More Best Practices
One of the challenges with continuous insulation (CI) is how to attach cladding. The cladding type and weight as well as the type and thickness of the insulation will determine the attachment details. In this post, I will identify related sections in the International Residential Code (IRC) and discuss how cladding and insulation manufacturers can help with prescriptive code compliance.
What the codes say
The IRC requirements for cladding attachment are found in Chapter 7, Section R703 Exterior Coverings. Section R703.15 “Cladding attachment over foam sheathing to wood framing” states:
“Cladding shall be specified and installed in accordance with Section R703, the cladding manufacturer’s approved instructions, including any limitations for use over foam plastic sheathing, or an approved design. In addition, the cladding or furring attachments through foam sheathing to framing shall meet or exceed the minimum fastening requirements of Section R703.15.1, Section R703.15.2, or an approved design for support of cladding weight.”
This section includes Table R703.15.1 showing fastener size and spacing requirements for foam thicknesses, based on the weight of the cladding when fastening directly through the insulation. A second table, R703.15.2, shows requirements when using wood furring or strapping for cladding support. Both tables show the maximum insulation thickness based on the cladding weight, and fastener size and spacing. (Note that this information comes from the 2021 IRC; local codes may differ.)
My big takeaway from Section R703 is this: If you are installing a lightweight cladding by fastening it through thinner exterior insulation, the code requirements are clear. When you start getting into heavier cladding or thicker CI, you may need some help from either the cladding or the insulation manufacturer, or from an engineer—especially if high-wind loading of the cladding also needs to be taken into consideration.
Cladding weights
Before we get into some specific…
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7 Comments
Great post , Randy
I have to wonder, at the end of the day does all of the extra labor and materials for exterior insulation pay off. For me in MN I will build a double wall without hesitation. Straight forward, uses standard materials and common building techniques. Too much headache with exterior insulation including windows and exterior drainage plane in general. Hope summer is treating you well !
Doug
Thanks Doug!
I agree with Michael, for new construction, it's about the builder's preference. I can see why some would want to build double wall, and some use CI. Existing homes though, this is where I think CI has the greatest potential, with the possibility of messing a home up. Minnesota hasn't yet adopted the CI requirements, but I'm seeing a few projects in my community where it is being used. Good to see, hopefully the right assemblies and products are being chosen.
Good info, Randy. I find that the preference for double stud walls vs. exterior insulation is highly dependent on the contractor. Some say double stud walls are twice the work and that exterior insulation is easy; others say that double stud is hardly any extra work and prefer aspects like traditional window details, and that exterior insulation is a lot more work.
I have used both approaches and find them pretty equivalent. Exterior insulation is really just two additional passes around the exterior, and a bit more fiddling around openings.
Hi Randy, thank you for the information! I'm about to start installing Royal Celect cellular composite lap siding over continuous rigid foam insulation. I'd like to clarify your statement that LP SmartSide lap siding specs southern pine with specific gravity of less than 0.55 -- I believe that should read greater than or equal to 0.55 specific gravity.
Hi Alan. Yes, you are correct, I will contact the editor and have her make the change. Thanks for catching my mistake.
We are building in Canada very close to Port Angeles, planning for 3 inches of Comfortboard 80 exterior insulation and 1x4 furring strips screwed through to the studs. I want stainless steel screws, because they have dramatically less thermal bridging than regular steel screws., but finding them is a challenge. Small Planet Supply has discontinues their.
Any suggestions on a source for 6-7 inch stainless steel screws with a nice broad head?
Stephen,
You may find this discussion useful: https://www.greenbuildingadvisor.com/question/do-screws-through-exterior-insulation-reduce-the-wall-r-value
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