Shallow Frost-Protected Foundation questions
Can a Thermomass (http://www.thermomass.com/overview/overview.htm) foundation be used in a Shallow Frost-Protected Foundation in a cold climate? My concern is whether the break in insulation caused by the outside concrete wythe separating the horizontal wing insulation and the vertical insulation in the wall is a problem.
Can/should the horizontal foam be extended underneath the Thermomass wall? If so, what type of foam should be used? With appropriate soils, does a Thermomass foundation need a footer for a single-story house? Can a low (~2’ high) Thermomass foundation be poured directly from the chute of a cement truck, or is a pumper-truck necessary?
In an article entitled “Foam Under Footings” (https://www.greenbuildingadvisor.com/blogs/dept/musings/foam-under-footings), Martin Holladay mentions Insulated Raft Foundations. Apparently preformed foam components for this type of foundation are available in Europe, but not yet in the US.
I’m wondering what problems would there be to make the raft out of XPS panels? What grade foam should be used – would Blueboard work? Is it necessary to tie to panels together like Isoquick system does? How can the vertical edge panels be tied to the concrete slab? Since the slab is a constant thickness without a thickened edge, how thick should it be for a single story house? Is it sufficient to reinforce the concrete with fibers, or should steel mesh or rebar be used?
If not using horizontal wing insulation panels, what configuration should the drainage gravel be? Could there a problem with insects or burrowing animals excavating the foam? Should some sort of barrier be extended into the soil some minimum distance? How would the foundation for an attached non-heated garage differ from that of the house?
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Replies
This particular question has had some discussion already:
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/18889/airtight-sheathing-thermally-isolated-double-?page=10
I know the title doesn't suggest it but if you scroll to the very end of the thread you'll see what I mean.
John: I'm glad you asked about TMass walls. I am thinking of fabricating my own, using #5 galvanized rebar, because I can not get them up here. It will be interesting to see what is said about them. FWIW, a gent at the Cooperative Extension Service, UAF, told me that he wanted to put foam under his footers, but the site engineer advised against it, saying he feared the ground underneath would go through freeze/thaw cycles, soften, and allow sinking. The heat loss there was not deemed large, so he gave in and skipped the foam. I think you will hear "do not rely on only fibers in the concrete", and "yes, bugs and animals love foam; protect it." A bug killer friend of mine says foam was made for ants. Shall see. j
John,
It makes no sense to use ThermoMass for a shallow foundation, unless, perhaps, you were building in termite country and could not use exterior vertical or shallow horizontal wing insulation (though in most termite zones, no wing insulation would be required).
ThermoMass is an excellent alternative to either ICFs or exterior insulated poured concrete foundations, either full basement or frostwall with crawl space or slab-on-grade. And there are excellent details for achieving insulation continuity
I just used ThermoMass for a walk-out basement with radiant slab for a super-insulated house in VT, and the 4"/4"/4" configuration is perfect for my 12" thick wall systems.
I've also built FPS foundations using a reinforced 20" deep (8" above and 12" below) grade beam that has 10" of bearing on the soil (plus 2" exterior XPS to match the 12" thick wall above). The slab is then floating inside the grade beam with 2" additional slab-edge insulation that is recessed into the top inside face of the grade beam to be continuous with the sub-slab XPS.
A slab must either be thickened at the edge and reinforced with rebar to serve as a footing or poured separately inside a grade beam which is wide enough to be the footing. Short fiber in concrete only prevents surface cracking from drying and plastic shrinkage and is not a substitute for welded wire mesh or rebar (but is an excellent supplement to them).
Here are some grade beam/slab options I drew up for a client:
John,
An unheated building with a FPS foundation requires XPS under the entire building, including foundation, and extending out a minimum of 30" on all sides (depending on air freezing index).
ThermoMass does require a pumper truck (another reason it makes little sense for a shallow foundation) as well as the proper mix for easy flow and compaction.
These are the specs I was given by my ThermoMass rep:
- use 3500 psi mix, 7" slump, mid-range/high-range water reducers
- 30%-35% fly ash
- ¾" aggregate OK, but mix of 3/8" and ½" is better, aggregate must be no larger than 2/3 of space
- rebar is placed 2" from form
John Klingel,
I don't understand where that site engineer is coming from, since all footings have to be designed to prevent freezing under them, either by making them deep enough to stay below the frost depth or by insulating outside them to keep the frost away.
Even conventional 25 psi XPS is more than strong enough to hold up any single-family residential structure (that's 3600 psf, which is more than some typical soils can handle).
I've done some design work with Thermomass (but we ended up with a more conventional insulated foundation wall for cost reasons in the end) and I learned that one can install high density foam insulation (like the Dow HD40) under the interior wythe of concrete allowing you to keep the thermal break continuous. In this scenario, my engineer (and the thermomass technical rep.) wanted to install #4 dowels every 24" or so. We decided that these should also be epoxy dowels to avoid any blow-outs in the concrete due to the expansion of rusting steel at this location (where the rebar is not entirely within the concrete.
But I'll say that I'm with Riversong on the expense of that system for a shallow foundation detail, and that I wouldn't feel satisfied with that site engineer's explanation of why one shouldn't insulate under the footings. The fact that the ground would freeze under a footing is not a reason to not install insulation under the footing, but a reason to design a new footing (make it deeper or install horizontal insulation)
Chris Briley, Architect
Unless you have unusually high loads on the interior wythe of concrete, there's no reason that the same 25psi XPS used in the ThermoMass wall system can't be used under the inner concrete for continuity. That's precisely what I just did for my walkout basement (see details below), even though the second floor bears on the inner wythe alone.
Also, by rough calculations, I figured that the ThermoMass wall would cost no more than an 8" concrete wall with 4" of exterior XPS and the stucco necessary to protect it, once labor costs were factored in.
John Hess,
You are asking so many questions about the design of your foundation that I wonder whether you are working with an engineer or architect. If you have so many questions, it's probable that you would benefit from hiring a consultant to help you.
1. I agree with several other people commenting: there is usually no reason to use Thermomass with a frost-protected shallow foundation (FPSF). Thermomass is usually used for full basements; it is a type of wall system. A FPSF is a type of slab; it is a horizontal foundation.
2. I'll try to address your questions about insulated raft foundations:
Q. "What problems would there be to make the raft out of XPS panels?"
A. None.
Q. " What grade foam should be used – would Blueboard work?"
A. If you mean Dow Styrofoam (XPS), the answer is yes.
Q. " Is it necessary to tie the panels together like Isoquick system does?"
A. No.
Q. "How can the vertical edge panels be tied to the concrete slab?"
A. If you use the vertical foam as a stay-in-place form, you could use a few fender washers attached to wires that extend into the concrete. (The washers and wires should obviously be inserted into the foam before the concrete is placed.)
Q. "Since the slab is a constant thickness without a thickened edge, how thick should it be for a single story house?"
A. The answer to that question will be determined by your engineer.
Q. " Is it sufficient to reinforce the concrete with fibers?"
A. Probably not.
Q. "Should steel mesh or rebar be used?"
A. Yes. Again, talk to your engineer.
Q. "If not using horizontal wing insulation panels, what configuration should the drainage gravel be?"
A. It should be deep enough to avoid frost heaving. In colder climates, or at sites with unusual soils, you will probably need wing insulation. Again, talk to your engineer.
Q. "Could there a problem with insects or burrowing animals excavating the foam?"
A. Absolutely -- especially in hot climates.
Q. "Should some sort of barrier be extended into the soil some minimum distance?"
A. Requirements to address termites are determined by your local code; these requirements vary widely. In many locations, it is unwise to install exterior rigid foam on your foundation.
Q. "How would the foundation for an attached non-heated garage differ from that of the house?"
A. Your question is too general. What is your specific question? Again, if you have no idea how to design a foundation, it may be best for you to hire someone with experience designing foundations.
Lucas, thanks for the link to the previous discussion of ThermoMass walls. I appreciate Robert Riversong’s diagrams of how to use a ThermoMass wall with footers and concrete slabs. Since the consensus advice is not to use Thermomass Walls for Frost-Protected Shallow Foundations I’ll not considerate further. I did note in Robert’s drawings that he shows the inner wythe having the insulation break above the footing. If one is using a ThermoMass wall such that the footer is not below the frost line, and that horizontal wings are needed, then it might be better to put the insulation break under the outside wythe. In fact, the more insulation is used under a slab, the more susceptible the bottom of the foundation (footer in this case) is to being lifted by frost. So, ironically, the more insulation used under the slab, the more insulation may be needed in wing panels. I found the document “Frost heave in Swedish slab on grade” helpful. http://www.supergrund.se/files/document/tjale.pdf
Again, thanks to Robert for the beam grade diagrams. That type of foundation had been my first-choice until I read Martin’s article about raft foundations. Were the second and third diagrams depicting two different sections? The only difference I could see is that the slanted insulation on the third diagram seemed to be further inside the wall than in the second diagram?
Thanks to Martin for wading through my list of raft foundation questions. I understand that details of slab thickness and reinforcement will need to be checked by an engineer, but since this type of foundation is apparently seldom used in the US I might guess that finding an engineer willing to design such a system might be difficult. So I hoped that one of the engineering folks on this forum might be willing to stick their neck out and give some technical advice.
I live in a cold enough area (New England) that AFAIK termites are not a problem. However, I’ve had ants get into foam, and I wonder if moles or chipmunks might also be attracted to exterior foam? How deep into the soil must a foam-protecting barrier be?
My questions regarding an attached non-heated garage pertained to the raft foundation scenario. I’ve seen diagrams of how to insulate such a garage that was part of a beam-grade-type frost-protected shallow foundation, but I in the information Martin referenced in his article I didn’t seen any details on how to do that with a raft foundation for the main house. It seems the two areas of concern are where the garage attaches to the house, and where the garage door is.
John,
If you have an attached unheated garage adjacent to a house on a slab foundation, the most important detail is to be sure there is a thermal separation (a layer of rigid foam) between the garage slab and the house foundation.
Other than that, you are free to design a variety of garage foundations. Many people are happy with a so-called "floating" slab (thickened-edge slab-on-grade) for an unheated garage; however, since such foundation might move slightly with respect to the nearby house, it's probably best to separate such a garage from the house with a breezeway.
If you want to be sure the garage slab doesn't move, put your garage footings below frost level. Build a perimeter frost wall up to the level of the top of the slab. Then fill the area inside the frost wall with compacted gravel or stone. Pour a slab on top of the compacted fill.
@ John Hess
Thank you for asking this, and for the clarity of your questions. This is pretty much the same question I was trying, in my ham-fisted and badly worded way, to ask a couple of weeks ago and ended up only frustrating the people who tried to help out. Robert, especially, when through the Vermont equivalent of hades trying to understand the query ... yours is slightly different, and I'm in Zone 3 but I do finally grok why it would be a bad choice for me to try using a thermomass approach to a slab design here.
Joe W.
You're contradicting yourself. The ThermoMass wall requires footings below the frost line, otherwise it's a shallow frost-protected foundation.
The Guide to Frost-Protected Foundations specifies that their standards work for total slab R-value up to 28. Beyond that, the specs for an unheated building must be used. You can, of course, simply increase the R-value of vertical and wing insulation and the extent of the wings, but you would need thermal engineering to determine how much and to justify it to a code official.
Yes, as you noted, the difference is in how much bearing surface there is between grade beam (not beam grade) and ground and between wall plates and grade beam.
Martin said:
This is a common misuse of terminology which leads to a great deal of confusion about slab types.
Any slab that is at or near ground level is a slab-on-grade (as opposed to a full basement slab or a walkout basement slab).
Any slab that is supported only by the ground (or compacted fill) is a floating slab.
Any slab that is supported at its edges by a foundation or footing is a supported slab.
Any slab that is a combination of floor and footings (by the use of thickened and reinforced edges) is a monolithic slab (created in a single pour). This is sometimes called an Alaskan slab.
Any slab that is designed to float on expansive soils, often with a waffle-like configuration to concentrate the weight on a smaller area of soil , is called a raft slab.
"You're contradicting yourself. The ThermoMass wall requires footings below the frost line, otherwise it's a shallow frost-protected foundation. "
Robert, while I may not have expressed myself clearly (not being an accomplished editor :), I didn't contradict myself. I was attempting to comment on, and thank you for, the excellent diagrams you posted, even though they didn't exactly address my question -- my question being whether a ThermoMass foundation sans footers could be used as a Frost-Protected Shallow Foundation. My current understanding is that ThemoMass is not the best choice for that application.
Can you offer any engineering specifications for using rebar/welded-wire mesh to reinforce a constant-thickness-concrete insulated raft foundation for a single-story house, on well-drained compacted soil?
John,
The contradiction was that you went right back to a question about a ThermoMass FPSF immediately after saying that you won't consider it further.
And it appears that you're contradicting yourself again. What you had asked in your last post was "If one is using a ThermoMass wall such that the footer is not below the frost line" and now you're asking about ThermoMass sans footer - and after I explained that a ThermoMass foundation requires a footing and is incompatible with the FPSF approach.
You cannot build a load-bearing wall on a constant thickness slab unless the slab has the same thickness as a footing (generally 8" minimum). A raft foundation is for expansive (clay) soils and requires engineering. They are generally 12"-14" thick at the ribs and perimeter and require a great deal of steel reinforcement, sometimes post-tensioned.
In further study of the Isoquick documentation (http://www.isoquick.de/en/downloads.html) it appears that there may be four or more inches of insulation outside of the edge of the concrete slab. To get the outside of the house wall in line with that insulation may mean cantilevering the wall out further than the 1" or 2" that are typical of the foundation insulation that I've seen. I suppose the 12" double-wall cellulose-filled frame wall could be designed to do that? The inner wall would be load bearing, and the outer wall would be totally suspended from the inner wall. Would the gussets that space the two walls need to be larger than 1"x4"@3'OC that appears to be typical of the Larson/Riversong Truss?
You wouldn't use a double wall (which is two independent stud walls, each requiring bearing), but a single load-bearing wall, sheathing and then a Larsen (not Riversong) Truss hung on the outside, as it was originally designed to do.
The IsoQuick documentation (http://www.isoquick.de/bilder/downloads/12_IQ-Datenblatt_engl.pdf) shows they are using slab thicknesses from 8” to 12”, and foam insulation of 6” to 12”, which corroborates Robert’s statement that single-thickness slabs should be sized the same as a footing. IsoQuick is promoting their raft foundations not only for expansive soils, but also for just about any other use for which a superinsulated foundation is desirable, including full basements. (http://www.isoquick.de/bilder/downloads/ISOQUICK_Flyer_E.pdf)
PS. Not only do I contradict myself, but I am also unverified. I’m wondering if that makes me an unverified contradiction? LOL