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Frost Protected Shallow Foundations (FPSF)

by Chad Ludeman on December 9, 2008 · 20 comments

in Building Science,foundation

What is a Frost Protected Shallow Foundation?

A frost protected shallow foundation (FPSF) is simply a more practical construction method used in cold climates as an alternative to standard footer foundations. Traditional foundations require a footer to be placed at the frost line with a foundation wall and eventually slab on top of that. FPSF’s use additional insulation on the outside of the foundation walls that actually change the frost line underneath and around the perimeter of the structure by bringing it up to shallower ground. This allows foundations to be poured as little as 12″ deep in areas that would normally require 3′ or more for a traditional foundation to reach the frost line for that area.

Frost protected shallow foundations can also be poured in one monolithic concrete pour once the insulated forms are created underneath and around the perimeter as required. Most FPSF’s will save time, money and material costs while yeilding a better insulated foundation. This sounds like something we might be interested in.

Lastly, the shallow foundation is able to withstand poor soil conditions due to the fact that the weight of the building is spread over the entire structural slab. This is muy bueno for us as almost every vacant lot in Philly has poor soil conditions.

History of the FPSF

Frost protected shallow foundations have been used for over 50 years in some of the most extreme climates. Most got their origin in the Scandinavian countries like Sweden, where hundreds of thousands have been built. Many have also been built in Canada and Frank Lloyd Wright was using them in the 40′s and 50′s on his Usonian homes in the US.

Benefits of the FPSF for the 100K House Model

The diagram above is a good illustration of what our detail would most likely look like. In our climate, it is recommended to use only 1″ (R-5) of rigid on the exterior of the foundation wall to a depth of only 12″. We will probably beef up this insulation level for thermal performance reasons, but it is good to know that the minimum requirements are very reasonable for our climate.

Below are the basic reasons why the team is excited about the possibility of FPSF’s in all of our future 100K House inspired homes:

  • Less labor for foundation work
  • One concrete pour vs. three for traditional foundation
  • Less concrete used in foundation
  • Superior insulation value compared to traditional
  • Less excavation and hauling of existing soil
  • Able to withstand poor soil conditions
  • Potential for prefab insulation forms (Legalett)
  • Faster
  • Cheaper

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{ 20 comments… read them below or add one }

1 lavardera December 9, 2008 at 12:35 pm

Great research as usual Chad. I think you may find that there are some issues using this in an urban situation that you would not face in the typical single family freestanding home.

First of all you don’t need to protect from frost along a party wall (going on faith the house next door is occupied and heated). Thats good and actually reduces the exposure of the foundation wall. But you may have some complications at conditions where the back of the houses are offset and your new house is extending further to the rear of the lot than the neighboring house. The exposed wall facing the adjacent lot would require the perimeter insulation which would need to be placed under your neighbors rear yard. It may not be practical to install in that situation.

Similar issues may exist at the front of the house. If the house is placed at the front property line as common for Philadelphia row houses then the perimeter insulation will want to extend under the sidewalk. That may be easier to install, but any street work by utilities or the city could ruin the insulation skirt with no obligation for them to replace it. A front setback as with the 100k house is helpful for this, but it can break with the urban pattern, and it can increase the offset at the rear which is its own problem.

Just some thoughts to anticipate things you may wrestle with. You know I want to see you use this in the future.

2 chad December 9, 2008 at 12:46 pm

Excellent points Greg. I was thinking that neighboring houses should improve our position as you stated.

In regards to the insulation extending out to the sides, I was also worried about this. Upon further investigation, I don’t think it is something we actually need. If you look at this design guide from the US Gov’t, it tells us that in our climate we do not require any horizontal insulation at all. The detail would look like the second image in this post, rather than the first.

We are just starting to work on this with our engineers next week and hope to have more info soon…

3 lavardera December 9, 2008 at 1:06 pm

Ahh, the milder climate here works in our favor. More northern locations will have to work through this however.

4 chad December 9, 2008 at 1:31 pm

Bingo. Yes, doing this in a tight infill lot in Boston or Chicago could be a different story altogether. We’ll worry about that when we get to it… ;)

5 Kevin D December 9, 2008 at 6:34 pm

One thing not discussed much is the water content of the soil. A brand new house should always have really good drainage, and therefore dry soil. Dry soil means never any frost heave even if the perimeter insulation were compromised. Using a footing form like Form-A-Drain run to a dry well would provide a third line of defense.

The main thing is that future home inspectors/buyers will have to know this stuff if these systems become more popular.

6 Sarah December 10, 2008 at 4:02 pm

Are there termite issues in Philadelphia? Down here on the Gulf Coast, we build monolithic slabs all the time (locally called “turn-down” slabs), but it’s not a good idea to provide any munchable material that will provide a sheltered path from soil to wood. When I detail wall sections here, I make a sunlight gap in any polystyrene – that area marked “per local code” in your Fig. 8.

Obviously frost heave is no issue at 30dN latitude, but I wonder if, for example, there’s somewhere in the middle of the country where both frost heave and termite risk have to be accommodated.

7 Denis Du Bois December 10, 2008 at 10:04 pm

Just try to get a conventional builder to consider this. They’ll say Planning won’t approve it, and they’re probably right. But in truth the builders won’t risk the unknown.

We had to pick our battles when planning our green home. Some are easier than others. We studied up on FPSFs several years ago and loved the idea, but found it too hard a sell.

“Get a smarter builder,” you might say. In the non-urban county where we’re building, we sought out the best. They’re very expensive and still don’t like SIPs, tankless water heaters, passive solar — they barely understand hydronics.

Time will change minds, but we’re building in this century.

8 chad December 10, 2008 at 10:18 pm

Kevin – Good points on water protection. We will be looking to improve upon our current methods the next time around.

Sarah – Termites are not a huge problem in Philly. We are taking a couple of measures recommended by LEED and non-toxic to address any concerns here.

Denis – Unfortunately this can be the case many times. We are very fortunate to have a great builder who is open minded to our new designs and able to carry them out once proven. We are actually all sitting down next Monday to discuss FPSF’s in more detail with the builder, architect and engineer.

We have discussed with our builder how many GC’s out there are simply not willing to try new things. It’s often hard to blame them as it can be a very big risk to their bottom line. It’s also hard to trust what others are telling you as a GC for a new system. Product and system manufacturers may bend the truth trying to make a sale and others may simply be uninformed.

One of the biggest things that may help builders to consider new methods is actually our struggling economy. In order to keep business going, they may need to try new methods that the green developers who are still struggling forward are implementing.

9 goran December 11, 2008 at 9:52 pm

I’m so glad I found this site. I really want to build a house using what I learn here, in the next couple of years. Sorry if these topics have already been covered, I just started reading.

Great info on Legalett. The idea of using a slab as a heat reservoir with air to transfer heat, reminds of Japan’s OM solar ( They use a slab under a plenum/subfloor. The solar collector roof omsolar describes uses air as a working fluid, which would be a good match with the Legalett system. The solar roof look simple enough: metal roof, painted black, over a plenum, on top of SIP, with a clear radiation shield over the top 10 feet of roof. Does anyone know of a source for an air based solar roof, and controls, in the US? I’ve contacted OM Solar, but they’re not selling to the US.

Also, Legalett is clear the slab can’t be used for cooling in the summer because it would result in condensation and mold, which would produce air quality problems, so in the summer, the air has to be maintained cooler than the slab (or must be dehumidified), be some separate means. OMsolar claimed they could use their slab for summer cooling, I guess because it was covered with a plenum and wood floor, so no clammy floor. Probably they also had to dry out the circulating air to prevent mold.

I guess with Legalett, you could still use a separate geothermal heat sink and heat pumps with that. A separate geo sink could also be used to pre-heat air in the winter, provided the rest of the house was tight. But now it starts to get complex. Before it was just air as a working fluid, now its air for the Legalett floor, hydronics for the summer cooling, plus a heat pump, and dehumidification, and a fluid to air heat exchanger. Is that manageable? Is there a simplier way?

10 Kevin D December 12, 2008 at 9:00 am

Yes, simplicity is highly important.

Solar space heating systems were fairly common in the late 70′s and early 80′s, air and liquid. One lesson learned then was that the active air systems (with blowers) are impractical.

Most people investigating geothermal heat pumps for small, highly efficient homes, find out quickly that they are too expensive.

Photovoltaics are the future as costs come down because of their relatively huge output in summer. Solar thermal is needed more in the winter when its output is less.

Chad’s choice of mechanical systems is prudent for now. All-electric is the next step as PV comes into its own.

If I were building the 100k, I’d consider a small TTW (through the wall) heat pump instead of the gas boiler, since it’s less expensive, provides cooling, and runs on electricity which can be supplemented by PV.

In-floor heating is overrated in a superinsulated house because the heat load is so small that the heat comes on rarely. So your slab feels cold (even though it’s 70F) almost all the time.

11 chad December 12, 2008 at 9:54 am

Wise words Kevin. You’re on the money here. We are actually looking at eliminating the radiant floor in the next home and moving to all electric for the reasons you stated.

As far as the thru-wall heat pump for water heating, I’ve looked and not found. Could you point us in the right direction? The only heat pump water heaters I’ve found are completely internal. During the winter they will suck heat out of the house to heat our radiant floor, which is heating the house. You can see how this could become a vicious cycle…

12 tlynch December 12, 2008 at 10:37 am

Great article on heating a well / super insulated home:

13 goran December 12, 2008 at 4:56 pm

I’ve just started reading some of the archive info, and its amazing how many options you’ve looked into for HVAC, and how much info is here.

14 Kevin D December 15, 2008 at 4:27 am

There are some possibilities for a through the wall heat pump for hot water (HPWH), but all of them are semi-custom. One-off solutions should to be avoided when trying to keep costs down, but OK if they are truly simple.

One possibility is to put the HPWH in its own vented room and duct the cold air straight outside. Freezing the pipes is the big fear for that scenario, but avoidable. The other big drawback is that now it’s hard to use it for summer cooling. There would have to be a damper to direct the cold air from the heat pump into the dwelling instead of outside.

But that would require a biannual task to be performed by the occupants, and that’s is a no-no. That’s one of the nice things about boilers, you never even have to change a filter.

So, there is some needed technological development in this field. That is, a self contained HPWH that gets air from outside or inside, depending on the need for cooling. Another feature they could easily add would be to preheat the hot water with ambient air during the summer.

15 mike April 23, 2009 at 11:34 am

I’m a homeowner who is in the process of planning an outdoor bluestone patio to contain a curving circular low profile sitting wall (27″ +/- tall) enclosing the outer area of the patio. Since I will have natural stone veneer faced piers (42 ” tall+/-) with manufactured columns above constructed within this same patio space to hold up a deck above, I naturally would like to use the same veneer stone for the construction of the sitting wall.
I do not want to go through the labor and expense of constructing a full depth (below frost level – assumingly 42″ in the hudson river valley of NY) concrete wall to support the “wet set wall”, and i do not want to construct a dry laid wall either, since i want it to match the stone pier look. I’ve explored using the rubble footing concept utilized by Frank Lloyd Wright in some of his Chicago homes of the 50′s, but I still would need to excavate down to the frost depth, and then after filling the cavatity with clean crushed stone, would still need to add perforated pipe within and drain to daylight, which would be about 200 ft away. (a drywell will not work with my siol type either).
I’ve reviewed many segmental block wall types, but these just do not cut it in terms of trying to match the real stone appearance.
My question: Is there a possible mechanism in utilizing this insulating technology to construct a shallow foundation “wet set stone veneer” sitting wall, that will not be subjected to frost heave and consequent damage?

16 lavardera April 23, 2009 at 1:56 pm

dont think this works without a heated interior space to prevent the earth below the footer from freezing

17 mike April 23, 2009 at 2:37 pm

That’s what i initially thought, but I see a detail invloving the insulating material that is specific to un-heated buildings also, whereby it utilizes the geothermal heat from the earth below to keep the footing from freezing. Unfortunately the detail fails to show how far horizontally from the footing wall the insulating material needs to run outward from the wall to function properly… on a simple seating wall situation it would not be practical to have to excavate more than a few feet on each side.

18 mike April 23, 2009 at 2:43 pm

IE…Unheated structure
Unheated garages and other outbuildings require a different approach. A shallow foundation for one of these structures relies on heat from the earth. Ground temperature remains fairly constant around the local average annual temperature, which is between 40 and 50 degrees in the U.S. The footing bears directly on a layer of foam. The foam must extend beneath the entire structure, and it must bear on 6 to 12 inches of gravel.

19 lavardera April 23, 2009 at 3:40 pm

The horizontal extension would depend on your location and how deep the frost line extends. Well, thats if if it is similar to a conditioned house’s configuration.

20 Terry June 17, 2009 at 7:38 pm

I have an existing home in Taberbash, Colorado (outside Winter Park)and i am going to build a new one in the near future. i want to use FPSF because of water table issues during the seasonal runoff time. The question i have is: What about moles? My current house has a standard foundation/crawl space and has moles digging around the yard and definitely along the foundation. It is difficult to keep them in check. Will they dig through the foam? How deep do moles go?

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