WRB type with exterior insulation
I am re-siding a house in Salt Lake County, UT (Zone 5B) and adding some exterior insulation. I was thinking of fully re-sheathing it with Zip-R but for cost reasons I am reconsidering that approach. The house is currently sheathed with 5/8″ OSB, 16 in OC 2×4 studs.
The new process I’m thinking of is:
1. Air seal the bottom plate and end of the sheathing to the foundation with Zip liquid flash.
2. Install a peel and stick WRB, possibly Vycor-enV-S.
3. Install 2 layers of 1″ EPS, lapping the seams. Possibly taping the outer layer seams?
4. Either install vertical strapping with screened vents at the bottom, then install horizontal (since the siding will be vertical), or 1×3 strapping at 45 degree angle.
5. Install siding.
My two main questions are:
1. Should I use a vapor open WRB like enV-S, or should I use a less permeable option?
2. Are there any glaring issues with the system that I’m missing?
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
In terms of vapor permeance, the product you mentioned was measured at 15 perms, which is around the 'goldilocks' zone of 10-15 perms that is generally recommended. Supposedly you might be able to optimize this for specific climate zones and internal loads, but a middle of the road perm value is generally safe in most climate zones. More permeable is not always better (e.g. 50 perms in a humid environment or with reservoir cladding), but the minimum of 10 perms is usually necessary.
I would double check with the manufacturers to see what order you would put the WRB or foundation liquid flashing on. I'm not too sure what order is 'best', but they might know which stuff is more likely to stick in what order. My gut says it will be easier to put the liquid flashing on top of the WRB than vice versa, because applying self adhered membrane to a textured surface doesn't seem like fun.
Since the self adhered membrane is presumably your air control layer on the sheathing, I don't think it's necessary to tape the foam joints but someone else might chime in here.
If you are going to the trouble of installing two layers of 1" foam, why not 2 x 2-4"? Or however thick it can get before it gets too tedious to land screws into studs.
Lastly there are many options for furring strips, so if you can't find 1x3s for some reason, others have ripped strips of plywood, and there are also plastic products with ventilation/drainage holes that could let you skip a layer of furring.
Andy, thanks for the detailed reply!
I was looking at liquid flash because its easy to get, but it looks like I can get Vycor PRO Flashing tape which is also rated for concrete. It seems like it might make sense to use that instead to avoid figuring out if zip liquid flash is compatible with enV-S.
I would definitely prefer a thicker layer of EPS, but because the overhangs are pretty small I would start to run into issues with the extra thickness.
Are you referring to a product like Benjamin Obdyke Slicker? So I could just install that, and then horizontal furring directly on top?
For tape flashing on concrete I would probably test a strip of the tape somewhere, or generally investigate the application, even if it is rated. I have not seen a lot of examples of tape onto concrete, and I don't know why. It could be that tape works fine, and their marketing game is just not as aggressive as liquid flash.
But I think liquid flashing might be more popular due to the 3D nature of the seam, i.e. the concrete isn't perfectly smooth, so the sheathing doesn't lay perfectly against it, the sheathing isn't perfectly flat, rolling/pressure-activating the tape against a relatively more complex geometry than a sheathing butt joint, etc.
Seems like tape could do the job, but with potentially more hassle and required attention to detail.
RE: furring, no I think the brand is called Cor-A-Vent or something. It's basically plastic battens with drainage/ventilation holes. You could probably use that Obdyke rainscreen mat as well, since you might not be putting thick enough foam down to require the support battens provide for siding.