I have a high performance thermal design exterior wall requirement, and a conventional 6" metal stud wall with a panelized rainscreen system. I’m looking for options to add rigid insulation to the exterior, and one of the contractor’s requests is to to omit the gyp sheathing cost in lieu of the added rigid insulation.
I might be able to simply do that (omit the sheathing), but I haven’t resolved that with an appropriate air barrier membrane yet. What I was thinking instead, is to use an insulated gyp sheathing board, sort of like an insulated structural concrete panel.
A quick search found products from China and listings on Alibaba, but nothing conventionally available and produced locally. Any suggestions?
Isn’t the sheathing required to cross brace the studs? Or stablize the flanges? Or some other structural thing that this dumb architect doesn’t recall anymore?
Perhaps one of the structural engineers (Mark G?) can refresh my memory and explain…
Nathan, be careful with the foam exterior. Make sure that it is tested with the NFPA 285 test. Make sure that the sheathing is tested as an assembly. I have heard that Thermax is the only one that has passed the test. I also have heard that metal panel is the system that fails the most.
Whoops, I think that Thermax Sheathing has only passed the NFPA 285 test in a masonry wall assembly. Thermax (ci) Exterior Insulation has passed it in lots of assemblies, but I’m not sure you could use it instead of sheathing.
I have clients who use Thermax (ci) as the exterior insulated sheathing & air barrier system, attached directly to exterior stud framing & with sprayed-foam insulation in the stud cavity. The sheathing panels are available in multiple thicknesses, have a metal facing & joint treatment accessories to create an air barrier, and the entire assembly (including spray insulation) are marketed & warranted by a single entity (Dow).
Ownen Corning has a new thing, I believe to compete with the Dow Thermax system. It’s the Owens Corning CommercialComplete Wall System. It has been tested (NFPA 285) as part of a 1 " thick closed joint terra cotta rainscreen wall assembly, but not a metal panel rainscreen wall assembly. What is your rainscreen veneer material?
Jeff, you are correct. Dow does have the system. They are the ones that do and others may not. You will have to verify with the others if they have tested it. BTW, that means the contractor cannot substitute with something else. IBC 2603.5.5 hits the exterior use of foam quite hard. The material that you talk about is a composite material and by introducing foam as the insulation may bring into question the NFPA test.
Does not the building code require gypsum sheathing (reference is California Building Code, based on International Building Code)?
2603.4 Thermal barrier.
Except as provided for in Sections 2603.4.1 and 2603.9, foam plastic shall be separated from the interior of a building by an approved thermal barrier of 1/2-inch (12.7 mm) gypsum wallboard or equivalent thermal barrier material that will limit the average temperature rise of the unexposed surface to not more than 250F (120C) after 15 minutes of fire exposure, complying with the standard time-temperature curve of ASTM E 119 or UL 263. The thermal barrier shall be installed in such a manner that it will remain in place for 15 minutes based on FM 4880, UL 1040, NFPA 286 or UL 1715. Combustible concealed spaces shall comply with Section 717.
I do believe interior gyp board qualifies as the code-req’d thermal barrier separating foam plastic insulation from bldg interior, so exterior gyp sheathing is not specifically req’d by code.
Presuming the interior gypsum board can be installed to provide a continuous separation. Depending on the detailing and construction sequence that’s not as easy as it sounds.
Thank you John, John, Jeff and Jordan I had not factored that portion of the code into my exterior wall design.
Currently, our proposed air barrier membrane is Henry’s AirBloc 33MR, and they have confirmed that they can direct apply to rigid foam, and are UV stable in direct exposure conditions. I am meeting next week with the lead engineer for SwissPearl to resolve mounting methods in this type of assembly. NetZero buildings are so much fun!
The issue, I believe, is not separation between the foam plasic (EIFS) and the occupied interior but protection of the structure, particularly wood framing, from fire originating at the exterior.
I don’t believe that gypsum board provides “lateral bracing” value … at least, no longer does it. Evaluations after the 1994 Northridge earthquake showed that the seismic movement elongated fastener holes through the gypsum board, reducing the value (if any at all) to resist lateral movement. Lateral bracing value is only good until the first good shake. Perhaps this is just one of those wacko California code requirements; earthquakes and other significant lateral loads don’t happen east of the San Andreas Fault.
The bottom line, I guess, is what is required by the applicable ICC Evaluation Report for the exterior finish system/assembly.
Gypsum board is inappropriate to resist earthquake loads since its strength deteriorates so fast.
Significant earthquake loads do occur east of the San Andreas Fault. There are some parts of the country where seismic loading is a significant concern.
Wind can be a significant lateral load.
Compliance with ICC Evaluation Reports is not a code requirement although many people accept them. ICC Evaluation Reports are typicaly written by the manufacture and reviewed by ICC staff. With probably rare exceptions the manufacturer provides the test results to ICC staff thus raising concerns whether the reviewers have all the data.
The Architect’s Standard of Care is not established by compliance with an ICC Evaluation Report.
To all, we have solved the challenge in my particular application by using semi-rigid mineral wool insulation (Thermafiber), located behind the outer layer of gyp sheathing. We accomplished this by using 2.5" continuous horizontal Z-clips across the face of studs. The Thermafiber occupies the cavity of the z-clips, and the outer sheathing is attached directly to the z-clips. The rainscreen system and membrane is hung outboard of the sheathing.
The trick was solving the thermal bridge between the z-clip and the metal studs, in an inexpensive manner. We did not want to add another layer of sheathing to the assembly! After prodding him rigorously, our energy consultant has calculated that we can use a “premanufactured co-polymer resinous thermal decoupler”, or what might otherwise be called, a 1/4" plastic shim
I had not factored that portion of the code into my exterior wall design.