Here's a hoop for Dan to jump! (Public domain image)

Dan the *Enviro-Tamer had a pretty big hoop to jump through when he went to the City of Seattle for a building permit for his Passive House.  He wanted to use structural foam underneath the load bearing portion of his foundation, but that use had not been permitted in Seattle before on a residential project.  Armed with a lot of research from Insulfoam (the structural foam manufacturer), the backing of his Structural Engineer Carissa Farkas, and a lot of persistance, they made a few requested tweaks to the plans, everyone was satisfied and the permit was issued.

This was a very critical step in Dan’s project.  Without the structural foam underneath his foundation, his plan failed the requirements of Passive House by not reducing the heating energy needs to less than 15kwh/m2/yr.  In other words, too much heat would escape through the concrete and into the soil that the footing was resting on – the curse of the dreaded thermal bridge.  (cue spooky music)

Let’s take a closer look at that foam.  There are 3 types of closed cell foam being used under the slab on this project, two are made of EPS (Expanded PolyStyrene) and a few sheets of locally available XPS (Extruded Polystyrene) were needed to fill in some gaps at the end when piecing together the foam came up short.  (Dan made sure to get the most environmentally friendly XPS that he could).   Of the two types of EPS, 0ne can support Structural loads and is called Geofoam.  The other EPS foam is an underslab Insulfoam product that is not designed for structural loads.  Dan is not an engineer and  engineering was needed to determine just what density was required in the structural applications where deformation had to be 1% or less, so that is where Carissa Farkas and her expertise stepped in.

The density and thickness of the Geofoam affects its strength and R value – the thicker and denser the stronger and more insulative.  The non-structural insulfoam’s R value is also affected by its density and thickness.  As you would expect the cost increases accordingly too and that is why Dan used the more expensive Geofoam under the 3 spread footings and the perimeter grade beam where the structural load bearing strength was important, and the less expensive non-structural foam under the rest of the slab and next to the stemwalls and slab edges.  The non-structural EPS came from remnants a roofing company had left over.  Dan had heard that the 8′ sheet of foam could shrink a small amount (3/8″+/-) and did find that some of the sheets had shrunk from their original size.  He considered that would mean less voids, though he did spray foam the joints together anyway.

One of the sustainable features of this material, according to the Geofoam slideshow is that one truckload of Geofoam is equivalent to 12 truckloads of soil – that translates into less transportation costs, less impact on the environment, and because of the lightweight nature, fewer workers and less equipment needed to place the material.  There is quite a bit more information available on the manufacturers website.  Your structural engineer can help you decide if these products would work on your project.

The Central Puget Sound chapter of the Eco Building Guild recently held a presentation that focused on Spray on Polyurethane Foam insulation that I have blogged about on the Passive House Northwest blog if you would like to read more about foam.

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