Foam Under Footings
Yes, you can build your house on rigid foam — as long as you can convince your local code official to let you do it
A wide variety of residential foundation types, including monolithic slabs, crawl space foundations, and basement foundations, can lose heat due to poorly detailed insulation at the concrete footings. That’s because many construction details, including some details on the GBAGreenBuildingAdvisor.com Web site, fail to address thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through foundation footings.
There are several possible ways to address these thermal bridges, including:
- ignoring the problem (based on the theory that the heat leaks are trivial);
- on some types of foundations — those with stemwalls insulated on the exterior — switching to interior wall insulation to allow for an uninterrupted thermal barrier;
- altering the construction details to include insulation under the footing.
In general, deep footings will lose less heat during the winter than shallow footings. Whether the amount of heat leaking through a concrete footing is enough to worry about depends on your climate and your performance goals; if you hope to achieve the Passivhaus standard, such a thermal bridge is clearly a no-no.
Is there any reason NOT to put foam under a footing?
Traditional wisdom taught builders to place footings on undisturbed soil below the frost line. If footings are properly designed for the soil at your building site — an exercise based on either soil testing and engineering calculations or rules of thumb and local knowledge — then such undisturbed soil should be able to support the weight of the building, with a healthy margin of safety.
SOURCES FOR INSULATED RAFT FORMS
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Tomorrow’s Energy Ltd.
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Engineers explain that good soils should be able to support 3,000 lbs. per square foot (20.9 psi). As it turns out, common extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) insulation like Dow Styrofoam or Owens Corning Foamular has a compressive strength of 25 psi. That’s more than many soils that are routinely used to support a footing and a house. Moreover, it’s possible to order XPS or EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. with densities exceeding 25 psi. (Dow will be happy to sell you XPS with a compressive strength of 40, 60, or 100 psi; these products are likely to exceed the performance requirements needed for a residential project.)
In short, the weight of a concrete footing plus a concrete wall plus a two-story house isn’t going to compress the foam insulation under a footing — not by a long shot. So logic doesn’t support the traditional assumption that foam is squishier than undisturbed soil. It isn’t.
Building scientist John Straube points out that when rigid foam supports a load, it can suffer from “creep” or deflection. “Over 50 years, the foam can shrink by 10%,” Straube notes. However, as long as the creep is consistent, the building sitting on the foam shouldn’t suffer harm. “The real problem isn’t settling, it is differential settlement,” says Straube.
(For further engineering notes related to the design of buildings that bear on rigid foam, see Comment #42 by John Klingel, and Comment #45 by Josh Golek, below.)
It's been done
Many builders have successfully installed foam under residential footings. For example, Thorsten Chlupp, an Alaskan builder and author, has installed as much as 12 in. of 25-psi foam under residential footings.
Rigid foam is often used under the footings of buildings with frost-protected shallow foundations. For example, the NAHBNational Association of Home Builders, which awards a Model Green Home Certification. Research Center publication, “Revised Builder’s Guide to Frost Protected Shallow Foundations,” recommends the use of XPS under the footings of unheated buildings.
Convincing local code officials
Just because rigid foam can support more weight per square inch than excellent soil, doesn’t mean that local code officials will understand the use of rigid foam under footings. Several architects and builders have had to engage in negotiations over the issue with local code officials; among those who have been successful are several in Washington state, including architect Rob Harrison of Seattle, Tessa Smith of the Artisans Group in Olympia, and Dan Whitmore of Blackbird Construction and engineer Carissa Farkas, both of Seattle.
According to a blog by Linda Whaley, Dan Whitmore “had a pretty big hoop to jump through when he went to the City of Seattle for a building permit for his Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates.. 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 persistence, they made a few requested tweaks to the plans, everyone was satisfied and the permit was issued.”
Insulated raft foundation systems
To avoid the problem of thermal bridging through concrete footings, many European Passivhaus builders are now using “insulated raft” foundation systems. As typically installed, an insulated raft is a load-bearing flat slab on grade. The slab has a uniform thickness rather than a thickened edge. The concrete thickness and the rebar schedule are designed to support the loads imposed by the perimeter walls and any interior bearing walls.
The EPS forms for an insulated raft foundation resemble a big rectangular tray. There is a continuous horizontal layer of rigid foam under the entire slab, as well as vertical insulation at the slab perimeter; all of the foam locks together. After the concrete is placed, the foam forms stay put, just like the forms of an ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure. wall.
An insulated raft foundation differs from a frost-protected shallow foundation:
- Insulated raft foundations have a uniform thickness rather than a thickened edge.
- Unlike many frost-protected shallow foundations, insulated raft foundations always include a continuous horizontal layer of insulation under the entire slab.
- Insulated raft foundations usually have no wing insulation, depending instead on a deep layer of crushed stone to address the problem of frost heaving.
European manufacturers of EPS forms
Isoquick forms are made of EPS manufactured by BASF. The foam panels interlock with an egg-carton configuration that the manufacturer calls “pyramid-shaped lugs.”
Isoquick forms can be used to assemble an insulation system that is either 5.9 inches (R-23) or 11.8 inches (R-46) thick.
Tomorrow’s Energy forms are rated at R-38. The insulation and the slab have a total thickness of about 15.8 inches.
For energy nerds obsessed with insulation details and the problem of thermal bridging, insulated raft foundations are aesthetically satisfying. I predict that the growing interest in the Passivhaus standard in the U.S. and Canada will eventually lure European manufacturers of insulated raft forms to begin distributing their products on this side of the Atlantic. In the meantime, North American builders who want to build an insulated raft foundation will need to use ordinary XPS or EPS panels and removable perimeter forms.
To see a video of workers installing rigid foam at a foam-under-footings job, click here.
Last week’s blog: “All About Glazing Options.”
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