Sheathing for Exterior Insulation and Finish System (EIFS)
I am designing a house in climate zone 4a (Virginia) and my client wants to use EIFS. This is my first time working with EIFS, so I have read a number of GBA articles about it, but I have a question about what is the best sheathing to use.
My EIFS contractor likes to install EIFS over top of DensGlass Fiber-faced gypsum. He creates a drainage gap by applying adhesive over the liquid-applies WRB, using a notched trowel to create vertical grooves behind the rigid insulation.
Since I typically would use Zip sheathing, I contacted Huber Engineered Wood about the glued-on approach in conjunction with Zip. Huber said that such an installation would void the warranty over Zip sheathing. They recommend instead installing a drainage mat and mechanically fastening the rigid insulation through this drainage mat.
I would assume that, if done correctly, either of these assemblies would work well. Does anyone have any experience or counsel regarding either of these approaches? Thanks.
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Replies
The FG-faced gypsum is probably more rugged than ZIP. Your EIFS contractor's approach is one of the best, assuming that the fluid-applied WRB is done well, and that they don't screw up with the ribbons of adhesive going sideways instead of up and down. Drainage details are mandatory at the base of the system, above windows and doors, and any other horizontal terminations of the system. That said, a good ZIP installation with taped seams, a drainage mat, and mechanically fastened insulation is also a good and rugged EIF system. With EIFS, as in everything, the devil is in the details. Once you and your contractor settle on a system, you should go online and download the EIFS manufacturer's installation details, learn them, and make sure your contractor uses them.
From what I'm seeing around me, fluid applied WRB over gypsum sheathing seems to be the standard in new suburban builds. I'm guessing mostly because of cost, it also helps that it doesn't have the moisture issues of OSB.
One way to bulletproof the install is to spec grooved EPS such as DuRock Puccs. Since the drainage channel is formed right into the foam, the installer can't mess it up.
The nice part about EIFS, compared to installation cost, the cost for extra foam thickness is pretty minimal. By specing thicker foam you can get a pretty high R value assembly for very little additional cost.
As Peter said, the devil is in the details. Don't let the installer skimp on any.
Peter and Akos, thanks for the helpful tips. I'll definitely read the instructions and make sure the contractor is following them!