Dow SIS

Over the past decade, we have been involved in the design of hundreds of residential homes.  HERS ratings have been dropping and the efficiency of homes has been rising.  It can be a bit frustrating sometimes from the perspective of a Structural Engineer because we can not always play a big role in making a home energy-efficient.  We use engineered lumber, specify studs at 24″ on center and simplify design whenever possible to limit the use of unneeded resources.  But we are always looking to work with like minds and be more involved in the process of making a home more energy-efficient.

A couple of years ago, several of our clients asked us to explore using Structurally Insulated Sheathing instead of the traditional OSB exterior sheathing.  This panel can replace OSB, exterior rigid foam and house wrap.  These panels can be fastened directly to the outside face of the exterior stud wall.  The seams can than be taped and according to Dow, no additional house wrap is required.  They typically come in two different thicknesses, 1/2″ and 1″, with an R value of 3 and 5 respectively.

Our clients wanted to try this product because they were having trouble achieving the required/desired insulation in the exterior wall cavities.  They had already switched to a stud pacing of 24″ o.c., but with 2×6 studs, there is a limit to the maximum R value that can be achieved.  In addition, exterior rigid foam interrupts thermal bridging, (cold from conducting through adjacent wood products).

The product proved challenging from a structural and building perspective.  Our clients were frustrated with the ‘SIS’ panels because they were easily damaged at the site.  The installers punctured them and the panels de-laminated quickly.  All of our clients continued to use house wrap because they felt that simply taping the seams wasn’t equivalent.

From a structural perspective, the panels do not have the same capacity as OSB sheathing for resisting lateral shear.  The panels can be installed with both nails or staples, however staple installation tested better.  To increase lateral shear capacity, drywall can be glued and screwed on the inside face of the stud.  Manufacturers of these types of panels typically provide good spreadsheets illustrating values for these installations, however we were unable to find any testing on forces perpendicular to the wall, such as suction.

We found the biggest structural challenge was educating the installers.  Fasteners must penetrate the foam layer of the panel so that they adequately penetrate the wall stud.  This is easier said than done for 1″ thick panels fastened with staples.  In addition, framers who typically install OSB sheathing are not used to installing with staples. They would install the panels with nails, reducing the lateral capacity by approximately 30%, from a staple installation.  Adding additional fasteners did not seem like a viable option because the fasteners were already installed at 3″ o.c. and adding more fasteners compromised the integrity of the panel.  Our designs typically included more interior shear panels or required the installation of OSB sheathing at certain locations.

If you have used the ‘SIS’ panels or a similar product, we would like to hear your thoughts.  What were some of the successes and obstacles you encountered?

For more on green building or residential construction, visit our green building or wood framed construction categories of our blog.

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