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Issues or success with rigid insulation under strip footing

Kim Walton | Posted in Green Building Techniques on

I wonder what experience anyone has had with installing rigid insulation under continuous strip footings supporting a standard concrete foundation wall.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Kim,
    If you haven't seen it yet, I suggest that you read this article: Foam Under Footings.

  2. Kim Walton | | #2

    Thanks Martin. When ever I have an important question about building the first place I look is at the archives for your blog! I believe that you have written (well) about anything that matters!
    I have a client that is doubtful about installing insulation under a concrete strip footing. Interestingly, my client quoted the bible story about building on sand just like Super Structure Builder did after your blog! My client's regular engineer will not consider insulation under the footing at all. We can find an engineer that will work with us, but resistance has set the stage.
    Most of the discussion for the "Foam Under Footings" article has to do with a monolithic or raft foundation. We will be pouring a traditional footing ~ 9" below grade for the proposed project. I am fishing for examples and recent experiences that deal specifically with wrapping a deep below grade footing.

  3. User avatar
    Mike Eliason | | #3

    kim,

    they build freeway on/off ramps on top of rigid foam these days...
    http://www.djc.com/stories/images/20110603/ASCE_SR519_big.jpg

  4. Kim Walton | | #4

    Wow! looks like an Escher drawing! Thanks!

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Mike,
    In a few decades -- during the post-automobile era -- it will be possible to build a Passivhaus on that on-ramp. After all, the site already has the necessary 24 inches of rigid foam under the slab.
    .

  6. Ron Keagle | | #6

    Kim,

    What reason does your client's engineer give for opposing the use of foam under the footings?

  7. Kim Walton | | #7

    The engineer is concerned with the potential for EPS to absorb water and harbour mould.
    I did not speak to the engineer so this is 3rd hand information... I believe that the concern is the potential for degradation of rigid insulation over time.

  8. Ron Keagle | | #8

    Kim,

    I suspected the concern was due to compressive failure, but I am surprised about the concern over mold in the foam. Couldn’t this be avoided by the use of closed cell XPS foam as opposed to EPS which you mentioned?

    I have some thoughts on this subject in general. I understand that the compressive strength of the foam is sufficient to support the load. However, care must be taken to assure that the foam board is placed on stabilized soil with a surface that is absolutely flat in order to prevent the foam from bridging, whereby it would lose support in some areas and thus possibly increase the loading beyond its compression strength in other areas. Concrete poured directly on top of the soil will flow to match any small irregularities in surface elevation, but foam board will bridge those areas and transfer the load to areas that are not bridged.

    This uneven loading will not necessarily cause the footing to crack if it is reinforced. But rather, the point is that it might cause overloading in certain areas, and exceed the compressive strength there. It should not be much of an issue if the foam is say 2” thick because that thin of a sheet will flex to distribute the bearing over soil irregularities when the footing is poured on top of it.

    I also understand that there is a possibility of cold flow or “creep” that can occur in the extruded polystyrene under compression over time. Proponents of foam under footings say that the settling resulting from compression creep will be small and that it will be inconsequential because it will be uniform. They point out that in is differential settling that poses structural problems such as cracking in the structure, and that uniform settling will not pose this problem.

    However, even if a building settles uniformly, I wonder how it can do so without creating a stress or break in the sanitary sewer line.

    Furthermore, if creep settling occurs, its amount will depend on loading. Therefore, in order for the creep settling to be uniform, the loading must be uniform. And while the loading is based on a minimum support pressure requirement that is uniform, the actual load of the building might vary from one portion to another. So it seems reasonable to expect creep settling to vary according to the actual pressure applied to the footing by the varying weight of the building in one part versus another.

  9. Kim Walton | | #9

    Thank you for taking the time to write Ron. I especially appreciate your thoughts about "creep". The point about footings spanning sewer lines is a good one! (one that I have had anxiety about in the past!)
    I have read and heard a few times in the past month or so that there is evidence that XPS degrades over time. The information has been generated by EPS proponents, but the claim is that XPS looses 52% of it's original stated R value. Apparently XPS is more prone to water absorption over time than EPS.
    After seeing the photo that Mike Ellison has offered with the copious use of EPS for some serious concrete construction, it seems that EPS has gained significant credentials with engineers.
    I wonder about the merits of using XPS at all.

  10. Dennis Dipswitch | | #10

    Since part of this discussion deals with the bearing capacity of rigid foam,has anyone incorporated rigid foam on top of a foundation wall and then placed a sill plate on top of that?

  11. Expert Member
    Malcolm Taylor | | #11

    Kim. it does seem there may be legitimate concerns about using foam under footings, but I don't think cracked sewage lines are likely to result from creep. Sewer lines are bedded in compacted sand and any settlement that the foundation they pass through experiences will be nothing like the settlement that occurs in their excavated and re-filled trench. Sewers do break most frequently at foundations and at their connections to tanks. These invariably show it is the pipe that has sunk not the object it is connected to.

  12. Ron Keagle | | #12

    I would not conclude that this cannot work. But I also do not think that success or failure will depend solely on using foam of adequate compression strength. We are told that even foam of the proper compressive strength can settle over time due to creep, either from the loading and/or from chemical changes in the foam.

    This potential settling is estimated to be less than problematic. The prediction that such settling will be uniform is also an estimate. I would add that the vitally critical bedding of the foam against the ground has the potential to cause differential settling by overloading the foam due to uneven bedding. A foundation is not very accommodating of any settling, and completely intolerant to differential settling.

    I understand the desire for a thermal break under the footing, but in weighing that objective against the risk, I would never place foam under the footing. There is enough that can go wrong with the construction of a foundation without adding more risk. And what could possibly be more costly to correct than sub-footing foam causing a problem by settling in ways beyond what is estimated?

    To weigh the risk of foam under the footing, one has to place a value on achieving a so-called “thermal break” under the footing. Two inches of foam would only reduce the heat loss there. It would not eliminate it as the term “thermal break” implies. So even though two inches of foam would help, four inches would be better.

    Another way to cut heat loss from the footing is to bury a foam skirt outward from the wall all around, just above the footing elevation. The primary heat loss path originating under the footing is outward and upward into the progressively colder ground. Considering the heat loss that could be stopped with foam under the footings, what percentage of that heat loss could be eliminated by forgoing the sub-footing foam and installing a foam skirt around the foundation instead?

  13. Dennis Dipswitch | | #13

    Ron,I'll keep it brief,I agree with you completely.A foundation failure, is a complete building failure.I can't see anything riskier to mess with.

    Foundations in these parts are getting more design and steel reinforcing than ever,and everything has been poured concrete for at least the last 40 years.All footings,for the most part,are getting vertical rebar to tie in the foundation wall,so the talk of thermal breaks are the opposite of where things seem to be going.

    But there must be areas where the foam would even be considered.I mean at what soil temperature would someone start to be thinking along these lines?

  14. Kim Walton | | #14

    Carissa Farkus has sent a link to videos that demonstrate techniques for installing EPS under strip footings.
    http://hammerandhand.com/_blog/Field_Notes/post/Passive_House_construction_time-lapse_a_mountain_of_foam,_delivered/#.UVhanqtASWs There are 6 videos showing the progress for the pour for this project.
    The techniques used for the preparation for the insulation under footings for the Karuna house are the same as the details that we use for classic PWF wood footings. (This is a GOOD thing!) The advantages to this type of subsoil work and careful consideration of drainage will ensure that the foundation stays dry and any unknowns about moisture issues are not issues since there is no standing water to worry about. This is the type of (visual) information that I was looking for!

    If the work is done as shown in these videos, the insulation under the footings does not look like a risky proposition to me, but I am not a structural engineer.
    I have specified under footing insulation for monolithic pours (Raft foundations) many times but have not specified under strip footing insulation.(It's easy to guess that the project that wants under footing insulation is a Passive House proposal!)

    My understanding is that protecting the footing from frost with an insulating skirt is especially important when the foundation is isolated from the surrounding soil. If the foundation is not contributing to the modification of the temperature of the backfill and the frost is driven deeper than the underside of the footing, the skirt will prevent frost penetration next to the building. I know that it seems crazy, but around here, the frost can be as deep as 8' some years.

    The experience that I have had in the past with sewer and water lines is that they have been literally "shoved" under footings without proper sub preparation or support around the sewer pipe or under the footing. The footing might span 18" of ABS and uncompacted gravel in the after the fact excavation.

  15. Ron Keagle | | #15

    The video shows the addition of an independent sub-base of granular material between the top of the excavation grade and the bottom of the foam. That sub base is precision graded by screeding along a guide form, and then plate compacted. It would create a perfectly uniform bearing for the foam, and so it seems to me that it would be an essential detail in placing foam beneath the footings. It would eliminate the possibility of settling due to uneven bedding. Is this detail considered to be a necessary requirement for placing foam under the footings?

    As I recall, it was estimated that foam could settle 10% of its thickness over time due to creep. So for 12” of foam, that could be 1.2” of settling. Does the 10% figure apply to any density of foam, or can the density be high enough to preclude settling due to creep? Is creep settling anticipated for the house shown in the video?

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