I live in an area that normally requires a 44″ frost footing. Want to build a seasonal cabin without a basement & without elevating on piers. I’ve seen references to the use of rigid foam insulation horizontally around the perimeter of the foundation to reduce or even eliminate the need for so much depth.
Assume 2″ of foam buried at least 12″ underground, 4′ wide all the way around the foundation.
Assume that convincing an inspector is not an issue 🙂
Does anyone have a feeling for the amount of benefit in terms of making the frost footing shallower?
Thank you.
Replies
Check the NAHB reserch center http://www.nahbrc.org/ for a copy of "Design Guide for Frost-Protected Foundations"
You might also do a search at HUD http://www.huduser.org/ or do a google search on frost protected foundations
View Image
What you describe does work for a heated, inhabited house. But the idea is to keep the heat in. If this is a seasonal house, where is the heat coming from? Deep in the earth only.
So will the shell of the building be insulated?
It might still work if not but I would go to great strides to be sure that the ground will be able to drain any moisture out of it also. It is not the cold that does the damage. It is when moisture in the ground freezes and expands.
No water - no expansion - no damage - no doubt!
Excellence is its own reward!
"The first rule is to keep an untroubled spirit.
The second is to look things in the face and know them for what they are."
--Marcus Aurelius
My understanding, so far, is that they can be used for heated or unheated buildings, as they mainly rely on ground heat. But you do need thicker insulation for an unheated building.
So far we've only researched, no practical knowledge. View Image
Barry E
Edited 12/10/2002 11:41:03 PM ET by Barry E
Thanks for all responses.
Yes, the home will be well insulated.
No, the idea is to be able to leave the home unheated during the winter and get by on the occasional winter weekend with a wood stove - perhaps bundling the plumbing in one area and keeping it from freezing with a small electric heater, but that's another topic all together.
I was hoping that someone might say "yea, we did .... with our cabin and it's worked well even through some rough winters"
I don't want to sound cavalier about an important aspect of a house, but my impression of the issue is that frost footing code requirements are often overkill - that in 9 out of ten winters frost wouldn't be half as deep - that with lots of attention paid to good water drainage, it could easily be 99 out of 100 winters and that one horrible winter might not move any part of the house more than a 1/4" anyway. You'll have to parden me for sounding like a heretic, but 1/4" every 100 years I could live with.
We had a horrible winter here in 1991 when 28" of snow fell on Halloween and we didn't see the grass again until mid-April. The funny thing is, in some areas the ground didn't freeze all winter despite numerous sub zero nights - you see, all that snow acted like a blanket....
44" frost footings are deep, but if you have a machine digging for you and consider the cost of styro, why not do them to that depth. You could cheat that depth by a few inches aswell. I've used the syro method before, it works and it's kinda a pain. Make sure the inside is protected against frost aswell. I've also had frost problems before...and it was more than 1/4 ".
good luck
You have a good point under conventional circumstances, but here's what I'm envisioning;
1) A sprawling, single level building with 2-3,000 sq.ft., but possibly more than 400' of perimeter.
2) Doing all the work myself with little more than a bobcat with attachments.
3) Take a reletively flat site - scrape off the first 12" or so of earth - pile up excess around the perimeter - build "on grade" either with treated wood or 3-4 course block on conventional (surface) footing with standard framing above that - lay rigid foam around perimeter (& up the wall to the height of backfill) - then backfill with excess dirt sitting around perimeter - the result would be a gentle mound effect next to the house which could be disgiused with plantings, shed water well (assume 30-36" roof overhangs everywhere) and effectively insulate against frost heave without making the house look like an earth home or make you feel like you were living in a 1/2 basement while inside looking out.
4) To be effective aesthetically, the backfill could not be anywhere near 44", which is why I'm here. I was hoping for something in the neighborhood of 18-24" including the foam.
Under the circumstances, this seems like an inexpensive, easy, and uncomplicated approach - but you folks might think differently and I would welcome your opinions.
JLC has had several articles on this. I think the term is "frost protected" foundation. Go to http://www.jlconline.com and then Reseach. I believe that there is at least one free article over than and a couple of pay ($5) articles.
Also there might have been some in FHB. Go to the magazine button at the top of this page. Then on the right side is a link to the magazine index where can search for it.
You can now by FBH articles online also.
Thanks Bill,
I'm not averse to doing a little research. I actually have the FHB artical that came out a couple of years ago, which inspired this inquiry. There are a couple of important differences here, though.
1) The FHB story focused on continuously heated spaces.
2) the FHB story involved a fair amount of engineering, lots of concrete, lots of labor, etc made easy because they were working with a pro crew and they were working with an otherwise conventional design that had the interior floor higher than the earth directly next to the house - a big difference from what I described in my previous post above.
My plan - assuming it's sound - would be simpler, but a little unconventional and therefor, likely not addressed directly in a magazine article anywhere.
Here's an online version of that Design Guide. With a little effort, you should be able to determine the depth and insulation neccessary.
http://www.cs.arizona.edu/people/jcropper/desguide.html
"Listen, strange women lying in ponds distributing swords is no basis for a system of government."Jon
Wow, I cant thank you enough. That pretty much answers my question, although I would certainly welcome opinions from others regarding pros & cons.
If you dig down 2 ft and install your footing directly over SM and add 2 ft to the outside perimeter, you should be fine as long as you DO have good drainage.
THe area of concern is that directly beneath your footings. In an unheated circumstance, you want to make sure that the underside of a shallow footing is protected.
Gabe
It's true that the snow acts like a blanket and prevents deep freezing in the soil and you don't need to worrry about sounding heretical when it's your nickle. When we professionals build, it needs to be good enough to protect our reputations, meaning we pay attention to Murphy's law.
Good drainage is the most important thing. I believe that I remember a study done by some university or other outfit here in Maine, for dealing with posts and lighting footings etc in parking lots, to determine whether deep footings werer needed below frost level and how to modify that. In my memory, they concluded that one could place two inch foam board one foot below the surface and four feet out around the footing (Footing pad 18"x18"x18") with well drained gravel base and bare ground, and not experience heaving where a four foot deep footing is normally required.
I have also seen numbers of three foot deep piles heave up and out in wet areas with no protection.
Ice lense formation in the spring is another subject that can impact this but I'm not sure it would apply to your structure..
Excellence is its own reward!
"The first rule is to keep an untroubled spirit.
The second is to look things in the face and know them for what they are."
--Marcus Aurelius