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Home on Foam? “More Structural Support than Soil”

Guest Blogger: Zack Semke

Image courtesy of Holst Architecture

It’s always cool to be on the cutting edge. Earth Advantage builder Hammer & Hand has joined forces with Holst Architecture to build the Karuna House, an ambitious green building project in Yamhill County, Oregon that aims to meet the world's most demanding green building certifications, including Passive House, Minergie-P-ECO, & LEED for Homes Platinum.

The Karuna House's client, a leading proponent of smart climate policy and sound land use, is pursuing the project as a case study to shed light on the ways that the leading green building certifications and standards complement one another, and the ways that they may conflict.

"It's a great challenge," says Hammer & Hand’s Sam Hagerman, who also serves as president of the national Passive House Alliance.  "When you line up the various aims of the standards, like the superior energy performance of Passive House, the sustainable materials requirements of LEED, and the lifecycle concerns of Minergie-P-ECO, you're left with a fairly narrow path to navigate through in design and construction.  It'll be an interesting journey, one with pretty exciting environmental rewards… and lessons."

One of the first steps is the installation of what is generically called geo-foam. Because Passive House and Minergie design and construction focus on super-insulating the envelope of a building on all six sides -- the four walls, roof and foundation -- the Karuna House will be placed on a continuous layer of this EPS foam.  The higher density foam will sit under the footings of the house and will bear significant weight, while the lower density EPS will go under the slab and will hold up just the weight of that slab. 

Image of EPS foam, Ministry of Transportation, Ontario, Canada.

"The intersections where the 'six sides' of the building envelope connect are the most critical parts of the building and detailing them properly is the difference between a Passivhaus and an average house," says Skylar Swinford of Hammer & Hand.  "Neglecting to create a 'thermal bridge free' connection between the wall and foundation system is a big mistake. It would be like trying to build a watertight bucket that has gapping holes in the bottom-the energy just pushes out at the point of least resistance."

Structural foam like this EPS provides a nice solution (one that we also employed at our Super-Efficient ADU recently).  And if you have any lingering doubts about the wisdom of building a house on top of foam, rest assured that because Geofoam has more structural strength than soil, it can be used in all sorts of heavy construction applications, even highway construction (below).  For more, see this article by Green Building Advisor's Martin Holliday: "Foam Under Footings."

Now, you might look at that pile of foam in the time-lapse video and think, "it looks like a huge stack of Styrofoam to me.  How is that 'sustainable'?  Global warming impact, anyone?"

If we were using conventional extruded polystyrene, or XPS, you'd be right to be concerned.  According to Green Building Advisor's Alex Wilson, XPS is made with a blowing agent that has a global warming potential (GWP) of a staggering 1,430, meaning that as a greenhouse gas it is 1,430 times more powerful than carbon dioxide.  That kind of global warming impact would quickly negate any global warming benefits of super-insulating a house with XPS.  Unlike the potent greenhouse gas utilized in XPS, EPS uses a pentane blowing agent which is relatively benign, with a GWP of just 7.  So the energy savings conferred by super-insulating with EPS easily dwarfs the modest global warming impact made by manufacturing that foam. 

We also took care to be as efficient with the EPS as possible.  In order to reduce waste and fit all of the foam into one truckload (reducing the environmental and financial costs of transport), Hammer & Hand Lead Carpenter Scott Gunter created a cut list from Holst's plans and ordered the EPS pieces custom fabricated to the precise dimensions and quantities needed for the project.  That means there will be virtually no waste onsite, and any waste that we do generate will be recycled into new EPS products.  We'll save money on materials and labor, speed up construction, and minimize environmental impact.

And lest you be daunted by the sheer quantity of foam pictured, Skylar reminds us, "EPS is mostly air, so while that mountain of foam looks impressive, it's mostly a pile of air bubbles." 

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