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Helpful? 1

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, GBA Advisor

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


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

Supergrund
Aerobord
Askeaton
Co. Limerick
Ireland
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.)

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

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. 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.”


Tags: , , , , ,

Image Credits:

  1. GreenBuildingAdvisor
  2. Building Science Corporation
  3. PATH / NAHB Research Center
  4. Isoquick
1.
Fri, 12/10/2010 - 07:11

Insulated Raft for Double-Wall Construction
by Interested Onlooker

Helpful? 1

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 ?


2.
Fri, 12/10/2010 - 09:41

Response to Insulated Onlooker
by Martin Holladay, GBA Advisor

Helpful? -1

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.


3.
Fri, 12/10/2010 - 10:04

Clarification
by Interested Onlooker

Helpful? -1

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.


4.
Fri, 12/10/2010 - 10:20

Second response to Interested Onlooker
by Martin Holladay, GBA Advisor

Helpful? 0

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.


5.
Fri, 12/10/2010 - 10:58

Larsen trusses
by Interested Onlooker

Helpful? -1

Martin,
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.


6.
Fri, 12/10/2010 - 11:03

Third response to Interested Onlooker
by Martin Holladay, GBA Advisor

Helpful? -1

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.


7.
Fri, 12/10/2010 - 13:14

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

Helpful? 3

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: http://bit.ly/hITEEV 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"...)


8.
Fri, 12/10/2010 - 13:45

Basement Slab over Rigid Insulation
by AJ Quinter

Helpful? 1

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?


9.
Fri, 12/10/2010 - 13:54

Foam under footings as a capillary break
by Doug McEvers

Helpful? 0

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?


10.
Fri, 12/10/2010 - 13:54

Edited Fri, 12/10/2010 - 14:02.

Response to AJ Quinter
by Martin Holladay, GBA Advisor

Helpful? 2

AJ,
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:
http://www.toolbase.org/Technology-Inventory/Foundations/wood-foundations


11.
Fri, 12/10/2010 - 13:56

rating a blog
by aj builder

Helpful? 2

I rate this blog AAA

Great topic.


12.
Fri, 12/10/2010 - 13:56

Response to Doug McEvers
by Martin Holladay, GBA Advisor

Helpful? 1

Doug,
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.


13.
Fri, 12/10/2010 - 14:18

Improvement to a conventional foundation
by Doug McEvers

Helpful? 1

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?


14.
Fri, 12/10/2010 - 14:23

Response to Doug McEvers' second post
by Martin Holladay, GBA Advisor

Helpful? 1

Doug,
Yes.


15.
Fri, 12/10/2010 - 15:11

Seismic Areas and Larsen Trusses
by Timmy O'Daniels

Helpful? 0

Rob,
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'..." ?


16.
Fri, 12/10/2010 - 15:51

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

Helpful? 3

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)..



18.
Fri, 12/10/2010 - 17:09

Response to Skylar
by Martin Holladay, GBA Advisor

Helpful? 0

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


19.
Fri, 12/10/2010 - 18:17

What about termites!?
by Brad Buser

Helpful? 0

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?


20.
Fri, 12/10/2010 - 23:29

Suppliers - response to Dickerson
by Dan Kolbert

Helpful? 1

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.


21.
Sat, 12/11/2010 - 13:57

Edited Sat, 12/11/2010 - 13:58.

ICF manufacturers
by j chesnut

Helpful? 0

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.


22.
Sat, 12/11/2010 - 18:04

Sourcing EPS custom cut
by Al Cobb

Helpful? 1

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.

http://www.epsmolders.org/PDF_FILES/C578%20Chart.pdf

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.


23.
Sun, 12/12/2010 - 18:15

Is there such a thing as
by Super Structure Builder

Helpful? -1

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.


24.
Mon, 12/13/2010 - 18:20

A Few Details + Larsen Trusses in Seismic Zones
by Aaron

Helpful? 3

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


25.
Tue, 12/14/2010 - 00:10

Not so super
by Skylar Swinford

Helpful? 0

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.


26.
Wed, 12/15/2010 - 12:06

level ground
by Tim Geiger

Helpful? -1

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?


27.
Wed, 12/15/2010 - 12:14

Response to Tim Geiger
by Martin Holladay, GBA Advisor

Helpful? 1

Tim,
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.


28.
Wed, 12/15/2010 - 15:38

Heated buildings need insulated footings too
by Kevin Dickson

Helpful? 3

"“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?


29.
Wed, 12/15/2010 - 16:03

Response to Kevin Dickson
by Martin Holladay, GBA Advisor

Helpful? 0

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


30.
Wed, 12/15/2010 - 17:24

Insulated Foundations and Slabs
by Cory Johnston, PE

Helpful? 0

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.


31.
Wed, 12/15/2010 - 21:18

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

Helpful? -1

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.
http://www.research-alliance.org/pages/FFF/Press%20ReleaseFFF.pdf


32.
Thu, 12/16/2010 - 18:18

Soil Moisture
by Doug McEvers

Helpful? 1

Kevin.

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.


33.
Fri, 12/17/2010 - 09:48

Insulated foundations
by maison-passive

Helpful? 0

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.


34.
Fri, 12/17/2010 - 10:02

Edited Thu, 12/01/2011 - 11:23.

Merci bien!
by Martin Holladay, GBA Advisor

Helpful? -2

Maison-Passive,
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!


35.
Fri, 12/17/2010 - 14:41

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

Helpful? 1

Doug,

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.


36.
Fri, 12/17/2010 - 18:42

Polystyrene under footings is goodness
by Dave

Helpful? 1

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 http://www.carnationconstruction.com/Techniques/003-Techniques-Polystyre... 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.


37.
Tue, 12/21/2010 - 15:41

Lateral/seismic testing
by Gavin Farrell

Helpful? 0

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.


38.
Wed, 12/22/2010 - 00:54

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

Helpful? 0

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.


39.
Tue, 12/28/2010 - 14:45

question on basement interior foam
by Jim Bannon

Helpful? 0

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?


40.
Tue, 12/28/2010 - 15:10

Response to Jim Bannon
by Martin Holladay, GBA Advisor

Helpful? 2

Jim,
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.


41.
Thu, 12/30/2010 - 19:31

ants
by R Scott Campbell

Helpful? 0

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.


42.
Sun, 03/20/2011 - 00:04

Edited Sun, 03/20/2011 - 00:06.

Misleading numbers
by John Klingel

Helpful? 0

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.


43.
Thu, 12/29/2011 - 19:15

Re: Misleading numbers
by Carissa Farkas

Helpful? 0

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.


44.
Wed, 02/01/2012 - 19:41

Gavin Farrell
by John Klingel

Helpful? 0

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.


45.
Thu, 03/27/2014 - 22:39

Foam and Foundations more thoughts
by Josh Golek

Helpful? 0

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.


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