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Green Building News

Prefabricated Foam Forms for Slab Foundations

Developers look to Sweden for techniques on forming frost-protected slabs faster and with far less labor than conventional foundations

Image 1 of 3
Forms for insulated raft slabs are being offered by a New Jersey company seeking to introduce North American builders to products and techniques developed in Scandinavia.
Image Credit: Bygghouse
Forms for insulated raft slabs are being offered by a New Jersey company seeking to introduce North American builders to products and techniques developed in Scandinavia.
Image Credit: Bygghouse
The top of the vertical leg of the WarmFörm pieces is tapered. This type of foundation requires an 8-inch-thick base of crushed stone.
Image Credit: Bygghouse
WarmFörm sample. This sample photographed by Greg La Vardera shows a corner section.

Borrowing an idea from Swedish builders, a New Jersey company has begun marketing prefabricated foam forms that make it possible for two workers to install a frost-protected shallow foundation in a single day with little or no digging.

The New Jersey company, Bygghouse, got its start when Scott Hedges studied building materials and techniques while working as a carpenter in Sweden. There, expanded polystyrene (EPS) forms are routinely used for slabs (often called “insulated raft slabs”) as an alternative to deep footings and concrete foundations. To create an insulated raft slab, concrete is placed in a shallow bathtub-like enclosure made of EPS foam.

Hedges and architect Greg La Vardera developed a prototype for their product, WarmFörm, in conjunction with Bensonwood, the New Hampshire-based firm led by Tedd Benson, and placed its first raft slab in the ground in 2012. The company began providing quotes to potential customers this summer.

A WarmFörm foundation would typically be placed on an 8-inch-deep base of crushed stone, with the top of the crushed stone at about 8 inches below finished grade.

A conventional foundation usually includes concrete stem walls on footings that are located below the frost line so they don’t heave in winter. That could be 48 inches or more below grade in colder parts of the country.

The approach is decades old

Frost-protected shallow foundations aren’t new. In fact, this type of foundation was used by Frank Lloyd Wright in the Chicago area during the 1930s. The NAHB Research Center (now the Home Innovation Research Labs) published a paper on how to make them a decade ago, although builders had to cobble them together with sheets of rigid foam insulation and conventional concrete forms.

GBA senior editor Martin Holladay wrote about insulated raft slabs in a blog in 2010. In it, he listed several European manufacturers of EPS forms intended for raft slabs.

Lay out the forms, place the concrete

La Vardera mentioned WarmFörm in a GBA post earlier in October. He said the firm hoped to see the system gain the kind of popularity here that it has in Sweden, “where nearly all new houses are placed on this kind of foundation system.”

“The Swedes have refined this to an extremely efficient, and economical way to build. They build this way not only because it is energy-efficient, but because it is faster and less expensive than digging footings below grade,” he said.

The forms sold by Bygghouse are made from two types of expanded polystyrene (EPS) insulation. Horizontal sections which support the weight of the slab and a reinforcing grade beam are Type IX foam. Vertical pieces are made from BASF Neopor, a graphite-modified Type VIII EPS with an R-value that approaches the R-value of extruded polystyrene (XPS), LaVardera said.

The outside of the vertical part of the form, part of which would be exposed after the slab is finished, is protected by a polyester-reinforced cementitious coating. The vertical part of the form is 4 inches thick except for a tapered section at the top. The horizontal leg under the thickened edge of the slab is also is 4 inches thick, while the field of crushed stone in the center of the building would typically be covered with 12 inches of horizontal EPS, La Verdara said.

Bygghouse isn’t in a position to stockpile an inventory of foam shapes, but works with a fabricator and coater in New Jersey and is ready to supply forms to builders who want to use the system. Forms cost $25 or less per lineal foot.

The forms are too bulky to import from Europe

Bygghouse had considered importing forms from Europe and selling them here, but there were a couple of problems. For one, forms made in Europe are manufactured to unfamiliar metric dimensions that don’t coincide with the 4-foot increment that U.S. builders use. Also, shipping the lightweight but bulky foam pieces didn’t look attractive financially.

“There’s so much to learn from them that they’ve already vetted and done well,” La Verdara said of Swedish builders. “It’s actually very frustrating to us to see how we sort of flail around trying to invent this stuff when there are already very well vetted solutions sitting there.”

He added that at least one other North American company, Legalett, sells a similar product used in conjunction with its hot-air radiant system, but doesn’t appear interested in selling the slab forms separately.


  1. eyarmak | | #1

    Frost-Protected Shallow Foundations
    This system certainly has uses in the industry but it is not a frost-protected shallow foundation system as defined in ASCE 32 or by the pdf linked from Home Innovation Research Labs in the article.

  2. user-2069108 | | #2

    Ed. Why
    Why is it not frost protected, Ed?
    Although the photo shows no perimeter insulation, the line diagram does. Presumably, it is added after the initial slab pour.
    I'm not calling you out, Ed, but could you be more specific?

  3. eyarmak | | #3

    I’m certain that this system will give a nice warm floor and will be very comfortable with a heated slab. I visualize many applications where this system will work well.

    A frost-protected shallow foundation (FPSF) is one where the footings, etc. do not extend below the frost line and building (or geothermal) heat is used to prevent the frost from undermining the footings. Typically, insulation is used to limit frost penetration. By putting insulation beneath the building interior, you are limiting heat flow that might be used to prevent frost penetration next to the structure. Check out the typical detail in image 2 of 2 and there is no perimeter (or skirt) insulation with this system unless required by local codes. If you were use this system in my climate (Anchorage, Alaska) on frost susceptible soils, I would expect to see heaving around the perimeter of the building unless either a large heating element is incorporated into the grade beam or perimeter insulation with a thickness and width appropriate for an unheated structure is utilized. It's the perimeter (or skirt) insulation that does the frost protecting, not the insulation under the grade beam or the slab.

  4. Gregory La Vardera | | #4

    photo above
    In the photo above of the construction site, the work is in progress. Skirt foam is not yet installed in this photo. This project received skirt insulation per the IRC's recommendations.

    In proper sequence skirt insulation is placed and covered before the concrete is poured. This backfilling prevents the edge elements from breaking at the glue joint under the weight of the concrete.

  5. Gregory La Vardera | | #5

    Ed, our explanation
    You have raised a big question, one that is probably deserving of its own blog post, but for the sake of reassuring people that this is most certainly a frost protected shallow foundation system I wanted to offer some clarifications here in the comments.

    The answer to the skirt insulation question is yes, skirt insulation is required per the recommendations indicated in the IRC chapter 4. It varies per location by width and thickness. There are places where it is not called for at all. But "local codes" as you put it don't change the physics, the skirt insulation is still needed.

    But we need to talk a little bit more about how a frost protected foundation works.

    First of all, the situation you describe, where soils below the slab are are prevented from freezing due to heat loss through the slab certainly works, but is clearly not the best approach. I'll risk speaking for everybody here. We all want the heat we make inside the house, that we pay for or gather, we want that to stay in the house. We don't want it leaking into the soil, even with the good intention of stopping soil from freezing.

    Second, frost protected foundation systems do not have to rely on heat loss from the building to prevent frost. The main goal is to prevent the ambient air temperature during winter from freezing the soil, and putting the building between the air and the ground with a little help from a skirt in colder regions is all that is needed to do that. Case in point, houses in Sweden are on insulated slabs like this. Very little heat from the house gets into the soil. The soil does not freeze. And there are places in Sweden that get just as cold as Anchorage, and it still works. In fact, they put the same foundation systems under their garages which are not heated at all.

    So, if the slab is properly insulated, the ground under the slab will not freeze and the foundation will not experience frost. This is all discussed in the ASCE standard in the section on Unheated Buildings. If you don't believe me, you should talk to Jay Crandell about it. He is the engineer who served as the principal investigator in the ASCE report.

    One thing I have to point out is that the section on unheated buildings never made it into our construction code, and the code does suggest incorrectly that heat loss from the building is required to prevent frost. I'm afraid this is just another case of our codes getting it wrong.

  6. eyarmak | | #6

    Gregory - Skirt Insulation
    We agree that it’s the skirt insulation that does the “frost-protecting” on this system. IRC Chapter 4, Section R403.3 assumes that the building interior (with an un-insulated floor) is maintained at a minimum monthly mean temperature of 64°F during freezing weather. Because your foundation is using heavy insulation beneath the floor, the requirements for skirt insulation detailed in Section R403.3 are insufficient to provide frost protection below the perimeter of the building. Perhaps the Swedes use more insulation than the IRC recommends?

    I understand the desire to keep all the heat in the house and not use it to prevent frost heaving of the foundation. Warm floors and low utility bills make happy homeowners in winter.

    If your line drawing of the typical section detail showed skirt insulation being “as required for climatic conditions to prevent frost heave” instead of “as code req’d”, I doubt I would have had any issues in the first place.

    Regarding your final comment on codes getting things wrong, remember that codes are just minimum requirements. We’re all here trying to do much better than that.

  7. Gregory La Vardera | | #7

    Ed - closer consideration of skirt insulation
    Ed, yes, I think we are on the same page now.

    As far as the question of the size and R-value of the skirt insulation there are three variables in play. The horizontal dimension of the skirt, the R-value of the skirt, and its vertical position.

    In the IRC code again does not specify insulation for un-heated buildings, or applications where the slab is insulated. The ASCE report again does cover unheated buildings. You will note that the section on unheated building's discussion of skirt insulation indicates that the depth can be varied, and if there is less cover it can be compensated by increasing the R-value. Another factor indicated by the unheated building section is that the effective depth of the slab can be measured to the depth of the drainage fill, and that this is effectively the same as the depth of the turned down slab in the heated building examples. However if you compare the skirt insulation requirements of these two it indicates that the unheated building requires a wider skirt. But these skirt dimensions are predicated on a slab with no edge insulation, and the relatively thin skirt insulation extending all the way under the slab. This is not the same as the Swedish systems which behave more like a heated building due to the insulated edge, and the higher r-values under the slab.

    The Swedish systems are all placed on the surface of the stone drainage layers for the purpose of eliminating excavation. They do rely on the stone layers to create an effective system depth which is comparable to the progressive depth indicated in IRC and ASCE documents for colder zones. The difference is that the skirt insulation in the Swedish systems is placed at the bottom of the foam forms, which is higher than the placement in the IRC and ASCE documents. The foam r-value in the Swedish standard tends to be twice the values in the IRC, which is consistent with the description of increasing r-value with less cover in the ASCE section on unheated buildings. However where they differ is that the Swedish standards for skirt width tend to be smaller than the IRC recommendations for heated buildings in similar climate zones.

    We've not had the wherewithal to resolve these differences, but we've spoken to Mr. Crandell several years ago, and we worked with a Swedish engineer on sizing the skirt on our prototype. Where we stand now is that if we follow the IRC for depth by varying the underlying stone layers, and follow the requirements for skirt width which is greater than the Swedish standard, and we double the r-value required in the IRC to compensate for the higher placement of the skirt. If done this way we will have a conservative solution.

  8. Robert Swinburne | | #8

    frost protected slab under garages
    Frost protected floating slabs are fairly common under unheated spaces such as barn and garages in the Northeast. Sometimes they are called "Alaskan slabs" Even only 2" of foam makes all the difference in whether the slab will heave due to frost. Builders resist them for houses mostly due to unfamiliarity and the question of inaccessible plumbing runs.

  9. GBA Editor
    Martin Holladay | | #9

    Response to Robert Swinburne
    I agree with you. To get it right, you need (a) a site without topsoil or organic material, with crusher-run gravel fill, if needed, graded level and compacted; (b) good drainage away from the building in all directions, (c) at least 4 inches of free-draining crushed stone -- more is better; (d) at least 2 inches of rigid foam; (e) a thickened-edge slab with 8 in. to 10 in. of concrete at the perimeter and 4 in. to 5 in. of concrete in the center of the building; (f) a rebar grid.

    This approach works for garages in Vermont.

  10. charles3 | | #10

    Can this be used in termite country?

  11. Gregory La Vardera | | #11

    Charles - bug question
    Charles, we will be fabricating these with EPS that includes borates to resist insect intrusion. This is a strategy used with "geofoam" fill for the same reason.

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