Plywood sheathing as rainscreen
We’re building our house in North Idaho zone 6b (ish). We’ve noticed condensation issues on the plywood sheathing. We even made a test box with 6 mil poly, R30 rockwool (more than we have, but we still seem to have issues with R23), right on the sheathing. In a few days, it was a little damp, so through the winter would be a real problem. For the curious, R15 rockwool did the same thing without any cover, bare drywall, or plywood.
Our options seem to be adding a ton of insulation to the outside at this point, removing sheathing and doing alternative shear strategies, or… adding vertical girts on top of the horizontal girts (as rain standoffs) and using out sheathing as a rainscreen. We’ll put wood siding on top of the sheathing. Probably will use 2x4s for the vertical girts that will create air space and let moisture find its way to the ground, and should prevent vapor drive in the summer.
This is not an airconditioned structure. We use a wood stove. Part of the moisture issues are due to it being a daylight basement where the foundation contractor did not put a vapor barrier under the slab! We have a fix for this in the works. Currently, the humidity is around 60%, so it’s a good testbed for condensation issues. That said, our testing has made us think that there may be issues even in drier houses without damp slabs.
I have not heard of anyone turning sheathing into a rainscreen. I don’t see why it won’t work. It seems like the easiest option. Certainly, exterior insulation plus a rainscreen is preferable, but it’d be quite the expense. R23, especially with so little thermal bridging as a postframe, is plenty for us.
Hopefully someone can chime in and tell me if this is crazy or not. Hopefully someone else has already tried it?
On a stud wall, one could use 2x4s on the wide end mounted to the studs, to then mounting the sheathing to. You could put a WRB between the studs and 2x4s on the wide end as well, or use furring strips, but I feel a little more comfortable having a particularly solid connection to the sheathing.
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In your climate you want to have a vapor barrier or retarder on the interior and have a wall assembly that dries to the exterior. Just rockwool, sheathing and wood siding would work so long as there is some sort of barrier on the interior.
If there is moisture in the basement you don't want it drying into the house, only bad things can come from that. You want to block it at the source with a vapor barrier against the concrete.
Thanks for replying!
Right, we have poly on the inside. Just getting a surprising amount of condensation on the sheathing even with the poly. Should be about as much of a vapor barrier as possible. Probably a lot of that is the moisture coming up from the slab.
We will be putting down a dimpled plastic moisture membrane over the slab to act as a vapor barrier. Unfortunately, we didn't know how bad the situation would be until drywall was up (but not finished) in the basement. We'll use Stego tape to protect the bottom plates (already has Zip tape on the bottom), covering the sides.
I feel like having the sheathing spaced out would allow the condensation to drip down. We are already having issues with that condensation coming back into the window sills. We'll move the windows inward of the sheathing to prevent that. I guess we could do that either way, but I'm most confident with the rainscreen approach (at least so far.)
One reason is that the horizontal girts tend to catch the moisture and concentrate it. I think it doesn't help either, but shouldn't matter with vertical girts on top. If that makes sense?
That doesn't make sense.
In a finished house, the source of humidity should just be occupant behavior. If there are other sources -- rainwater leaking in, rising damp, groundwater -- they need to be stopped at the source. If you're seeing high humidity in an unoccupied house, you need to find the source of the humidity and control it.
The solution is not to make the walls better able to dry to the exterior. The solution is to find the source of the moisture.
IF you're still under construction, there might be a lot of moisture built in to the structure. The slab can certainly be a part of that, even if it had a vapor barrier under the slab. But there can also be a lot of moisture in the walls and other materials. It often takes a full year (sometimes more) to dry out all of that moisture and reach a more steady-state. That said, your condensation issue seems unusually bad. You mention 60% RH, but not the indoor temperature. If you are anywhere north of 60 degrees with 60% RH, then you have a TON of moisture in the house and you need to find out where it's coming from. Have you been heating with a gas-fired heater? DC is absolutely right - you need to address the source first.
Also, I'm not a fan of poly film anywhere south of interior Alaska. It seems to do a much better job of holding moisture in the wall than keeping it out. In your cold climate, you either need a bunch of exterior continuous insulation to keep the sheathing warm, or a really well sealed air and vapor retarder on the inside walls. Poly film can work, but the newer "smart" membranes are better. Regardless, these have to be detailed to be airtight, otherwise air moving through the walls can transport lots of moisture to condense on the inside of the sheathing and yes, that will cause significant damage over time.
No other moisture sources. Heating with wood. 60% humidity at 55-70 degrees (depending on the floor and how hot we let it get.) That's a good point that it may take a while longer to reach steady-state, but it sure seems high to me.
Would definitely prefer a smart membrane to poly. Not planning on using A/C and I feel like with an air channel, vapor drive from the siding hopefully won't happen.
I agree that exterior insulation is best, but is there any reason having the sheathing spaced out 1.5" more inches and acting like a rain screen wouldn't work?
With R23 on the inside, I think we'd need an extra R15 or so on the outside to be safe. Would be pretty expensive to do that.
I feel like addressing the sheathing condensation either way, even though it should be way less when the slab is sealed off. It seems like it may be an issue in the long run even if humidity levels come way down.
I agree that exterior insulation is best, but is there any reason having the sheathing spaced out 1.5" more inches and acting like a rain screen wouldn't work?
With R23 on the inside, I think we'd need an extra R15 or so on the outside to be safe. Would be pretty expensive to do that.
You're asking a rainscreen to do what it isn't meant to do.
To back up: an insulated assembly will be prone to condensation if it is vapor-permeable and the vapor can reach a cold surface. There are two ways to prevent this. The simplest is to make the assembly vapor-open to the cold side (the exterior in heating climates) and vapor-closed to the warm side. This is what you have.
The other way -- which you do if for some reason the cold side can't be vapor-open -- is to have enough vapor-impermeable insulation on the cold side that there isn't a surface that vapor can reach that's cold enough to allow condensation. That would be where continuous exterior insulation would come in. But that's not your situation, I'm only mentioning it for the sake of completeness. So it's really not true that "exterior insulation is best."
A rainscreen has nothing to do with this. The purpose of a rainscreen is to make the wall more resistant to intrusion by rainwater. That is a good thing. But it's not going to prevent condensation.
Let's back up a bit. You're observing moisture on your sheathing. How are you sure that it is condensation and not rainwater leakage? Where are you in construction? It sounds like you don't have siding on yet. Do you have insulation? Vapor retarder? Drywall?
I understand that this isn't the usual use of a rainscreen. It just seems like that for whatever condensation does form, if it can drop out of the structure without causing harm it's the best case scenario. While I think it may help the structure dry out from rain water, for some time it might function more as a "reverse rainscreen" to deal with excess condensation on the sheathing. I know it won't prevent it, it would just help mitigate it.
Yes, we are certain it's condensation and not rainwater leakage. We've noticed the condensation (as frost) for some time with the temperatures below freezing for a while. Then when it warmed up a little bit (and wasn't raining) when we spray foamed the windows we found lots of condensation on top of the windows that had migrated down from the inside of the sheathing. (The windows will have to be rehung inward if we do this sheathing thing, of course.)
No siding yet, planning to have rough cut siding. We are finishing drywall in the upstairs. We use 6 mil poly on the inside as a vapor and air barrier. R23 Rockwool batts in the wall cavities. R5 EPS foam around the outside of the basement wall and footer (daylight basement.)
We have a door opening cut in the top floor for a future porch door. The horizontal girts didn't have blocking around them, so there were some serious air leaks that were leading to condensation in the cavity. We blocked off the gaps between the girts (so it's just bare plywood) and ran tests directly on the sheathing. For a while, we weren't certain if the air gap between the exterior girts and the sheathing was causing some issues. One of the tests we did was with R30 to simulate if we had R23 batts up against R6 Comfortboard (R29, close enough) to get rid of the air gap. After just a few days, it was moist against the plywood and back side of the Rockwool.
I also tested with R15 (Rockwool) bare, with bare drywall, and bare plywood on top (interior side.) All three sections were damp in my test jig, the bare one being the worst as one might expect. We were hoping that with poly it might block enough moisture, but it seems not. Perhaps if it was perfectly sealed it might, but I don't want to be reliant on perfectly sealed plastic to ensure the integrity of the house.
Our temperatures have mostly been below freezing. We've had no rain for a while. We had temperatures in the negatives for a while. Don't see any way that it could be from rain water. We've inspected the sheathing in other areas and found frost (it was well below freezing then) in mild amounts all over it. The testing we've done indicates that this isn't just because of the air gap around the sheathing (which is a horizontal gap,) and doesn't seem like something we can prevent entirely if things are as they are. I think the damp slab greatly accelerates it and we are waiting on materials to seal over the slab, but I suspect that some condensation will accumulate on the plywood sheathing over a full winter even if that is addressed, given how rapid we have seen it form under these conditions.
You say "girts." Is this a pole building?
You can temporarily seal the slab with poly from a big box store.
How is the condensation going to drain out from the interior side of the sheathing without essentially moving it outside of the air and water barrier layer?
Seems like you are suggesting venting the interior side of the sheathing. In which case you would need to install some type of cold side air barrier or aren't you introducing lots of cold air into your wall assembly? And wouldn't you need to pull the already installed sheathing off to do that?
itchy,
"60% humidity at 55-70 degrees “
That may explain a lot of the problems you are having. With temperature swings that large, the dew point of the interior air will likewise be swinging wildly. 60% RH air at 70F can be expected to condense on surfaces when cooled to 55F.
I agree with DC. You have a very common wall assembly which shouldn’t experience condensation problems. The solution is to figure out what’s wrong, not to try and invent a solution no one else with the same wall has had to do.
Malcolm --
" 60% RH air at 70F can be expected to condense on surfaces when cooled to 55F."
That would show up as condensation inside the building envelope. Presumably there is less humidity inside the wall assembly, presuming the vapor barrier is doing its job. I mean, that's the whole point of the vapor barrier.
DC,
I'm really unclear on both what the wall assembly is, and whether it is complete with insuation and interior VB / air-sealing, or just the test box has those things.
Yeah, when he said it had girts alarm bells went off in my head. That means pole building to me, which means it could be anything.
I'm wondering if there's a bottom plate to the walls, or anything to keep moisture from rising out of the slab into the walls.
[Reply to #14]
How is the condensation going to drain out from the interior side of the sheathing without essentially moving it outside of the air and water barrier layer?
The idea is to keep the condensation from happening in the first place by having a vapor barrier or retarder on the interior to keep moisture out of the wall.
To me, trying to get the condensation out of the wall sounds like waterproofing your attic when the roof is leaking. Why not instead fix the roof and stop the moisture at its source?
Very interesting thought about moisture coming up into the walls from the slab! In this case, stem wall (light side of the basement) and basement wall. It's not sealed there, so adding poly there is a great idea.
However, the condensation appears to be more the higher up the wall you go, further from the concrete. Also the tests we've conducted were isolated from that.
I should probably add some pictures. But yes, it's a pole building. For now I'll try to describe it.
Posts 8' on center. Dry set brackets on the stem wall (bright side of the basement) and basement wall for the rest.
From inside of the wall to the outside:
Unfinished drywall
Interior horizontal girts
6 mil poly
R23 Rockwool
Exterior horizontal girts
Plywood sheathing
Tyvek WRB
I agree that moving the WRB to under the horizontal girts is a good idea, especially if proceeding with the rainscreen sheathing idea with a deliberate air gap.
The membrane that'll go down is on its way to the store. It sounds like it works better to have the air pockets, or would there be no issue just covering the slab with poly?
I agree that moving the WRB to under the horizontal girts is a good idea, especially if proceeding with the rainscreen sheathing idea with a deliberate air gap.
The point of the WRB is to protect the sheathing. If you put it inboard of the sheathing it can't do its job.
The membrane that'll go down is on its way to the store. It sounds like it works better to have the air pockets, or would there be no issue just covering the slab with poly?
If it's a Schluter type membrane, it's meant to be mortared down, the dimples are there to give more surface area so the mortar adheres better. Dricore type barriers are a scam.
Would this include Delta FL "being a scam?" Dricore makes some that have OSB on top. This seems like it should be a tight seal.
Joe (I'd have to look up how to spell his last name) recommends the membrane, albeit he talks about mortaring it down. I saw lots of promising reviews about Delta FL used with subfloor and then another floor on top. Not familiar with Schluter.
Are you suggesting that we could just put down poly (with taped seams, of course) and then another flooring on top to protect the poly?
I'm hoping that the siding will protect the sheathing, although if the siding is directly on the sheathing, it will get damp as well at some point.
The assembly seems fine in terms of moisture management.
It should have plates at the tops and bottoms. At the bottom there should be a capillary break between the stem wall and the wall. The plates should meet the bottom girts and be attached to them. Without plates the wall isn't fire-safe. (It probably also needs horizontal fire-blocking.) Without the plates and the capillary break there's nothing to keep damp from rising into the wall out of the stem wall. I don't see how you air-seal or vapor-seal the assembly without plates. And I don't see how you keep rodents out of the insulation.
What purpose would the top plate serve? I can understand the bottom plate (or just some poly) to retard vapor coming up from the stem wall.
Could you elaborate about the purpose of fire blocking and the top/bottom plates for fires? I don't see how the Rockwool would light on fire very easily. The posts could burn, as could the horizontal girts.
The poly is generally attached over the wall. It's not taped or attached at the bottom, it's loose so some air does get through. We're not going for completely air tight -- we aren't running any kind of central air or mechanical ventilation. I know this is less efficient, but we are going for something closer to off grid. We're able to run the wood stove without an outside air intake, which is nice (although one would probably be preferable, just difficult on the dark side of the basement.)
On the outside we will have siding and measures to prevent rodents. On the basement wall sections, we also have another horizontal girt below the wall, attached to the concrete. We will put Comfortboard on those sections, enoough to cover the exposed concrete wall and the bottom of the wall assembly, limiting thermal bridging through the bottom of the wall gap and the dry set brackets holding the posts.
"Could you elaborate about the purpose of fire blocking and the top/bottom plates for fires? "
It's a complicated subject, you should google for a complete explanation. But basically if you have an enclosed assembly that is not air tight it will act like a chimney in a fire and allow fire to spread quickly through the building. So if a fire starts in an open sidewall like you have, it will quickly spread up the wall and emerge at the top and catch the roof on fire.
Modern fire codes require that assemblies that are enclosed be air tight on all sides. They also limit the size of enclosed areas to 4' vertically and 10' horizontally so that if there is a flaw in the sealing the risk is compartmentalized.
I live in the city, they take fire safety very seriously here, any opening in a wall like a pipe or a wire has to be sealed with fire blocking foam or caulk. In addition to improving fire safety, this makes for assemblies that are better insulated, air sealed and soundproofed.
This is good to know, thank you! Definitely something to keep in mind.
It does sound like one weakness with my rainscreened sheathing idea is a lack of fire blocking, that there's a risk of fire coming up in that gap. May be an issue with other rainscreened sidings as well.
Just to be clear, I don't see the fire danger in the hypothetical rain screen, but in the wall assembly between the drywall and the sheathing.
And I wish you would stop being so fixated on the rain screen. It's not going to solve the problem of excess humidity in the wall, you already have enough drying ability.
[Reply to #14]
Yes, would have to pull the sheathing to do this. Would ideally move the WRB to under the exterior girts, or so I think. Might that cause an issue? Right now the WRB is on top of the sheathing, which is on top of the exterior (horizontal) girts.
I'm thinking about adding vertical girts on top of the horizontal girts, and moving the sheathing on top of that. WRB could go under the horizontal girts at the same time, if that seems wise.
My understanding is that moisture is not supposed to condense on a WRB, but I could be wrong. Hoping it'll allow the vapor to pass through, limit air transfer to some degree, and help keep rain out (although rain should be blocked by the sheathing and especially siding once added.)
Itchy,
The WRB’s primary function is to protect the wall from bulk water intrusion coming from the exterior. It has to stay where it is outside the sheathing. If you add the inverted rain-screen you are suggesting you will need another air and vapour retarder inside the gap.
You are seeing more moisture higher on the walls not because the source is there, but because humid air rises. That’s why you most commonly see moisture damage near the peak of roofs, not the eaves.
Again, although the assembly is a bit different than standard walls, there doesn’t appear to be anything unique about it that would cause the condensation you see. The source is elsewhere, and there is no building science reason to install this new inverted rain-screen you have invented.
I guess you get some condensation on the WRB, it is at the end a cold surface. For me the "how much" is interesting because some will evaporate to the ambient at the same time - that classic balance of wetting and drying what Mr. Joe talks about in assemblies. At very low temperatures not a lot will evaporate however, specially because the WRB is in the shadow and will not see any sun light to warm up.
You can argue that the wetting of the plywood in itself is not a big deal because the wood will (at these low temperatures) not get damaged - no mold grow. When it gets warmer it has to be dry. Question is how you can handle the condensate on the WRB when it wants to run off - where is that water to go without damaging the internal structure..
wastl,
When we talk about condensation on the WRB what are we really saying? The first condensing surface is the sheathing. That’s where the moisture will be accumulating, and it will be diffusing outward to dry to the exterior. Most WRB’s will be in the range of 20 to 70 perms. The plywood when damp is in the range of 10. So no interior moisture should ever be able to accumulate on the interior face of the WRB - and I don’t see how you would ever know it was. How do you see it? Right now the WRB is acting as the cladding. It’s more likely any moisture on it comes from the outside.
My feeling is that there is either:
- No problem at all with the walls beyond typical construction moisture.
- There is excessive moisture coming from the slab, exacerbated by widely fluctuating interior temperature.
- There is something else at play that so far we haven't been able to identify from the description of what’s there.