This is how they do it

[RIjake]

I sat with the lumber company last week and we went over my materials list, getting final pricing before I place the order and we get to talking about my options for a foundation.  This gentleman is also a small time independent builder who's M-F gig is selling building materials.  He's very familiar with the area where I'm building and understands the challenges we face in this part of Maine.  It's VERY boney in my neck of the woods! 

So he recommends a base for post foundations like this



The way he describes it is like this; you scrape of the layer of topsoil which is about 6-8" of composted leaves and shallow roots.  Under that we have a very dense layer of Berkshire soil that drains very well but is loaded with glacial till.  You don't disturb that but lay a thin layer of crushed stone, then a layer of 2" rigid insulation( he spec'd 25psi).  Then a precast 6" thick by 18" concrete pad sits right on top of the insulation.  The post stands on the pad with no mechanical fasteners, then the posts are braced all around with 6" PT.

He claims he's built many camps like mine using this method with no or very little movement.  We actually met in the model log home the company has on their property.  He built the model himself and used this method for the foundation.  There was no racking of the doors or windows in the model.

I'm considering going this route not only from a cost standpoint but the labor would be very little.  It is impossible to dig post holes by hand here and even with a machine it would be a nightmare to trench 4' down.

Any thoughts or comments on this method?

[Alan Gage]

Looks like the same principle as Frost Protected Shallow Foundations. Never done one but did lots of research on it when getting ready for my build.

Alan

[bayview]

  I would think sooner or later frost would get under the Styrofoam and lift the foundation. . .

/.

[MushCreek]

Maybe it's not very windy there, but I would want to know what's going to hold the building DOWN, rather than what is holding it up.

[Squirl]

Same general principle as an FPSF.  There is a different execution on how it is accomplished.  This method uses a lot less foam than fpsf. Also this method puts the foam under the footing instead of to the sides.    So if you have an 18" pad that is around 254" square inches with a load capability of 6358lbs.   I will use a popular building, 20x30 1.5 stories, as an example.  So you would have 600 square feet times 40 pounds per square foot times 2 stories, with 70 lbs snow load per sq. ft. X 600, which would be 90,000 lbs, without the weight of the building materials themselves.  Even So if you were to go with spacing 6 ft between piers you would have 12 piers to carry that load, or 7500 lbs per pier. I don't know your design so I can't say what yours would be, but my assumption is there is a heavy snow load in Maine to account for.

I did not see diagrams of stryrofoam under the footing of a pier in any diagram I had seen.  It was always under the slab or outside.

I completely screwed up and deleted my prior calculations from this post.  I mistook psi for psf. Sorry.

[JRR]

I would consider digging well below frost line, lining the hole with foam, installing rebar and pouring concrete posts in place ... forming and extending upward, not using pretreated wood at all near earth.  This scheme has no foam underneath supporting heavy loads.  The insulation foam board vertically around the concrete post should minimize the freeze-jacking of the earth.  The bottom of the hole can be large enough to support vertical loads ... downward and upward.

[rwanders]

I would place the foam on top and on sides of the footing. Ground freezes from top down not from bottom up. You can also place rigid foam on surface over the footers extending outward for a few feet if they are accessible in your crawl space for additional frost barrier. I have never seen a frost barrier other than compacted gravel for drainage under a footer in Alaska where frost line is a minimum of 48 inches below grade. You don't want to block any residual heat from deeper in the earth----even in very cold areas it is there.

Here areas covered by deep snows stay significantly warmer---areas under parking lots or other areas where snow is kept plowed freeze much deeper and quicker----insulate over your footers and extend outward as far as practical. Also helps to let the snow cover remain undisturbed around perimeter of building for 3 or 4 feet out as much as practical.

[Squirl]

Again, I screwed up my math.  

There was a reason I wasn't finding under the U.S.D.A. it was the U.S.H.U.D. that did the study.  Also most of the info can be found at N.O.A.A..  Here is all you ever wanted to know about F.P.S.F. for heated, unheated, stem walls, and slabs.

http://www.cs.arizona.edu/people/jcropper/desguide.html  

It looks like year round heated buildings use vertical insulation to trap the heat of the building in the soil below the footing to prevent frost, and unheated buildings use horizontal insulation to trap the heat of the earth to prevent frost.  This requires much more insulation.  

[RIjake]

Thanks for all the replies.  I had a bad feeling about this design and raised all the above concerns to him when we were going over it.  I'm going to do it right and deal with the excavation and concrete.

Thanks again

[Alan Gage]

I would worry most about the compressive strength of the foam.  I believe the average compressive strength of stryrofoam is 40 psi, but can go up to 100.  This is a LOT lower than the minimum compressive strength for concrete footings (3500 psi) and footing Soil (1500 psi). 

Footings and soil are rated in PSF (Pounds/square foot). The foam is rated in PSI (pounds/square inch). 25 psi foam has a bearing capacity of 3600 PSF, so it should have no problem supporting the footings. People use foam under footings all the time and it doesn't seem to be an issue.

The reason for foam under the post is that it traps the earths geothermal heat and keeps the soil above freezing temperature. That's how FPS foundations work. I've read that 1" of foam has the same insulation value as 4 feet of soil. I'd certainly do some more research but I wouldn't write off the idea just yet. Especially seeing as how you said you already have good draining soil, which will greatly reduce the chance of frost heaving. Personally I think I'd want to run the foam out past the edge of the footing a foot or so rather than keep it flush to trap the heat a little better.

Some good reading on how frost heaving works:

http://www.buildingscience.com/documents/insights/bsi-045-double-rubble-toil-trouble

And good information on Frost Protected Shallow Foundations:

http://www.cs.arizona.edu/people/jcropper/desguide.pdf

Alan

[Squirl]

Alan you are completely right. Concrete is measured in PSI, soil in psf.  I will edit my post so as not to perpetuate incorrect material. I am very sorry rijake.

[Squirl]

Alan, good catch.  I guess there would be a problem with 18" footings overall for that kind of foundation (if there were only 12 posts).  At 256 sq. inches that would be around 1.8 sq. ft.. If the soil is good and bears at 3000 psF, it would bear 5333 lbs.  That is very good.  It is still below the design calculations of two 40 psf clear span floors and a 70 psf clear span roof.  So I guess it could work for RIjake depending upon what his foundation plan and house plan is. I had read the same info about 1" of insulation and 4 ft.  All designs shown have some "wing" insulation extending out from the footing. So depending on the foundation and the data from Alan, it may work well.

Again, I am very sorry for steering you wrong RIjake.

[RIjake]

no problem, after reading today's posts I did some more research and found the psi/psf difference too .

I'm going with my gut here, I don't like the idea of this method and seeing as though in the future it may be more of a permanent home I'm going to get pricing for a 4' frost wall.

[rick91351]

no problem, after reading today's posts I did some more research and found the psi/psf difference too .

I'm going with my gut here, I don't like the idea of this method and seeing as though in the future it may be more of a permanent home I'm going to get pricing for a 4' frost wall.

[Squirl]

Ok.  I've spent most of the evening reading the HUD guides to FPSF and I think I have a better understanding and can differentiate the design the individual told you about from the building code and conventional practices.

First, the ICC only recognizes FPSF for year round heated homes.  I assume since you said this is a cabin right now and it is on piers, it would not qualify as a "heated structure" for the entire winter. 

There is a section on unheated structures.  According to the handy dandy NOAA map you are in the 2500 air freezing index area in Franklin County, ME.
According to step 1 of the guide you have to have a soil cover of a minimum of 10 inches. You also need a minimum of 6" of rock below that. So you would need to excavate a minimum of 16".  You CAN put the footing over the insulation. (sorry again) Next you jump to the end of the guide to table A8 to get the size.  You would need a minimum of 63 inches from the edge of each footing in all directs.  So if you had an 18" square footing you would need. 144 inches (63+18+63 ) in all direction.  This would be 12 ft. squared or 144 square feet of insulation for each footing according to the guide.  You would probably get some overlap, because you would hit another pier within 12 ft.  So figure 40x12 in insulation for each post wall. So if you have all your posts on two sides, it would be 960 sq. ft of insulation.  Since the average yearly temp is around 40 degrees the table minimum would be R-11.4.   
In conclusion the difference between the HUD guide would be you would be required to excavate 960 sq. ft. of earth 16" deep.  Add 18 yards of gravel. (960x.5 / 27) Lay down 2" of rigid foam. It is around $32 a sheet for 32 square ft or $1 per square for or $960.  Then cover this with earth.  On the plus side you would save some money with shorter posts.  I hope I didn't screw something up this time.

[Don_P]

Aside from cost, a nice warm soft layer 10" underground and a wood structure above, sounds like vermin condos 

For the cost I think I'd try to trench down to frost depth and be done with it.

[MountainDon]

Squirl, you are more than likely on the right track with those calcs. I believe the original example set forth is another one of those things that has been done, guided by a seat of the pants sort of feeling. As far as we know nothing has shown failure. Yet. But that doesn't make it good or acceptable practice in the long run.

[RIjake]

I'm waiting to get pricing back on a 4' frost wall foundation.  I'm also getting alternate pricing for, what the concrete contractor called a rat slab or a thin slab.  I'm very curious to see the price comparison between the two methods.  Since there is potential for this to be a somewhat permanent structure(7 months a year) I want a more permanent foundation with the house properly fastened to it. 

I believe the pad method that the lumber salesman was describing works in this area but I don't think it's for me.

When I see the pricing I can make a better comparison between all the methods. 

BTW the excavation work on this project has and will be done with rented equipment and a very good friend with the skills, who has generously donated his time to making this all a reality.  So far I've got about 800' of improved road in now and I think 1-2 more days of work I can get the septic and foundation excavated and my 20' shipping container moved about 50' and leveled.  I think that those concrete pad will be perfect for under the connex!

[RIjake]

I got the price for the 4' frost wall and rat slab.  It seems to fit in the budget and I know that I will never regret spending the money.

With the above foundation how necessary is insulating the floor?  Is this a project that I can take on later as I have the funds? Or will the cold floor be very noticeable?

[Alan Gage]

I put 4" of insulation under my slab and 3" around the perimeter to thermally isolate it from the stem wall. I've got 10" thick walls though so they do a nice job covering up the foam between the slab and foundation.

I know some people around here only insulate under the outer 4' of the slab and have been happy.

It's not something that will be easy to do later, especially under slab insulation. I wouldn't skimp on it.

Some reading:

http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11490

http://www.buildingscience.com/documents/information-sheets/slab-edge-insulation/?searchterm=slab%20insulation

http://www.buildingscience.com/documents/profiles/etw-minneapolis-profile/?searchterm=slab%20insulation

http://www.greenbuildingadvisor.com/blogs/dept/musings/can-foam-insulation-be-too-thick

I don't know if you'll be able to see this without being a member or not:

http://www.jlconline.com/cgi-bin/jlconline.storefront/4e34b5871384465f27170a323cb40674/UserTemplate/69?s=4e34b5871384465f27170a323cb40674&c=3395ad8cc0934a5a00d09453ada31a03&p=1

Here's one from Maine, but again, you might have to be a member:

http://www.jlconline.com/cgi-bin/jlconline.storefront/4e34b5871384465f27170a323cb40674/UserTemplate/69?s=4e34b5871384465f27170a323cb40674&c=cf6e9330206406dda640922be6253f18&p=1

Alan

[RIjake]

Oops, I meant insulating under first floor, ie under the floor joists.