The IRC now shows insulation as an accepted way to prevent frost movement, instead of a footing below frost depth you can insulate out from the perimeter of the building.
Has anyone been doing this? Done it at all?
I’m thinking I could build homes with a mono-pour foundation (like I did years ago in Arizona) and that it could save money on the construction and on heating the home in the future.
I would be building with ease, economy, and solar gain in mind, but I’m really just wondering if anyone is building foundations in that manner.
Thanks for your input!
Replies
This has actually been advocated for a long time, and I know it's been done occasionally in Minnesota, with an engineer's sign-off.
The schemes I've seen involving running foam insulation down the foundation wall to a depth of two feet or so, then outward from there for several feet. You need the insulation buried that deep to protect it mechanically, so obviously this technique is only a "winner" when the frost line is 4+ feet down.
Of course, with this scheme you need to be fairly confident that the structure will remain occupied and heated.
Another scheme that's been used occasionally in Minnesota is a grade beam foundation. Concrete beams are built more or less on grade, with the structure on top. The beams are sufficiently rigid (usually incorporated into a slab, with some sort of crushable filler in the middle) that the entire structure is guaranteed to move as a single unit, "floating" on the surface as it freezed and thaws. (Of course, utilities and such things as door stoops must be built to accomodate any possible movement.)
"Of course, with this scheme you need to be fairly confident that the structure will remain occupied and heated."You can do it with unheat buildigns also.But, if I am not mistaken, the data is the codes is for heated buildings so you would need to prove your design.
I don't follow. Why only if the frost depth is 4' down?
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
You have to put the insulation a couple of feet down, for mechanical protection. So if the frostline is two or three feet down it's simpler to do it conventionally.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
I'm still not quite clear. By "mechanical protection" do you mean protecting the insulation from physical damage?I specifically asked our inspector if the insulation had to be any certain distance below grade. He stood right there, looked it up and said "No".
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
Yep, that's what I mean. But I'm just talking about the systems I've seen/heard described, and this was about 15 years ago. I'm sure there are other systems.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
"you need to be fairly confident that the structure will remain occupied and heated."normal ave year round soil temp is 55-60°F in North America. The insulation keeps you from losing this to radiant loss so heating the building is not a necessity
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
This system is called the Frost protected shallow foundation and has been used for decades in Scandinavian nations.The deal is that cold air freezes the moisture in the dirt down a certain depth. Lets say 4 feet max. Below that the earth is a constant temp of say 45 degrees. The dirt is a poor insulator so it freezes down 4 feet in this case. Now lets take a really good insulator and put it on top of or close to the top of the ground (and cold air). What we are doing is keeping the cold air from reaching the warmer (50 degree) soil. Where the building and insulation is the heat from the earth will come right up to it and keep the soil right under the insulation from freezing. Of course you need sufficient insulating capacity there. I'm considering using this for my own garage here in Massachusetts next year. I'll go down 18-24" vertical then wing out at 45 degree angle down and away from the foundation for a length of 2 feet in all directions. That will put the insulation about 3 feet down total. I'll probably spray Corbond down something flat that I lie in place. Perhaps right on the dirt if that will work. 3" should work out just fine (~R20) I'm thinking.Stu
The deal is that cold air freezes the moisture in the dirt down a certain depth. Lets say 4 feet max. Below that the earth is a constant temp of say 45 degrees. The dirt is a poor insulator so it freezes down 4 feet in this case.
There's a common misconception here. The dirt just below frost line is not 45º or any other relatively constant temp. As you mention, dirt is a very poor insulator. The warmer dirt below frost line is going to lose heat to the colder dirt above it. Will not stay a constant temp.
You have to go down about 20' to find an almost constant temp. Here's an example from Montana:PAHS Designer/Builder- Bury it!
Tom,
You are exactly correct. I did say "below" that it's 45, just not how far!
Right next to freezing is of course 33 degrees and so on... a gradient down to the constant temp/depth. As I was writing I caught myself but tried to keep it simple and short. Cheers.
Stu
The important thing is that you, and everybody, understand what's really going on. Brevity is good too, not that I'm very proficient.
For PAHS, we use dirt for heat storage, it's that poor an insulator. Not that it's a particularly good storage medium, just has an attractive price and is readily available.PAHS Designer/Builder- Bury it!
As you know, our place isn't exactly normal, but we do have standard footings under exposed walls.
18" frost depth. I wanted a footing corner considerably shallower for less excavation on the rest. Asked for a variance. Supervisor looked at my drawings that showed a concrete patio slab underlaid by xps and crushed stone on that corner. He asked if it would work for me to consider the bottom of the slab "grade". Sure, saved me more than a few yds of concrete and a bunch of digging in rock.
Clear to him that the insulation prevented potential frost heave. He noticed, so I offered to explain why I was using insulation under an exterior slab and not under the interior slab. He wasn't interested, figured I must have my reason. Yup.
Your assumptions on construction and heating costs are correct. If you want to quantify, you can do heat loss calcs with varying ground temps. Or if you have enough to warrant, there's an engineer in the Denver area who offered to do similar modelling for me.
PAHS Designer/Builder- Bury it!
Thanks for the input, Tom. I was hoping you'd see this thread. I'll keep the engineer in mind.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
You're most welcome. I don't read most BT threads. If/when you particularly want my input, give a yell. Engineer's listed, with email and phone number (if they're still correct), at the end of that PAHS excerpt. http://www.axwoodfarm.com/PAHS/UmbrellaHouse.html
Know other places to track him down if necessary. I have no experience with him, have manually done my own calcs. Tedious but not difficult with the right book. PAHS Designer/Builder- Bury it!
Thanks again, Tom. I printed off the article.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
I have done a couple of houses and additions with shallow footings in Nova Scotia. Conventional footings have to be 4' down. An inch of XPS is considered equal to a foot of dirt. You want a foot of projection from the wall for every foot of dirt you are leaving off.
Put it in with enough slope to drain well and down deep enough that the dirt over the insulation will compact properly and not turn to mud every time it rains.
Practically, this all means that the footing will be 2 feet or more below grade with 2" XPS extending 2 feet from the wall starting at the top of the footing. This is fairly easy to install. More insulation/less depth or less insulation/more depth become more hassle than it's worth for me.
Ron
Thanks much for your input, Ron.I'm kind of surprised that the concept hasn't caught on more.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire