This post originally appeared on the Green Building Advisor website.
One of the challenges with continuous insulation (CI) is how to attach cladding. The cladding type and weight along with the type and thickness of the continuous insulation can affect the cladding attachment details. This blog post will go over a few of the code requirements and discuss how the manufacturers of both the CI and cladding can help with prescriptive code compliance.
The Codes
The International Residential Code requirements for cladding attachment are found in Chapter 7, Wall Coverings, specifically in R703, Exterior Coverings (this is also the section that covers water-resistant barriers). 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.”
Included in this section of code are a couple of tables. One table shows fastener size and spacing requirements for foam thicknesses, based on the weight of the cladding when fastening directly through the insulation (Table R703.15.1). The second table shows requirements when using wood furring or strapping for cladding support (Table R703.15.2. You can view the actual code references at: 2021 International Residential Code (IRC) – CHAPTER 7 (iccsafe.org). Be aware, this code reference is from the 2021 IRC, your local codes may differ.
TABLE R703.15.1
CLADDING MINUM FASTENING REQUIREMENTS FOR DIRECT ATTACHMENT OVER FOAM PLASTIC SHEATHING TO SUPPPORT CLADDING WEIGHT (a)
DR = Design Required
o.c. = On Center
(a) Wood framing shall be spruce-pine-fir or any wood species with a specific gravity of 0.42 or greater in accordance with AWC NDS.
(b) The thickness of wood structural panels complying with the specific gravity requirement of note (a) shall be permitted to be included in satisfying the minimum penetration into framing. For cladding connections to wood structural panels, refer to Table R703.3.3. For brick veneer tie connections to wood structural panels, Refer to Table R703.8.4(2)
(c) Nail fasteners shall comply with ASTM F1667, except nail length shall be permitted to exceed ASTM F1667 standard lengths
(d) Foam sheathing shall have a minimum compressive strength of 15 psi in accordance with ASTM C578 or ASTM C1289
TABLE R703.15.2
FURRING MINIMUM FASTENING REQUIREMENTS FOR APPLICATION OVER FOAM PLASTIC SHEATHING TO SUPPPORT CLADDING WEIGHT (a) (b)
DR = Design Required
o.c. = On Center
(a) Wood framing and furring shall be spruce-pine-fir or any wood species with a specific gravity of 0.42 or greater in accordance with AWC NDS.
(b) Nail fasteners shall comply with ASTM F1667, except nail length shall be permitted to exceed ASTM F1667 standard lengths
(c) The thickness of wood structural panels complying with the specific gravity requirements of Note (a) shall be permitted to be included in satisfying the minimum required penetration into framing.
(d) Where the required cladding fastener penetration into wood materials exceed ¾ inch and is not more than 1 ½ inches, a minimum 2x wood furring or an approved design shall be used.
(e) Foam sheathing shall have a minimum compressive strength of 15 psi in accordance with ASTM C578 or ASTM C1289
(f) Furring shall be spaced not more than 24 inches on center in a vertical or horizontal orientation, furring shall be located over wall studs and attached with the required fastener spacing. In a horizontal orientation, the indicated 8-inch and 12-inch spacing in furring shall be achieved by use of two fasteners into studs at 16 inches and 24 inches on center, respectively.
Both tables show the maximum insulation thickness based on the cladding weight and fastener size and spacing. My takeaway from this section of code is if you are using a light-weight cladding through thinner continuous insulation, the codes have the requirements well defined. When you start getting into heavier claddings, or thicker CI, you may need some help from either the cladding or CI manufacturer, or from an engineer, especially if high wind loading of the cladding also needs to be taken into consideration.
One question that is brought up often is the potential for sag in cladding systems installed over CI. This was covered by in an article by Scott Gibson on GBA back in 2016: How to Attach a Thick Layer of Exterior Insulation – GreenBuildingAdvisor. There are organizations that have tested (both long and short term) the effects of claddings when installed over CI. An informative paper can be found at Building Science Corp: BA-1404: Initial and Long-Term Movement of Cladding Installed Over Exterior Rigid Insulation | buildingscience.com
Cladding Weights
Before we get into some specific manufacturer requirements for CI and cladding attachment, let’s discuss cladding weights. The IRC lists cladding weights at 3, 11, 15, 18, and 25 psf. Where can we find the weight of the cladding we are using for a project? An internet search showed it’s not easy. A paper by Jonathan Smegal, John Straube, Aaron Grin and Theresa Weston on CI had these estimates:
Vinyl siding-0.6-1.0 psf
Wood siding-1.0-1.5 psf
Fiber cement siding 3-5 psf
Cement stucco 10-12 psf
Adhered stone veneer-17-25 psf
Smegal-Approach-to-Assessing-CI.pdf (rdh.com)
Most claddings will not have an issue with attachment through up to 4” of CI, the code doesn’t require engineering until you start getting into the 15 psf (heavy stucco and adhered stone veneer claddings) with thicker insulations. The best advice for designs using heavier claddings and prescriptive codes is to contact a cladding factory representative for the cladding to confirm weights.
Cladding Manufacturers
All cladding manufacturers should have installation instructions showing how to install their products over continuous insulation. I’ve selected LP SmartSide as an example. The installation instructions (below) show how to install LP’s lap siding over one inch or less thick rigid foam insulation (I would assume that also includes rigid mineral wool insulations). If the thickness of the insulation is greater than one inch, but not more than four inches the instructions say to follow the prescriptive siding attachment specified in Chapter 7 of the IRC.
The instructions also detail how to fasten the lap siding over furring strips. The furring attachment to the building’s framing follows the IRC codes or the insulation manufacturer’s instructions, but the siding attachment to the furring has specific details outlined by LP. SmartSide lap siding requires Southern Yellow Pine furring with a specific gravity of not less than 0.55 (the IRC requires SPF with a specific gravity of 0.42 or greater). Two nails per furring strip installed ¾” below the siding edge are specified. The lap coverage must be increased to not less than 1 1/8” and a hot dipped, ring shank nail not less than 0.120 in diameter used to fasten to the furring strip. These installation instructions are more rigorous than the IRC code language.
lp-smartside-lap-siding (lpcorp.com)
Other cladding manufacturers, such as vinyl cladding, have requirements for a smooth, flat surface for installation and many manufacturers limit the insulation thickness to 1” or less. For CI installations greater than 1” when using vinyl cladding, it’s recommended that an additional wall sheathing be installed over the CI (a home-built nail base panel). Certainteed’s Vinyl and Polymer Siding Installation Guide discusses the options (page 31 of the 162-page document). 9d84d03b-0d55-496e-b1d5-10fb921fd476.pdf (widencdn.net) There is a technical evaluation report that outlines how to build this second sheathing layer over CI. files (drjcertification.org)
Insulation Manufacturers
Insulation manufacturers may add to the installation details, but not always. Owens Corning’s Foamular Extruded Polystyrene (XPS) product discusses how to fasten the insulation to the structure and seal the insulation to prevent bulk water intrusion, but I had difficulties locating any reference to cladding attachment, most will differ to the cladding manufacturer’s installation instructions. There was one exception, Rockwool has several details that aid in CI and cladding installation, most of the information has to do with the installation of wood furring (or strapping). There are many more detail drawings and information on the Rockwool website:
ROCKWOOL – R-Class Builder Program – United States (English)
board-insulation-attachment-guide.pdf (rockwool.com)
With both so many cladding and rigid insulation options, it’s best to study the installation manuals of the selected products for the CI design. Prescriptive codes can help with some installations. Heavy claddings, thick layers of CI or geographical locations that include heavy winds or seismic activity might require some help from an engineer. As more and more CI projects are completed, and new products and ideas are developed, continuous insulation installations should become as familiar as installing a modern WRB, something we’ve been doing for more than 30 years.