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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

Posted on Dec 10 2010 by Martin Holladay

UPDATED on September 30, 2016

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 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.


Placing a Concrete Foundation on Rigid Foam Insulation

Prefabricated Foam Forms for Slab Foundations

Raft slab detail

Vermont House Uses Only Half a Cord of Firewood

"On PHirm Footings: A Survey of Passive House Foundations in North America"


45 South Centre Street
Merchantville, NJ 08109

Legalett Canada
103 Warner Drive
Long Sault, ON, Canada K0C 1P0
866-299-7567 or 613-936-0518

Isoquick GmbH
Am Hellenberg 26
56651 Niederzissen
Tel: 49 (0) 26 36 / 94 12 02

Co. Limerick
Tel: 1 800 604605

Tomorrow’s Energy Ltd.
Penallta Industrial Estate
Unit 2 North Road
Ystrad Mynach
Hengoed, Wales CF82 7SS
United Kingdom
Tel: 0844 800 0811

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. Note that for some projects, engineers will recommend aiming for a maximum of 1% deformation rather than 10% deformation. For a useful chart providing guidance on deformation of EPS, see EPS Geofoam Data Sheet. Here is a link to a GBA Q&A thread with further discussion of this issue: EPS foam for load-bearing applications.)

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 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 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.

North American manufacturers of EPS forms for raft slabs

Three North American companies are now distributing rigid foam forms for raft slab foundations:

  • Bygghouse, 45 South Centre Street, Merchantville, NJ 08109; Phone: 856-662-4909.
  • Legalett Canada, 103 Warner Drive, Long Sault, Ontario, Canada K0C 1P0; Phone: 866-299-7567 or 613-936-0518.
  • EcoCor, P.O. Box 359, Lincolnville, ME 04849; Phone: 207-342-2085.

European manufacturers of EPS forms for raft slabs

Among the European manufacturers of EPS forms for insulated raft foundations are Isoquick, Supergrund, and Tomorrow’s Energy.

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. North American builders who want to build an insulated raft foundation can choose from two approaches: they can purchase components from Bygghouse in New Jersey or Legalett in Canada, or they can cobble together foundation forms using ordinary XPS or EPS panels held in place by conventional removable foundation 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.”

Tags: , , , , , ,

Image Credits:

  1. GreenBuildingAdvisor
  2. Building Science Corporation
  3. PATH / NAHB Research Center
  4. Isoquick
  5. Image #7: Hayfield House
  6. Image #8: Chris Corson
  7. Image #9: Legalett
  8. Image #10

Dec 10, 2010 6:11 AM ET

Insulated Raft for Double-Wall Construction
by Interested Onlooker

Presumably an insulated raft design for a double-wall building would need a ring beam with a conventional footing outside the edge insulation of the raft to support the loads from the external walls and the roof ?

Dec 10, 2010 8:41 AM ET

Response to Insulated Onlooker
by Martin Holladay

Interested Onlooker,
Q. "Presumably an insulated raft design for a double-wall building would need a ring beam with a conventional footing outside the edge insulation of the raft to support the loads from the external walls and the roof ?"

A. Not necessarily. A wide variety of walls could be placed on an insulated raft foundation -- including an ICF wall, a wall framed with I-joist studs, a stud wall with exterior Larsen trusses, or even a conventional stud wall with thick exterior foam sheathing (i.e., a REMOTE or PERSIST wall) -- without a "ring beam with a conventional footing."

As long as the roof and wall loads are transferred to the concrete foundation, the foundation will work.

Dec 10, 2010 9:04 AM ET

by Interested Onlooker

I was concerned about the thermal bridge which would occur if the outer frame were to sit directly on the raft. I am envisaging a construction in which the outer frame carries all the live and dead loads on (and of) the roof, the exterior walls and the windows.

Dec 10, 2010 9:20 AM ET

Second response to Interested Onlooker
by Martin Holladay

Interested Onlooker,
Obviously, the load path from the roof and walls needs to bear on the foundation. If you are choosing an insulated raft foundation, it only makes sense to plan your walls so that the load paths are well supported.

If you want to build a double stud wall with the outer wall bearing the roof load, then obviously the outer wall needs to bear on concrete. That means that your siding won't be coplanar with the stucco coating used to protect the foundation foam, unless you install rigid foam of the same thickness on the exterior side of the outer stud wall.

It's certainly possible to have the siding NOT be coplanar with the foundation stucco -- I've seen it. Whether or not the detail bothers you is a matter of aesthetics, not building science. Needless to say, durable metal flashing is required at such a transition.

To me, it makes more sense to have the inner stud wall bear on the concrete footing, and to use Larsen trusses or exterior rigid foam to thicken your wall.

Dec 10, 2010 9:58 AM ET

Larsen trusses
by Interested Onlooker

Thanks for a prompt reply. I have accepted Robert Riversong's advice against using Larsen trusses in seismic areas (the house is destined for Cascadia) and have followed his suggestion of using independent load paths for roof+exterior wall and floors+interior wall.

Would it be sensible to build a Thermomass dwarf wall to carry the wall loads and sit this on the insulation with the inner wythe of the Thermomass wall acting as the form for the floor slab? Rebar protruding from the inner wythe would tie slab and dwarf wall together.

Dec 10, 2010 10:03 AM ET

Third response to Interested Onlooker
by Martin Holladay

Interested Onlooker,
Properly detailed, that would work.

This isn't really a question of what is "sensible," however. There are hundreds of ways to build foundations and walls, and each way has advantages and disadvantages.

Dec 10, 2010 12:14 PM ET

A Few Details + Larsen Trusses in Seismic Zones
by Rob Harrison AIA

Pierce County didn't blink an eye on our Passivhaus permit application. Have a look here, if you like, at the envelope details from our permit set: On that project we used a bearing stud wall with rigid on the exterior--similar to REMOTE/Persist. On subsequent projects we've moved to a stud wall bearing conventionally on the foundation, structural sheathing as air barrier, with Larsen trusses blown with cellulose hung off the exterior. The foundation details remain essentially the same.

An aside to Interested Onlooker: I'm curious as to why Robert Riversong advises against Larsen trusses in seismic zones. Can you (or he) point me to that discussion? Seattle is zone 3, and almost all Passivhaus building designs I've seen in this area use we we're calling Larsen trusses. (I recall that technically they are not actually "Larsen trusses"...)

Dec 10, 2010 12:45 PM ET

Basement Slab over Rigid Insulation
by AJ Quinter

What is the advantage of having a basement slab over a heavy layer of rigid insulation? I would think if a layer of 25psi foam (3600 psf capacity) was laid over a well compacted layer of gravel, the slab could be deleted and a plywood floor could be used instead. Would you be concerned that the foam and plywood would not be able to span over soft spots in the underlying gravel and soil? Any thoughts?

Dec 10, 2010 12:54 PM ET

Foam under footings as a capillary break
by Doug McEvers

It seems to me, one of the benefits of insulation on the outside of the building foundation is moisture control. Isolation of the building from the damp earth would help greatly in controlling humidity in an airtight building. If a conventional footing and foundation wall is used, would one have to wrap the footing with rigid insulation?

Dec 10, 2010 12:54 PM ET

Edited Dec 10, 2010 1:02 PM ET.

Response to AJ Quinter
by Martin Holladay

Advantages of a slab compared to plywood:
1. Greater durability
2. Greater resistance to radon.

I'm sure there are other advantages as well. But plenty of people have built wood foundations, including basement floors of wood:

Dec 10, 2010 12:56 PM ET

rating a blog
by aj builder

I rate this blog AAA

Great topic.

Dec 10, 2010 12:56 PM ET

Response to Doug McEvers
by Martin Holladay

Q. "If a conventional footing and foundation wall is used, would one have to wrap the footing with rigid insulation?"

A. I don't understand your question. No, you don't HAVE to wrap the footing with rigid foam. Most conventional homes do not place any rigid foam under the concrete footing.

Dec 10, 2010 1:18 PM ET

Improvement to a conventional foundation
by Doug McEvers

Martin, I was thinking more along the lines of a modification to how the majority of new homes (full basement) are built in MN. I understand the warm side (inside of the foundation wall) and under the slab, rigid insulation system for a full basement, my question is on exterior foundation wall insulation. I like to insulate with rigid on the outside of the foundation wall and under the basement slab, could this system be improved by wrapping the footing with rigid insulation from a thermal and moisture control standpoint?

Dec 10, 2010 1:23 PM ET

Response to Doug McEvers' second post
by Martin Holladay


Dec 10, 2010 2:11 PM ET

Seismic Areas and Larsen Trusses
by Timmy O'Daniels

Entering "seismic" and "Larsen" in the GBA Search Facility threw up :
"Airtight Sheathing & Thermally Isolated Double-Stud Walls" and
"Simple One on Thermal Bridging" as relevant hits.

BTW I'm intrigued in what way are your Larsen trusses "not actually 'Larsen trusses'..." ?

Dec 10, 2010 2:51 PM ET

How about something similar in the NE USA?
by M. Steven Dickerson

I am building right now, and need foam products for a passive haus (home) which I am building. I would love to use this type foam products, but am finding it tough to get a supplier for something similar in the US (Massachusetts)..

Dec 10, 2010 4:09 PM ET

Response to Skylar
by Martin Holladay

Thanks very much! What a great resource! I really appreciate all the links.

Dec 10, 2010 5:17 PM ET

What about termites!?
by Brad Buser

I look at these systems, get really excited, then realize this is impossible to protect the stud wall from termites. I live in Oklahoma, one of those crazy mixed climate areas, that needs termite protection and could really benefit from slab edge protection on our monolithic edge foundations, but the termites keep us from doing it. Has anyone really determined a good way to address this on a shallow frost protected foundation?

Dec 10, 2010 10:29 PM ET

Suppliers - response to Dickerson
by Dan Kolbert

Shouldn't be a problem getting foam. I found 25 psi EPS from several suppliers, including Foard in RI, and Beacon Sales in ME. Call around. XPS at 25 psi is available almost universally.

Dec 11, 2010 12:57 PM ET

Edited Dec 11, 2010 12:58 PM ET.

ICF manufacturers
by j chesnut

Thanks Martin.
Timely topic for us also.
A couple days ago we sat down with a local ICF manufacturer and asked about having custom formwork created for footings. According to the rep the ICF form manufacturer can make custom profiles to any design out of their base EPS stock which if I recall correctly comes in 4'x4'x8' sheets.
We are currently working on another design to meet Passivhaus standards, this time planning on slab on grade. We may have an opportunity to cost compare between the systems you outlined vs. local costume manufactured formwork.

Dec 11, 2010 5:04 PM ET

Sourcing EPS custom cut
by Al Cobb

The physical properties of EPS are defined by ASTM C578 Standard Specification for Rigid, Cellular Polystyrene. This chart is found below at the EPS molders Association.

The density of type IX EPS is referred to as two pound density. However, the specification allows a minimum of 1.8 pcf. This material has the 25lb compressive strength.
The EPSMA site also lists its members who can provide a custom cut form of any shape you can draw. The computer cutting machine (CNC) uses a hot wire to cut out the special shape. Most block molders mold billets up to 16 feet. However, most computer cut shapes come in eight foot lengths.
Most EPS block molders also have the capability of incorporating a borate solution during the manufacturing process to assist in the protection against carpenter ants and termites. The borate treated foam usually comes with a very small up-charge.

Dec 12, 2010 5:15 PM ET

Is there such a thing as
by Super Structure Builder

Is there such a thing as getting too carried away? When I was a kid, one of the bible songs I learned was about the three guys who built three houses, with the foolish man building his on sand. Dow can make their foam 1000 PSI and I still wouldn't go near setting my house on foam. The ground is continuous, going straight down non stop, the foam is not. There could be some sliding or some settling, despite what the mfr might say. If you've built an air tight house, the heat loss through the slab should be negligible. I'm putting insulation underneath the Advantech that will go on the slab and that's gonna have to do. Foam that thick will prob run 30 a pop. For a 2500 SF foot print, you're talking $2,000 or more. How about just making your slab thicker with the money? Foam mfrs might claim the product is waterproof, but can it really withstand 10 years on moist soil? I'll pass on this technique.

Dec 13, 2010 5:20 PM ET

A Few Details + Larsen Trusses in Seismic Zones
by Aaron

To Rob Harrison:
It would have been nice to have a higher resolution drawings in your link.

Dec 13, 2010 11:10 PM ET

Not so super
by Skylar Swinford

To Super Structure Builder:

"If you've built an air tight house, the heat loss through the slab should be negligible." Yes, if you build a home that is airtight with no insulation, the amount of heat loss through the slab will be less than walls and ceilings exposed to ambient air. However, in an air-tight and super-insulated home, the heat loss through an uninsulated slab or perimeter thermal bridge could easily match or exceed the heat loss through the roof of wall system.

Dec 15, 2010 11:06 AM ET

level ground
by Tim Geiger

How important is it to have all the voids under the foam filled? I have not found an excavator who can dig perfectly flat and level. Would the strength of the faom be altered if the surface under it is not flat and level?

Dec 15, 2010 11:14 AM ET

Response to Tim Geiger
by Martin Holladay

It isn't necessary to have an excavator who can prepare your site perfectly. You have a layer of crushed stone under your foam, right? If you use 3/4-in. crushed stone -- not larger -- it should be easy to rake. An ordinary garden rake works fine.

Use a long, straight 2x4 and a level to check your work.

Dec 15, 2010 2:38 PM ET

Heated buildings need insulated footings too
by Kevin Dickson

"“Revised Builder’s Guide to Frost Protected Shallow Foundations,” recommends the use of XPS under the footings of unheated buildings."

This is a major problem because it implies you can't do it for heated buildings. Who can we bug to get this omission corrected?

Dec 15, 2010 3:03 PM ET

Response to Kevin Dickson
by Martin Holladay

I suppose you could try to find an interested researcher at the NAHB Research Center, which is responsible for the document.

Dec 15, 2010 4:24 PM ET

Insulated Foundations and Slabs
by Cory Johnston, PE

As usual with these types of discussions, many subjects get involved and ideally will all be integrated to get the best overall performance. Placing foundations on insulation is structurally viable but has some issues to be aware of. Concrete placed in a standard excavation automatically forms to the unavoidable iregularities of an excavation. Foam will not, or at least not as well. Excavations and foam placement must therefore be done more carefully if the full bearing capacity of the soil and foam is to be used. On a raft foundation, this is not quite as critical as wall loads are generally spread over a larger area, with a corresponding lower bearing pressure, assuming the raft foundation is designed correctly. However, building code required frost depth can come into play and make a raft type foundation problematic unless it is part of a basement, versus a standard slab at grade. One also has to consider the thermal mass of a raft type slab/foundaton as it can easily change the overall thermal performance characteristics of the structure.

I am currently working on a passivhaus design in Michigan and looking at all these considerations with the architect. I appreciate the article and comments to see how these issues are being addressed by others.

Dec 15, 2010 8:18 PM ET

Comments on Frost Heave
by Kevin Dickson, MSME, P.E.

Some very interesting research from the manufactured home industry is worth mentioning here. It wasn't intended for slab foundations, but I believe their findings are applicable to on grade slabs.

It shows us that the "wing insulation" discussed in the FPSF guidelines can be ignored, you don't have to dig down at all, (except to remove organic material), and any gravel intended to drain water away is unnecessary.

Here's the reasoning:

Depending on the type of soil, frost heave will only occur when the moisture content is above roughly 15%. It turns out that it is really easy to keep the soil under your home this dry. The home itself protects the soil underneath it from water, and the surface water at the perimeter must be directed to a point at least 10 feet away.

Since the wing insulation is only to keep the soil below the shallow footing or slab edge warmer than 32F, if the footing or edge rests on on dry soil, you don't need to keep it warm.

If all the perimeter surface water is directed away from the house, no water gets underneath to freeze. Gutters help, but aren't required. Since you don't have any water under your foundation, it means you don't need any gravel and french drains to take it away.

This approach also means you don't have to rely on the heat from inside the home to prevent foundation freezing.

Note that you should still completely insulate the bottom and sides of the slab for energy reasons.

Here's the press release for the report, but I guess you have to pay for the actual report.

Dec 16, 2010 5:18 PM ET

Soil Moisture
by Doug McEvers


Lot's of "ifs" in your post, I like gutters on all of my projects with long extensions to get rainwater away from the building. Proper grading is also a top priority as you have stated. In the Midwest we have had some hellish rains in the last few years so I would say whatever foundation type you choose, install gutters with 3" x 4" downspouts and extensions.

Dec 17, 2010 8:48 AM ET

Insulated foundations
by maison-passive

I've seen some visitors from your site on my blog ;-)
I am glad it may interest. But it's in french.

So if you read french, there is a special category here for insulated foundations products

As I wrote two years ago (with a lot of hope): Welcome to the passive house movement!

Luckily, there are people like you who show the right way to go: "passive house once, passive house for ever" as our austrian friends from Vorarlberg say.

Dec 17, 2010 9:02 AM ET

Edited Dec 1, 2011 10:23 AM ET.

Merci bien!
by Martin Holladay

Merci pour l'information sur le système d’isolation périphérique.

Merci aussi pour cette perspective historique: "En outre, nos amis américains ont été semble-t-il, impressionnés par l’équipe allemande lors du dernier Solar Decathlon. Il faut dire, qu’historiquement, ils ne sont pas étrangers au concept de maison passive. Car le mouvement vers plus d’efficacité énergétique a été amorcé au USA à la fin de années 1970 et au début des années 1980, grâce à des pionniers tel que William A. Shurcliff ou encore de la construction du Rocky Mountains Institute par Amory & Hunter Lovins. Depuis, la Waldsee Biohaus a vu le jour dans le Minesota, le standard prend racine et les sites d’informations se multiplient aux USA."

Aux deux bords de l'océan atlantique, nous travaillons ensemble pour le même but. Bonne chance!

Dec 17, 2010 1:41 PM ET

Soil Moisture
by Kevin Dickson, MSME, P.E.


Under the SBRA guidelines, water management maintenance remains extremely import for long term success. I fixed a home last year that had sunk 5" because the missing downspout extension had been ignored for about 40 years. The foundation was down 3' per code, and there was never any frost heave, but it failed nonetheless because it was on mud.

Wing insulation may have helped deflect that water out a couple more feet and prevented the damage.

Another advantage of a raft-style slab is that it is high up on the site, above the mud that might occur from long-term bad drainage. That mud, frost heaving or not, my damage a stem wall or basement wall.

Dec 17, 2010 5:42 PM ET

Polystyrene under footings is goodness
by Dave

Excellent article Martin. I totally agree with putting polystyrene under the entire foundation (including the footings) to reduce heat loss. I am doing this with ICF construction (ie under a very heavy concrete house). My rationale for it is written up at and in the surrounding implementation pages. I have just added a link to your excellent article to help persuade people that I am not crazy.

Dec 21, 2010 2:41 PM ET

Lateral/seismic testing
by Gavin Farrell

Well, I'm sold that the load bearing capacity is no problem for foam, but how about lateral behavior? This would only be a concern in seismically active areas. As noted above, concrete fills all the little nooks and crannies a little better, setting up load transfer nicely. I see one of the pics of the insulation system shows a nice egg crate interlock between layers of foam which would prevent any lateral movement, but how about the bond between the foam and concrete above and the foam and gravel below? Is there enough keying or friction developed to prevent any lateral movment problems in, say, an earthquake? I would bet the answer is 'yes', but I am intrigued if it has been tested as such.

Dec 21, 2010 11:54 PM ET

Tim Geiger, grading
by aj builder, Upstate NY Zone 6a

I grade tennis courts to 1/8" tolerances. Look up laser grading. We know how to perfect planar surfaces preconcrete or in this case foam.

Dec 28, 2010 1:45 PM ET

question on basement interior foam
by Jim Bannon

Well, it's too late to consider foam under the footings for our 18X21' bump out addition, we poured them about two weeks ago. Site is located in Southern NH, climate zone 5. We are planning 2" of XPS under the slab, and a continuous layer of 2" ridgid foam on the interior side of the new foundation walls (as shown in the first illustration at the end of the article). The rigid foam on the interior walls will run up to the band joist which will sprayed with closed cell foam. I was originally planning to use 2" of foil faced Thermax on the walls, but note that the illustration at the end of the above article specifies "vapor semi-permeable insulation". I presume this is to allow vapor from the concrete walls dry to the inside. But wouldn't it be just as well to keep the vapor in the concrete walls?

The foundation wall has conventional damproofing applied to the exterior (Premier brand masonry coating applied with a roller) and of course footing drains on both sides of the footings (exterior and under slab). Site is properly graded, soils are well drained sandy gravel, roof runoff is managed via gutters and downspouts that discharge well away from the foundation. The existing basement is as dry as any that I've ever seen, and has never had any moisture issues.

I was planning on foil faced polyiso (Thermax) vs. XPS for the walls simply because of the higher R-value, and to keep any atmospheric derived vapor (e.g. say someone left the bulkhead door open on a humid summer day) off the concrete walls. Is this asking for trouble?

Dec 28, 2010 2:10 PM ET

Response to Jim Bannon
by Martin Holladay

I think you'll be fine. You're right that the experts at the Building Science Corp. believe that a concrete basement wall should be able to dry to the interior, but I think that the importance of this detail is exaggerated. After all, the soil on the outside of your basement wall is usually damp. If you encourage a basement wall to dry to the interior, you are inviting a lot of moisture into your home.

Lots of people have insulated the interior of basement walls with Thermax, and found that the walls perform very well. I haven't ever heard of any problems arising from the use of Thermax in that location.

The minimum code requirement for basement insulation in your climate zone is R-10. Just a few miles north, in climate zone 6, the code minimum is R-15. If you end up using 2 inches of XPS, your wall will barely meet code. A little extra insulation never hurts.

Dec 30, 2010 6:31 PM ET

by R Scott Campbell

Does anyone know of any studies about ants tunneling in below-grade foam? I have seen amazing infestations in XPS in contact with wood framing (flat roof used as balcony). The ants don't eat XPS but they find it awesome for tunnels. Perhaps below-grade, against concrete, is no issue, and way below-grade even less of an issue. I hope so, as I've used XPS in that application many times, including my own house.

Mar 19, 2011 11:04 PM ET

Edited Mar 19, 2011 11:06 PM ET.

Misleading numbers
by John Klingel

Be careful w/ the "25 psi foam". For EPS, I believe that advertised number is at 10% compressive resistance, ie, deformation. Beyond 5% compression is where you start to get out of the elastic limits of the foam, and by 10% the foam will not rebound. The "creep" referred to above is mentioned on the DOW web site, and they recommend a 3:1 ratio to avoid it (static loads; 5:1 for dynamic loads). I got info on this from the Insulfoam (EPS) web site, and by talking to a cat who does compression testing of foam. For example: EPS foam with a 40 psi compressive resistance (I believe that is what is called "40 psi foam") supports 35 psi at 5% but only 15 psi at 1%. I would not be happy building on something that I know will creep 10%. Just "bear" that in mind when you build and pick a compressive value that you are comfortable with.

Dec 29, 2011 6:15 PM ET

Re: Misleading numbers
by Carissa Farkas

Thank you, John, for stating an important point. It is crucial to not run with the "x psi" numbers on the packaging. These are very rarely the appropriate design values. There are numerous issues to take into account when considering placing foam below footings. Take care to hire an experienced structural engineer.

Feb 1, 2012 6:41 PM ET

Gavin Farrell
by John Klingel

Gavin: Lateral may not be a problem, according to an engineer here. If I recall the numbers he quoted, a house pushes sideways w/ about 10% of it's weight during seismic activity, and the shear plane between the foam and a slab would w/stand about 20% of the house's weight. However, I did not ask about the consequences of putting the vapor barrier right under the slab. I can't see the shear plane taking much load in that case. I'll have to ask about that next visit.

Mar 27, 2014 9:39 PM ET

Foam and Foundations more thoughts
by Josh Golek

John: Thank you for pointing out something not many people realize. The listed capacity on foam is usually not a design value-- it is the load the foam can take right before it fails or deforms to an unacceptable limit. Design values may be much lower depending on how much settlement the engineer deems acceptable, which will be driven largely by the thickness and density of the foam used as well as the buildings framing scheme. We sometimes change densities under a foundation depending on location in an effort to minimize differential settlement. On the other hand, bearing capacities of soil such as the ones listed in IRC Chapter 4 are design values whose full capacity can be used when sizing a footing. So, a bearing capacity of 2,000 psf (common in VT where silty soils are prevalent) has a 14 psi design capacity or allowable load. "25 psi" foam after dividing by 3 has about 8 psi. Unless you consider snow, wind and live load to be cyclical loads. Then a factor of safety of 5 is commonly used , leaving us with a 5 psi design value for those loads. By the way I have no problem putting foam under a foundation I do it all the time at the firm I work for with several different foundation types. If you are going to build with thick foam under a foundation, it needs attention and forethought. Think about your utilities a bit, if the house settles 3/4" uniformly and suffers no harm, will the septic tank still be hooked up? Maybe, maybe not. Bottom line: until building codes can correlate all the variables for you in an understandable table for the most common foundation types, I recommend that you have your design checked by an engineer.

Feb 7, 2015 1:41 PM ET

FPSF with foam blocks
by Anders Lewendal

Martin: I volunteer with Bozeman's affordable housing advisory board. Our goal is to create more affordable homes partly by using creative new building science. FPSF's are not new. Can we make a case for using 12" thick -2# density foam block extended 3' past the structure to act as the foundation? No concrete.

My engineer thinks we can. Thoughts?

Feb 7, 2015 3:02 PM ET

Response to Anders Lewendal
by Martin Holladay

I'm not an engineer. You have apparently already consulted with a real one. Believe him or her.

Feb 8, 2015 11:39 AM ET

by Anders Lewendal

You are right, we can likely get an engineer to approve it, but that does not address the economics and energy efficiency of the idea. In zone six we mostly install footings and four foot walls. If everyone is doing it, it must be the best solution. I am not so sure. I'll keep working on it and report back.

Feb 13, 2015 9:31 PM ET

Termite Protection
by Patrick Perry

I have 15 years field experience in the pest management industry as well as owning my own pest management business. I really like GreenBuildingAdvisor articles, and have for a good twoplus years, especially the insulation methodology embraced. Having said that, soils in most areas should be poisoned chemically at the proper times, volumes, & rates with termiticide. The entire process could be another article. Suffice it to say, subterranean termites can easily bore through foam to wood or other cellulose items, whether it be wood studs, beams, moldings, shelves, comic book collections on a shelf, pictures on a wall, or bibles next to their activity. I say that because we, myself included, often forget it's not just the wall studs, and not just the material value but time and emitiin correcting the problem. Most termite "pretreats" are done by the lowest bidder, and often that price doesn't even include treating the exterior backfill which may come months down the road. As well, it rarely includes treating before the footer but only after. There are some in my industry against foam near or at the soil, and encapsulated crawlspaces as well, who will not warranty work around foam. Myself consider it like any other inaccessible areas and slab homes, and feel it simply requires additional care and regular monitoring. If you decide to use foam, we have today exceptional products to control termites. Unfortunately not only product choice, but volume of total diluted should be very high, perhaps in hundred or hundreds of gallons, at the right stages, and to do absolutely best, on repeat visits after different soil disturbance (pipes, backfill, mulch, etc) periods and construction phases (additional backfill in porches and garages before pour). If, like 99% of the calls I get, the question is how many cents per square I charge, and can you match a penny difference with xyz, you're not asking the right questions. Tell them what products you might like, how many gallonsyou think it might take after you've read a product label (most are the same), inquire about followup needs, explain about your structure with foam, and tell them you're not made of money but you're willing to pay as long as it is done by the book. Okay, all that and one more thing: Borates. Extremely low toxicity and when applied to wood or porous building materials, and kept dry from rain and moisture, can last 20 to 30 years or more. You can inquire to have this done or do yourself, though like everything in life even the simplist things aren't so simple. The borates, in most of my area, would be supplemental to liquid termiticides, though for some uses can technically qualify alone. The concern I have, which can be mostly mitigated, would be the foam being "crushed=removed" because it was eaten by termites foraging looking for cellulose for nutrition. If you poisoned the soil prior this should protect you for a good 5, 10, 15 years depending on variables. After that you could supplement with a retreat/booster/bringing back to standards at ground level, inject more below the foam with a carefully designed system of injection pipes (must be done before construction), and/or a baiting/monitoring system service (stations in the ground that have to be checked and serviced typically every quarter (some 3x and some yearly). Some of the better baiting systems are Sentricon (not my choice because of being tied as a biz owner to DOW), and Advance system, among a few others. Patrick Perry, Perry's Pest Control.

Mar 5, 2015 12:12 PM ET

Termite Protection
by Buzz Burger

Question for Patrick Perry (or anyone). How far below grade will termites/ants go? With a full basement and interior foam on poured concrete walls that is continuous with sub slab foam, would there be any reason to be concerned? Assuming the poured concrete wall is sound an insect would have to descend 8' or so to reach any foam insulation. Or, again with a full depth basement, could exterior rigid insulation be protected using a membrane such as Bituthene?

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