Wall Assembly in Climate Zone 5 with stucco
EJDesigns
| Posted in Energy Efficiency and Durability on
I’m building an addition in Wisconsin and eventually will be retrofitting the rest of the house. My wall assembly is as follows: 2×4 walls with R15 mineral wool batts, zip sheathing, 3” XPS foam. For the stucco layer I was originally planning on installing 1/2” CDX with 6” structural screws over the foam, 1 layer tarpaper, hydro gap, lath and 3 part stucco. My question, is the CDX directly on top of the XPS ok? Or should I add an air gap like 1/2” or 3/4” furring?
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Replies
Eric,
If your water management details and installation at the tar paper layer are great, then yes I think the CDX directly on XPS would be fine. I'm not familiar with using HydroGap for stucco though. I see that their product data recommends a second WRB layer on the outside before the lath and scratch coat. seems like a lot of layers...
On a past stucco project, we used integrated furring/XPS (Certistud) over the first layer of foam and then a drainage mat with integrated filter fabric (MTI Surecavity) which helped reduce some weight, depth and trips around the building. There are likely other products that work in a similar fashion.
Most folks here will advise you against XPS for its high GWP blowing agents. If EPS or polyiso can meet your project goals, explore that route.
Best,
j
Jason,
Thanks for your response. You’re correct, I had the Hydrogap and WRB reversed. I know XPS Is frowned upon for the reason you listed. For budget and wall thickness reasons I went that route. If money wasn’t an option rockwool comfort board would’ve been my first choice. My main concern was condensation between the plywood and outside of the foam layer. Technically the condensation point should be in the middle of the foam but I am by no means an expert.
Eric,
Having the condensation point inside the foam is exactly where you want it. Especially with XPS, which is a vapor retarder itself, any humid air flowing through the assembly can't get to the layer of foam where it could potentially condense. This keeps the plywood dry. The cold outside surface doesn't see any interior moisture because it can't get through the foam. This is a very durable wall system. Just make sure to pay attention to detailing the Hydrogap and WRB properly to prevent bulk water from getting to the plywood.
One quibble: The IRC does not allow stucco lath to be fastened to plywood. The fasteners are supposed to penetrate to the studs. I call this a quibble because most of the stucco installations I see are fastened to the plywood and not the studs with little/no problems. It can allow more flexing/cracking of the stucco though.
Using synthetic stucco would solve this problem and probably save money. It can be applied over EPS or XPS foam directly. In this stackup, your Hydrogap and WRB would go between the foam and ZIP sheathing. In fact, with ZIP sheathing, I would use a single layer of something like Barricade Plus Drainage. The surface finish is slightly less durable than cement stucco, but overall lifecycle maintenance is similar. Again, attention to details is key.
In my area 3 coat stucco has pretty much completely disappeared in favor of EIFS. It can also save a bit on install cost as the exterior foam is part of the stucco costs. Normally it goes over 1" of EPS, but the upcharge for thicker foam is not that much.
In your case, it would be a much simpler install as you reduce the number of layers significantly. The key items with EIFS is well detailed WRB and window flashing on top of your sheathing with grooved foam for the stucco.
There is also more heavy duty impact resistant version you can get for areas such as walkways or carports.
Thanks Peter, good point about the lath fastening. I could definitely throw some longer fasteners in to the studs for added strength.