Envelope construction in VA with stucco
Was considering ICF, with traditional stucco, but cost to performance ratio in VA might not be ideal. Now considering 2×6 advanced framing, e.g. http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies/high-r-wall-02-advanced-frame-wall-construction?topic=resources/high-r-value
With this approach, we are considering fiberglass in walls, and 1″ Dow SIS exterior. Still doing three-course stucco outside.
I’d really like to increase the exterior insulation, but not sure how difficult that is considering the stucco, and needing attachment points for the wire mesh/basecoat.
I’ve seen very similar questions answered for NC, but VA is colder and I’d really appreciate any feedback!
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Replies
Roger,
You can definitely install traditional stucco over 2 in., 3 in. or 4 in. of rigid foam. Details can be found here:
To Install Stucco Right, Include an Air Gap.
From the article:
"If you’re installing stucco over foam, here’s how to create a ventilated rainscreen gap:
* Install 1x3 or 1x4 vertical strapping on top of the foam, screwed through the foam into the studs.
* Fasten paper-backed metal lath to the strapping, and proceed with a standard installation of three-coat stucco.
If your local building inspector raises questions about the number and size of the screws you plan to use to fasten your 1x4 strapping, you may need to get an engineer to review your details. As long as you have an engineer’s stamp, your local building department should be happy."
For more information on attaching furring strips through foam, check out:
Fastening Furring Strips to a Foam-Sheathed Wall.
Rodger,
The perm rating of Structural Insulated Sheathing (SIS) is a bit scary.
From Dow's website:
As tested per ASTM E96, Procedure A, the perm rating of STYROFOAM™ SIS™Brand Structural Insulated Sheathing is approximately 0.03 perms.
@Martin: Thanks for your answer! I had already reviewed the 'To Install Stucco Right...' article, somehow missed that paragraph. Also, appreciate the link to fastening furring strips.
@Brett: I also just found that the cost of Dow SIS is a bit scary too. Since I'd prefer to go with > 2" of exterior insulation, do you have a recommendation about which type of rigid foam to use? I'd really like to achieve > R30, but stay fairly cost conscious.
@Martin: My stucco guy is concerned about attaching wire mesh to 24" OC furring strips (since exterior framing is planned to be 2x6 @ 24" OC); they are too far apart. I tend to agree! He usually does attachment points at least every 12". How is this usually handled?
Roger,
I'm not sure what others have done. But it shouldn't be that hard to install furring strips 12-in. on center if that's what he wants. Just money and time.
Roger, install the strapping horizontally. Not as good for drainage, but still effective as a capillary break.
The gasketed non-metallic spider lath we use behind stucco requires 6" on center each way fastening schedule. that's what I usually see on wire lathe as well. keeps the lath from bouncing all over the place when you're applying the base coat. If you don't want to use OSB you could use plywood or planks. just remember that stucco is a system, it needs weep screed and expansion joints and at least two layers of tar paper under it on top of your taped house wrap or foam. (it wrinkles when the wet base coat hits it and creates an air space in front of the substrate)
if you want to do foam under the lathe you just need longer staples or screws. I know we can get 16 gauge galvanized 1" crown roofing staples in 1 3/4" length and i also keep the 1 3/4" X 1/2" crown galvanized sheathing staples but they don't seem as rugged as the 1" crown so I'd stick with the wider ones fro side wall over 1" foam.
ABC supply has some awesome deals on screws and they sell the R-12 Hunter-panel 2" polyiso at $26 per 4x8 sheet. it's nasty stuff to work with and you have to water proof it because it's intended as a rubber roofing substrate but a great value if you can get the details right. Just cause you don't like what you see at Home Depot doesn't mean that you can't find a way to giterdunn if you don't mind digging a little.
I found that my question had already been asked here: https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/17635/stucco-lath-over-4-foam
The New Mexico related answer seems sensible, which is to say: you don't put traditional/three-course stucco over 4" of exterior foam. My hunch is the NM code is logical, since hanging lots of weight on protruding screws is a bit concerning.
I'm considering double-wall construction, to get the R value, plus have a more traditional exterior sheathing. Thing is, I really like the concept of 'exsulation', since it solves many of the thermal bridging issues.
Does anyone have experience with Dow SIS? They claim it can take a three-coat stucco finish. Does it require any further strengthening in order for wire mesh attachment? This might give me a little exterior insulation, for thermal breaks, but without the stucco hanging so far from the sheathing.
Also, -- dragging on a bit, sorry! -- would it be advisable to do: Sheetrock | BIB | closed-cell spray | Dow SIS | stucco layers? Would in-wall moisture/mold be a problem if the stucco application is done correctly, ie. is this set of layers a no-no?
Thanks for the input, it's much appreciated!