Passive House and SIPs

Thanks to a recent reader comment, we’ve taken the time to address the question (one we often get) about structural insulated panels (SIP) and why we don’t use them. Read on if you are curious.

Comment from reader: From the photo it appears that you’re not using structural insulated panels (SIP) on this project. Can you tell me what design you’re using? Thanks.

Reply from Tessa Smith, Certified Passive House Consultant | The Artisans Group

Great question! We chose not to use SIPs on our Certified Passive House for several poignant reasons. We found on the North project that we could achieve better energy performance for less cost with a Larsen Truss wall assembly. A Larsen Truss is essentially a double 2×4 wall with plywood gussets at three locations that make the outer wall cantilever off of the interior core, which is load bearing and lateral. This also simplified our floating slab assembly, because we can’t have any thermal bridging in a Certified Passive House, we needed to wrap our foundation in insulation at least 6 inches deep for performance reasons. A SIP could not be used in this instance, they are not strong enough to cantilever one of their skins off of bearing, and a continuous bottom plate would be an unacceptable thermal bridge. The Larsen truss bears on our edge of concrete and then cantilevers over the continuous foam beautifully. SIPs are very costly, and tend to get cut up at every lateral strap location and plumbing or electrical junction.

Another reason we have yet to use SIPs on one of our 12 Certified Passive House projects, is its intrinsic HIGH embodied energy. We try to use as little foam as possible, foam processing and energy consumption in the manufacturing process is not a pretty thing, not for human health and not for the environment. In our project we have air tight construction, paired with an open diffusion wall (a wall that can last over 200 years) with dense pack cellulose or fiberglass, both of which have extremely reduced carbon footprints and cost in comparison to SIPs. Also we don’t expose OSB to the weather side, an element of longevity in my opinion. There are many cases where SIPs don’t last the test of time on there outer skins. In our more recent Certified Passive Houses, we have gotten away from any foam at all, which is very exciting!

At one time, SIPs were a good upgrade to achieve a more energy efficient home, albeit expensive and not accessible for many people. It’s possible there are acceptable uses for them still but, I have yet to find an opportunity where I couldn’t get better performance for less cost and embodied energy on a project. The PHPP modeling software gives us the opportunity to model homes with an accuracy that can’t be touched with ANY other modeling tools available on the market, the kind of information it yields allows us to run iterations and see curves of return on investment with material choice and very subtle design tweaks (the kind that can’t just be rule of thumb). With that, we’ve determined our current construction method to be the most cost-effective, highest performing approach with the least environmental impacts. It just can’t be beat so far.

Posted on October 05, 2010

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