Structural EPS foam will super-insulate the Karuna House’s foundation.
Hammer & Hand has begun construction on the Karuna House, a showcase of sustainable design and building located in the vineyards of Yamhill County, Oregon. Designed by Holst Architecture, the house aims to achieve Passive House, Minergie-P-ECO, and LEED for Homes Platinum certification. Read more here about Karuna.
In this time-lapse video below we see the delivery of the EPS foam that will be used to super-insulate the foundation of the Karuna House:
This morning I spoke with Hammer & Hand’s Skylar Swinford, Certified Passive House Consultant, about the video. He said that the “Geofoam” you see here consists of a couple different densities of EPS (expanded polystyrene), both of which have more compressive strength than the soil they will be placed on.
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.
“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,” said Skylar. “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 Holladay: “Foam Under Footings.”
Image of EPS foam, Ministry of Transportation, Ontario, Canada.
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. (See Wilson’s article here.)
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.”
P.S. Here’s more technical info about the foam from Skylar, for those building science geeks among you. Good stuff…
“There seems to be confusion on the topic, but a single product can be both Geofoam and EPS. Geofoam is simply a generic name for foam used in geotechincal applications. The key difference between the EPS and Geofoam is that they are classified under different ASTM standards. EPS used as insulation falls under ASTM C578 Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation while Geofoam is listed to meet or exceed ASTM D6817 “Standard Specification for Rigid Cellular Polystyrene Geofoam”. The ASTM D6817 offers better information for selecting the proper EPS foam product for load bearing applications. The critical spec to look at when selecting structural EPS foam is the 1% compression rating. Utilizing the 1% rating will ensure that the foam does not experience excessive or permanent deformation under the designed load. The thicker the foam the more pronounced deformation can be. For example, under the psi load listed for 10% deformation a 16″ block of foam could compress by more than 1.5″.
“Under the footings we are using 2.40-2.50 lb/ft3 EPS39 (Geofoam title) or Type XIV (EPS title). Under the slab we are utilizing 1.35 -1.50 lb/ft3 EPS22 or Type II. The lower density foam reduces cost and the amount of polystyrene needed to manufacture the foam.”Back to Field Notes