Frost Protected Shallow Foundations (FPSF)

[windward]

Hello all. This is kinda long....so please bear with me....

I am looking to use a FPSF in a detached garage I am getting ready to build. I am planning to install radiant heating in the floor but am not sure that I will always be heating the thing. I may want to leave for a month or two in the winter.

I am putting in a bathroom, which is the primary reason I am trying to protect from heaving.

Since I cannot commit to always heating it, I am designing it as an unheated garage. (attached or pg 17 in the pdf). I will extend 2" XPS 48" horizontally, 10" below grade. In this diagram, it shows the insulation running continuously under the slab and grade beam. Since I will be heating it most of the time, I am also trying to take into consideration some design features of a heated space. So I plan to put in vertical insulation around the perimeter.

My question is this, does anyone see an issue with me setting the grade beam directly on the 6" layer of of "Gravel/Sand layer", then putting my under floor XPS directly under the slab?

If I have to put the XPS under the "Non-frost Susceptible Fill", I am worried that I am going to have to heat all of that material (sand or gravel) and it would lose some efficiency.


http://www.toolbase.org/pdf/designguides/revisedFPSfguide.pdf

 

[joes169]

I,m probably not far from you, and I'm a concrete contractor, so I've looked into this in the past. If you haven't found this yet, it should be a good resource:

http://www.engr.psu.edu/phrc/training/FPSF_webinar_March_09.pdf

Personally, I wouldn't place even the 25 psi foam under the gradebeam itself just to potentially save a fraction in heating, and we never have. For gradebeam foundations, we've always foamed the inner "hump" and the 45 degree bank, but never the horiz. section of gradebeam. It wasn't until recent years that many building inspectors wouldn't even let us pour the gradebeam on gravel/stone, it could only sit on virgin soil........

 

[bfarroo]

I am planning on doing just what your saying. Have the concrete guys get their grade set and forms in place and then go in and lay my foam down inside the formed area and down the angled sides of the grade beam area but leave the bottom open to the gravel. Then do the outside with 2" foam vertically and horizontally. I was personally only planning on 24" wings. I plan on heating the building continuously over the winter but it will be a few years before the system is up and running. I'm going to make sure that there is proper drainage when the sand/gravel fill is brought in to keep any moisture far away from the slab. This will also be easy although more expensive for me as the land I'm building on has a slight slope away from where the house will be so the entire area for the shop will need to be filled.

My question for the concrete guys is when pouring knowing that the outside is going to be insulated is it easier to set the forms 2 inches wider all the way around and pour the concrete against the foam or just attach the foam after the forms are removed? I've heard stories of the foam "Floating" but they always seem to be someones brothers, uncles cousin told them and not actual fact.

 

[koditten]

I don't like that. Why would you want to heat all that gravel? I have my FB between the gravel and the bottom of the slab, as well as the sides. If you do go that route, please cosult a pro. They are going to ask you to use bigger pex tube and space it closer together. The radiant heat goes down as much as up. I would want my heat to move to the surface as fast as possible.

If you completely encapsulate the sides and bottom of the slab, you will not have a frost issue. the ground is pretty much 50*. That gravel and what is below it will be holding that 50* heat under the slab. Second, if you have taken good pains to ensure good drainage of the gravel and foundation fill, there will not be much water to make the slab heave.

 

[windward]

bfarroo,

Take a look at the last page of the pdf at this link. I think this would be the best way to form it up.....use the foam as a form. Obviously you have to back fill first.


http://www.ferry-county.com/Planning/PDF_Files/Sample%20Plans,%20Span%20Tables,%20etc/Frost_Protected_Shallow_Foundations.pdf

 

[windward]

What is everyone's thoughts about sand vs gravel for under the slab? My excavator was going to strip the topsoil and add some type of sand that he said packs really nicely (it is cheaper than gravel). My first though was great, save some money. Then I started thinking about drainage. I suspect that gravel drains faster/better than sand, but maybe not. Maybe they are both pretty much the same.

For what it is worth, below my top soil is all beach sand, very fine and does not pack well, but should drain nicely.

 

[pstnbly]

w
We call FPSF's Alaskan slabs around here. We use washed crushed stone as a base and to form the hump. The entire surface is insulated, and yes you can line the formwork with insulation.

Crushed stone is used for drainage, I would not use sand as it will hold moisture possibly leading to heaving and cracking.

 

[BadgerBoilerMN]

You must build for an unheated FPSF, i.e. insulate under the slab, the shallow foundation and wings to the perimeter including extra attention to the corners.

25psi XPS or EPS are more than enough, and in many cases better than uninsulated footers since the consistency of the foam is nearly perfect unlike the soil.

The tubing does not go in the foundation section but in the field. Keeping such a structure above freezing is not difficult or expensive.

The Swedes have been doing radiant floor heating in Frost Protected Shallow Foundation for 50 years, and I with my Dad for 35.

Trust your concrete guy to pour 4000# concrete and finish it, leave the engineering to someone else.

http://www.badgerboilerservice.com/hrf.html

 

[windward]

You must build for an unheated FPSF, i.e. insulate under the slab, the shallow foundation and wings to the perimeter including extra attention to the corners.


http://www.badgerboilerservice.com/hrf.html

Badger,

I was looking at your "Badger Radiant Designs FPSF.pdf". I do not see horizontal wings on the exterior of the grad beam(shallow foundation). Is this drawing how you would design it? or were you saying I should follow the instructions for an unheated garage that were in the link from my original post?

Thanks
Matt

 

[BadgerBoilerMN]

I would reserve "wings" for certain climates, but more insulation never hurts anything but the pocketbook. I always design FPSF insulation for "unheated". It is a bit counter-intuitive but we have not even scratched the surface of this matchless technology let alone embraced the radiant floor aspects.

The combination demands the techniques illustrated above.

BTW this is common practice the southwest where basements are not part of the culture and frost is skin deep. Insulation makes it all good, all over, including on perma-frost where "footings" never were or will be. Thinking outside the box makes me feel good.

 

[jJon G]

Very Interesting !

 

[oilslick]

Can I build a 2200 square ft house fpsf like this. Not trying to derail thread,just trying to learn about pro's and cons! A house would always be heated ,if that affects planning. .