Frost protected shallow foundation
Salibonz
| Posted in General Questions on
I have a question regarding frost protected shallow foundations. Why are they not popular in the northeast US? They seem to be a more efficient way to build a slab foundation. I understand it’s not usable in heavy termite areas, but why is it not a popular option for climate zone 5 and 6 otherwise. I appreciate any input because I am considering it in northeast Pennsylvania in climate zone 6.
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I had a hard time finding anyone interested in doing one. If I wanted slab on grade, the only option was a 42" frost wall. At that point, you quickly start to approach the cost of a basement. We ended up with Superior Walls. I almost went with WarmForm but never found someone willing to install it. Even found a builder who had already done it nearby, but he said it was tough to get his concrete guy to do it.
I am planning on doing the foundation form work myself with monoslab ezform but having a crew pour the slab. You are saying your builder had trouble getting concrete guys to pour it? Why would that be? Also, I'm bewildered why fpsf aren't more popular and not become the norm in cold climates.
People are obsessed with basements. They still consider them "free square footage."
Yes, that builder didn't do the labor, he just hired out the concrete and form work. If you do the formwork yourself and have a crew pour, I think that's a great idea. In retrospect, I wish I would have. I did the labor to set 6" of sub slab foam, vapor barrier, rebar, PEX. I wouldn't say it was easy, but doable. My initial estimates on the basement cost were off by a lot. You could have an excavator flatten the area and put down stone and go to town.
I've used FPSFs frequently for 10+ years. They are different than what people are used to, concrete contractors can be nervous about things they haven't done before, many people consider slab foundations to seem "cheap," and like Tim said, they see basements as free space. A properly detailed basement is far from cheap but it's still relatively inexpensive square footage.
Many people just don't like the idea of a finished concrete floor, for aesthetic or comfort reasons; in those cases, a slabless slab can make a lot of sense: https://www.greenbuildingadvisor.com/article/minimizing-concrete-in-a-slab-on-grade-home. But those are much more rare than even concrete FPSFs so there is a good chance you'd need a licensed engineer to approve it.
Hey Mike, I have a question regarding slabless slabs. So basically the structure is carried by the stem wall and the interior section is just drainage stone, insulation and 2 layers of advantech. Can the stem wall be a frost protected shallow foundation where the depth is reduced by encapsulating it with insulation and then doing a slabless slab on the interior. I'm just curious if this can be engineered to work?
Yes, that's the only way I've done it; in the example I shared (behind a paywall, sorry) our frost depth is 48" and the footing was only 24" below grade. It could have been less but we had a slope to deal with.
You don't have to use two layers of Advantech; my approach is a layer of 2x4 sleepers and then a single layer of Advantech, which results in lower carbon emissions and lower cost. I've run the numbers many times and sleepers always work, though there are fine points to consider. But there are many more slabless slabs using two layers of Advantech.
A bit of an aside... Apparently slabless wood on grade floors were not uncommon before the widespread adoption of Portland cement.
Here's an illustration I happened across depicting a turn-of-the-century "erecting floor" in a factory: wood sleepers on compacted stone supporting wood flooring. (ICS Reference Library, Vol 51, 1905.)
If you add perimeter drainage, a vapor barrier, Roxul Comfortboard 110 frost wings, and Glavel you have a PGF (Pretty Good Factory.) ;)
Ok Mike, can you elaborate on doing a fpsf with a slabless slab, I appreciate the tips. Also would I need an engineer to design it? I am planning on using monoslab ezform for my fpsf... so would I then do the slabless slab on the inside of the monoslab forms. I'm sorry to sound redundant, I just want to wrap my mind around it, and see how I can go about using this foundation. And lastly why aren't all house foundations done this way. It seems so much easier and cheaper that the footprint could be increased to make up for the loss of a basement? Really thank for your help, I appreciate it
If you don't have or don't want a GBA subscription, you can probably read my article here for free: https://www.finehomebuilding.com/project-guides/foundations-and-masonry-work/minimizing-concrete-in-a-slab-on-grade-home. I think I covered the important points but let me know if you still have questions.
Slab foundations (with or without concrete) are good in many ways but they aren't so good if you want future access to plumbing, if you want to minimize your home's footprint (with mechanical systems in a crawlspace or basement rather than above grade), they are much more sensitive to proper site design and preparation than a foundation that extends below the frost line, they may not have enough uplift resistance in high-wind or flood-prone areas, and other reasons. They definitely aren't suited to a sloped site, which my projects often have. I still find them to be practical in many situations but every custom project is different.
Hey Michael, great article and very helpful. I just have a question....how deep was your stem wall? I see you packed in 4 (6 inch deep)intervals of compacted stone. Is a 24 inch depth of stone necessary? Monoslab ezform requires only 4-6 inches but do you think it is better to do 24 inches or was this to level the slope to grade? Michael you are a great help and I appreciate it
It was about 24" below grade. The IRC requires only 12-16" below grade here but that's for conventional foundations; when you have a lot of insulation under the slab, you need to make up for the lack of interior heat making its way to the ground. ASCE 32-01 is the document structural engineers can use to design non-standard FPSFs, or the simplified version attached:
And 1 more question, is this system more cost effective than a traditional footing, stem wall slab design? It is definitely more energy efficient. I am loving this design but I was thinking of using 6 inches of glavel and 2 to 3 inches of gps foam below slabless slab? Giving an r value of 20-25. Any critiques on this?
It's hard to beat a concrete slab as a finished floor on cost. But if you don't want an exposed concrete floor, it should be roughly equal in price to skip the concrete and use either two layers of Advantech or a single layer over 2x4 sleepers.
I haven't used your proposed assembly myself yet but I've consulted for others planning that approach and I think it should be fine. You will need to use a layer of sand or 3/8" stone to create a level surface.
If you want to go foam-free, which I encourage, you can just use a deeper layer of Glavel, do an extra-good job leveling the surface and shim between two layers of sleepers to get the surface level.
In northern New England, R-20 to R-25 under the slab is "Pretty Good."
Thank you mike, you've been a big help. I appreciate your expertise and advice
I would think that in places that get heavy snow on-grade construction in general isn't very practical, you want to be able to open your doors and get out after a snowstorm, and snow piling up against outside walls and then melting is going to be a problem.
With both slabs and conventional foundations I keep the framing 8-12" above grade. Snow drifts so the height above grade isn't a major factor at doors. At least in my experience.
I have a question regarding heated vs non heated structures. If a home is required to maintain 64 degrees what happens if there is a power outage or if it becomes a vacation home and isn't heated in the winter? I understand that if you remove 1 of the 3 situations for ground freezing it wouldn't be an issue, but is that a safe bet? Isn't a safer bet to build all fpsf as an unheated space to avoid risks?
We just did a fpsf this year, very happy with it. We are in climate zone 7A. I will attach a few photos. We did all of our own form work, insulation (R20) poly(10 mm)and rebar. I hired out the plumbing and concrete. Concrete guys didn’t have any issue with the pour.. just seemed like a typical slab pour to me.
I put 1.5” halo interra above the slab, and two layers to T&G 3/4” plywood, glued and screwed. It really does have a nice feel under foot.
The slab was signed off by an engineer, and our local geo tech company recommended 8” or foam, 8’ out for our frost wings. Which seems crazy, but there have been foundation issues in our town, so they have went heavy to cover there liability.
Thanks for the pics northof60. It looks great. The halo interra and 2 layers of plywood is more for comfort, correct? Could the slab have been completely omitted to reduce cost increase speed. Is that doable? Or would the structure be compromised m
I think that would have been an option yes. Like other have mentioned a slabless slab is not overly common. I personally wasn’t comfortable taking the perceived risk of trying something I didn’t know much about.
The halo is indeed for comfort. I work in construction and didn’t want to come home to stand on concrete all night. We originally were going to do in floor heat with exposed concrete, but with our increased insulation levels (R56 wall / R90 roof)and air tightness targets (1ACH) it seems like the added expense of the infloor heating system was overkill. So between the infloor not being needed, and not wanting to stand on concrete for the next 50 years we decided to move towards halo and two layers to T&G.
I see, ok, I appreciate the help, you definitely gave me food for thought. I'm considering your method and like the idea of the interra and 2 layers of plywood. My thing is that I'm building a speculative home so I'm trying to find ways to lower costs and still be energy efficient. With fpsf I remove most escavation cost, cost of stem wall concrete(although I add in insulation forms) but that's an advantageous cost. Thanks for the help
Sailbonz,
The thing that usually precludes thickened slabs here and necessitates stem-walls are:
- Most lots aren't flat, so to use just a slab you have to flatten them, which creates problems integrating the house into the existing topography, and can use a lot of fill.
- Many lots have a couple of feet of overburden before you get down to a good bearing substrate. So you are doing a fair amount of excavation and filling with or without stem-walls.
Great article... I am not a big fan of basements for many reasons, not the least of which is the high imbedded carbon costs of the average basement. One of the lesser known or talked about risks assocaiated with basements is radon. Many scientists and health officials believe it is the second most prominent cause of lung cancer behind smoking. While there are radon mitigation strategies, I think the best strategy is not to build a house with a basement, particularly a basement where children will spend a lot of time.
Nick,
Basements seem to be one of those regional practices that people in other areas don't see the merit of - and yet where they are typically used people can't imagine building without one. I'm glad they aren't popular (and often aren't feasible) here on Vancouver Island. I know of one in our small community, and right now it's flooded.
Hey Malcolm, you stated that fpsf aren't doable because of sloped lots....but couldn't you use it even for a conditioned crawl space? It that way by the "revised builders guide to fpsf" you can have a 24 inch rise on one side to compensation for the slope. Although it won't only solve a 5-6 percent slope it can still be used. Is this doable? Or am I misunderstanding it? Thanks for your input in advance Malcolm.
Salibonz,
My comment as about thickened slabs, not FPSFs. No reason I can think of think of not to use a FPSF on a sloped lot.
My apologies I misunderstood. I'm new into home building, been doing commercial for over 20 years. So I can't understand why fpsf have not become more popular in my area. To most of the architects I mentioned, they were baffled and when I mentioned slabless slab they really thought I was making things up. So I guess I'm trying to figure why it hasn't become more common
Salibonz,
Interesting question. Inertia probably plays a big part. The other is that most places where you have to go down very deep have traditionally opted for basements. People now expect them.
I learned about concrete-less slabs here on GBA, and have to admit it took me a bit to get my head around the idea. Architects and builders are typically a conservative bunch. In some ways that's good when the stakes of failure can be so high, but it does mean innovation happens slowly.
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