Shallow foundation in a cold climate i.e., MONTANA

OK, it's been 3 years and THIS year is the root cellar year, by God!  I have two good possible locations, each with their own challenges, but both are the side of a hill facing North.  So I go studying (again) on how to start with the foundation.  Where I live, the ground is loaded with rocks.  Seems every time I garden, I get a bumper crop! It's daunting.  Just to put anchor pins in to keep my hay shed from blowing away took an impact wrench and 24" carbide steel rod, which actually STOPPED at 18". No, that's not low flying aircraft, it's me, whining!

So I know my frost line sits at about 3' deep.  The thought of attempting to dig a 3' deep x 24" wide x 56'(total length) trench-works in an area that no backhoe can access, well, I just want to start   .  So me thinks, I will look until I finally read someplace, anyplace that says I don't have to do that and I will go with what they said! (Who's with me?!)  And sure enough, I found it on Earthbagbuilding.com/faqs/foundation.htm:

Would it work if I just dig about 1.5, 2 feet rubble trench and start just below grade instead of 4 feet (frost grade) with first 3, 4 rows of stabilized earth hydro-isolated on the outside, and an isolation skirt around the perimeter of the house to prevent freezing?

Yes, frost protected shallow foundations have been proven to be quite effective in eliminating the need for deep foundations and providing better thermal protection for the house.

So dig (well, pry rocks out of) a trench about 1.5 feet deep and I am GOLDEN!

Then the paranoid evil twin takes over my brain and says "You, my dear, are an idiot."

Thoughts?

Some good info this doc:
http://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf

I know it deals with more "conventional" materials... but may provide some good insights.

Dale Walker wrote:Some good info this doc:
http://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf

I know it deals with more "conventional" materials... but may provide some good insights.

Great resource. Thank you!

Remember frost heave comes from the expansion of water freezing. if it is dry it will not heave. So as you pry and dig consider whether water will get under the foundation and your plan to prevent it.
The worst frost heave I ever saw was on the military road south of Fort Kent Main which had a spring under it and it was kept plowed at all times. ye nearby were houses built on ledge rock close to the surface that never moved.

Hans Quistorff wrote:Remember frost heave comes from the expansion of water freezing. if it is dry it will not heave. So as you pry and dig consider whether water will get under the foundation and your plan to prevent it.
The worst frost heave I ever saw was on the military road south of Fort Kent Main which had a spring under it and it was kept plowed at all times. ye nearby were houses built on ledge rock close to the surface that never moved.

That's interesting to think about.  Both my house and my big metal building 36'x48' are built on 6" monolithic slabs. All they have under them is about 4" of crushed stone I believe, and they have been solid for over 20 years. I have wondered if trenching would actually disturb all that rock and make heaving more of a problem than it is from rocks that have settled and are packed tight naturally.  I would think a moisture barrier under and around the foundation would keep the water from getting in and under. Not sure how insulation would help anything in the case of a root cellar. I'm not heating the space.

First off, a clarifying question...with a bit of a preamble:
You write this is for a root cellar. When I visualize root cellar, I visualize a dig into a hillside (or down into the ground, in flat country) that is intended to use the very stable earth temps below frost-line to keep a "root cellar" temperature year round. E.g. 55 deg. F or something in that range, maybe a bit cooler in your location, but probably not below 50 deg. F. The structure I visualize is only enough (height) to cover this excavation, assuming the hillside isn't steep enough for it to just be a cave with a Hobbit style door or hillside mine-shaft type entrance into it.

So, question is, am I misunderstanding what you are trying to do in terms of "root cellar?"

The "shallow foundation" doesn't seem to fit the named use.

If you build an unheated structure in St. Ignatius MT that does not go below frost line inside (by pure depth of excavation and/or earth-berm to raise frost line around it), I am pretty sure you will have a freezer in the winter, not a root cellar...regardless of whether your foundation heaves...

Or, perhaps, if you insulated a big enough area around the structure well enough, AND highly insulated the structure itself, you might create a PAHS type bubble and get sub-frost-line temps to persist inside. So there's a trade-off to consider, between excavating deep in your rocky backhoe-inaccessible ground, or buying a lot of petrol-based insulation for what would be IMO more of an experiment than a certainty. A third option, supporting shallower excavation, would be earth-berming around the structure to raise frost line (possibly with some PAHS type insulation around as well). Is it easier to move earth to berm with with than to dig deep, on your site?

Also of interest might be, how did the "old time" euro-american colonizers of that area build their root cellars, if any did, or anyone still remembers?
They didn't have backhoes or petroleum-based insulation.

I'll stop there, let me know if I'm totally missing your point/question or not, any other details that would clarify...thanks.

John Schinnerer: yes, I agree, a root cellar needs the earth to have a stabilized temperature., so my thought is, since the ground is almost a solid layer of rocks, hard packed, that if I go deep into the hill (where there are less rocks and more soil), then have the structure bermed on 3 sides completely with a deep soil roof, I should be able to "recreate" the in earth environment - I hope. I still plan on trying to dig as deep as possible, so some testing of that will come first.  It's more like using a large pry bar than digging.  But man, won't I have awesome rocks for the front exterior! So yes, my thought is to berm and use 2" rigid foam insulation around the foundation.

A note on the hill:  it is really more of a cliff about 15-18' feet in vertical height.  From the path, if I go straight into that hill, there is at least 7' of soil before you hit the surface if I dig a cellar 8' high.  There is at least 10' of soil on either side of the 10' area I will dig out.

I'm near Billings. My front garden is a strip just below the raised front yard, against a wall of rocks and and dirt about 3 feet high. Apparently having the yard looming over that garden strip is sufficient to keep it from freezing hard (and there are always "chimneys" in the snow with warm air wafting up from between the rocks). If that little protection does that well, seems to me your idea is a good one and the hill should provide plenty of thermal mass.

Danette,

Any relation to Wayne, from Camus Prairie?  Old friend from a previous life.

As I see the situation, you're planning to tunnel into alluvium.  Sounds like a mining venture.  You might want to consider how to stabilize the seven feet of overburden during construction.  As for protecting foundation walls from frost heave, the two side walls and rear wall should be sufficiently buried to prevent any disturbance from frost.  Only the base of the front wall, and perhaps a short return on the side walls will be close enough to the surface to be vulnerable.  If you go up Valley Creek a short ways and walk up the steep slope to the north, you'll find an old homestead.  Look for the lilacs.  Northwest of the house foundation you'll find a root cellar with a barrel-vault concrete roof.  It's pretty cool.  Are the rocks on your property predominately red?

It may not seem green but in this case a little bit of 2" rigid insulation goes a long way especially since you only have one face to protect - theoretically. We do FPFs a lot with my grain bin designs - also with the evil petroleum based pour foam - it is THE best product for this - we're not using gas in our Hummer to cruise the mall but to get the best building with the highest performance for the least labor which saves the most energy. I've done some "digging" such as you describe in Arkansas/lower Missouri - what a lesson in frustration. Maybe put an oversized overhang on your roof to get the water away from the foundation and a swale uphill to divert the water. In Kansas I have seen barrel vault cellars (storm/root) made of limestone - truly magical spaces especially in a hot summer. This is a labor of love - get started and do a little - do a little more - decide when you've had enough - think of it as therapy. You will get to the desired depth without the need for any band aides. You could use a section of a bulk bin as your vault and lay your stone on top of that (would recommend mortar if the stones are not a good shape for laying up) and then cover with your excavation material - it should be there for a long time. If you use the bulk bin for your form load it evenly from both sides and some bracing is a very good idea or it will become your final resting place. Be careful. You could build the vault with the bag method - see Dream Green Homes. Best wishes.

Mark Clipsham wrote:It may not seem green but in this case a little bit of 2" rigid insulation goes a long way especially since you only have one face to protect - theoretically. We do FPFs a lot with my grain bin designs - also with the evil petroleum based pour foam - it is THE best product for this - we're not using gas in our Hummer to cruise the mall but to get the best building with the highest performance for the least labor which saves the most energy. I've done some "digging" such as you describe in Arkansas/lower Missouri - what a lesson in frustration. Maybe put an oversized overhang on your roof to get the water away from the foundation and a swale uphill to divert the water. In Kansas I have seen barrel vault cellars (storm/root) made of limestone - truly magical spaces especially in a hot summer. This is a labor of love - get started and do a little - do a little more - decide when you've had enough - think of it as therapy. You will get to the desired depth without the need for any band aides. You could use a section of a bulk bin as your vault and lay your stone on top of that (would recommend mortar if the stones are not a good shape for laying up) and then cover with your excavation material - it should be there for a long time. If you use the bulk bin for your form load it evenly from both sides and some bracing is a very good idea or it will become your final resting place. Be careful. You could build the vault with the bag method - see Dream Green Homes. Best wishes.

I have some good instructions tucked away on how to frame a vault, so those will come in handy.  I also saw this product -Concrete Cloth!  Not "green" but when it comes to some types of construction that can be dangerous, I tend to give a little.  I wonder if I could Lay it over the vault frame.  But I think I may want to insulate the roof as well, besides the several feet of soil that will be replaced on top.  I plan on digging out the entire space, then replace the soil, no cave digging for me!!

Greetings.

Perhaps you finished your root cellar?

My thinking reading over your thread, is that few people read about shallow, frost protected foundations.  I read about these things for the first time  5 or so years ago (more?).

As I understand things, in some circumstances these involve little excavating.  Almost like scrape the sod off, add some gravel and other worth, pour the pad (nominally on the surface) and put up your building.  Of course, I am skipping all the detail about putting the waterproof insulation "wings" around the building.  The idea is that they abut the foundation, and that "joint" is sealed or flashed so that water doesn't fall between the foundation and the insulation.  The insulation is installed under the ground, with enough slope so that any water that falls onto the ground next to the building, percolates through the soil to the insulation, and then flows "downhill" to "fall off the end" of the insulation, at some reasonable distance from the foundation.

But thinking about how cold the air and surface is, doesn't really give you much idea about how these foundations work (note, this idea also applies to wood chips around trees).  There is a constant amount of heat coming up from the Earth's core all year long.  By putting a continuous "ring" of insulation around our foundation, this deep heat cannot escape the Earth where the insulation is.  So at some distance below the insulation, the heat flow bifurcates; some of the heat flows to the inside and passes to the foundation and pad, and some flows to the outside (and is "lost").  The amount of heat flowing through the footprint of the foundation is considerably higher than the native soil presents.

If we have an 8x8 shed, that is 64 square units of footprint.  If we put 8 foot insulation out from the foundation edge, we are increasing the thermal footprint from 64 to 64+4*64+corners(say 2*64) equals 448 square units.  The thermal footprint is 7 times the structural footprint.  Since about "half" of the heat flow is diverted to the "inside", the heat from about 256 square units of ground is going to flow through the 64 square units of the structural footprint.  Which is 4 times the normal heat flow.

So, the insulation increases how much heat comes up through the foundation and pad, and it diverts water so that it can't get into the ground near the building close enough to affect it.

If you happen to put your foundation on top of a spring, or other wet area, the insulation may reduce the tendency to dry and cause other problems.  But places like Montana and here in the Peace Country, we are on the leeward side of the mountains and in rain shadow for the most part.

I too need to do something for vegetable storage, and was wondering what people were doing where it gets cold in winter.

Addendum

I glossed over another detail.  As heat flows into this uninsulated (or underinsulated) centre, the temperature of the ground (and foundation/pad) increases, which decreases the driving force for heat flow.  Consequently there is a limit as to how much heat diverts to the centre.

For some people, they have a hillside available that transitions to level.  I just have one long hill.

Perhaps XPS or EPS insulation is cheap where you are, it isn't here.  Apparently you can replace the layer(s) of foam insulation, with a layer of gravel (for drainage), a layer of straw, a layer of black poly, more layers of straw and poly, a layer of gravel and then put dirt on top.  Keeping the water out of the ground under the building, and to the sides, will keep the ground dry, which makes it a better insulator.

How do I heat this building in winter?  Diverting ground heat will help, but how much at -30C or colder?  Looking at thermosyphon stuff and earthtubes (that was the name I seen today, I've seen other names), I have an idea.

In the centre of your building floorplan, you dig a post hole (big enough to take 4 inch pipe (the Earthtubes site, suggests that there is something lighter weight than schedule 35).  Legally, I think I have to consider the frost line here to be 9 feet down (I doubt it gets that low any more, it probably did when I was in high school in the mid/late 1970's).  And then a person cuts a trench that has a slope of say 2% that goes from the bottom of your hole, downslope until it exits the ground.

The air inlet (downslope) will let in cold surface air in winter, and as it flows uphill in this pipe, it is also getting further underground and picking up heat from the Earth.  It will be at its deepest, in the vicinity of where this trench turns vertical and comes into your building.  Hence, in this region is where it will gets its warmest, and between being warmer than the outside, and the now vertical pipe, buoyancy should allow it to draw air.  I doubt it will be 15C air here, but it may be warmer than freezing enough to "heat" the root cellar in the winter?  You do need an exhaust vent, and I suspect some kind of solar chimney would probably work the best.

I don't know what size of building to build, and the need to insulate the ground around it means a pretty big footprint is needed.  I've always like dry stacked, surface bonded CMU but never had the chance to build one yet.  That should cut most of the the costs down.  The building probably needs insulation on the outside surface and probably a wood truss ceiling that is insulated.  Much of the excavated earth can go around the building.  I think if you fill your hole/trench with gravel after putting the pipe in to a reasonable distance, that hole and trench could also serve as a drain for the building.

I am on a north facing slope.  I think it makes the most sense to have the ceiling joists oriented N-S.  I believe the maximum span for a SPF x12 joist is about 10 feet, so the N-S dimension of the building will be about 10'.  The wall height may end up being 8 foot tall, I am not sure.  Depending on the hill you are building into, the "height" of the building could be considerably less than 8 feet.  If it is less than 6 feet, it becomes very likely that Bambi could be on your roof, should there be something interesting up there.  I have watched moose (Bullwinkle) jump my 5 foot fence, so Bullwinkle could be up there too.  Especially since the moose are only around in winter, and then snow, ice, hard wind-packed layers and so on make the concept of fence height ambiguous.  Other farmers in the region have had elk attacking their hay, I've never seen elk on the farm.  I am going to assume an elk is just a moose with different antlers.  So it could be up on the roof too.

I am pretty sure that having an "earthtube" bring air into the building, should be able to supply some degree of heating in winter.  How much I am unsure.

People have been known to put barrels of water into root cellars, to act as a heat source.   One thing that occurs to me, is a person could have tanks (rectangular parallelpipeds - aka boxes) up against the wall (tall, wide and not deep), which have a fingers coming out the "back".  The fingers are attached to a "copper fin" on the inside, which is sized such that if all the water freezes, the copper fin deforms enough to take up the volume change of the water freezing.  The other end of the fingers fits into holes in the wall.  The idea is that these fingers are in better thermal contact with the outside air, than the inside surface of the wall.  If it starts to freeze outside, these fingers will bring the cold to the copper fin, which will start to cool the water and start to freeze it, which will release heat to the inside of the cellar.  The question then becomes will the amount of water last long enough to protect the food which is inside?

I was thinking about partitioning the room so that one part could be apples (which I grow here) and the other part could be potatoes.  And maybe a person needs a 3rd part for squash?  Which has a different T/RH regime than potatoes.  Apples and other fruit which emit ethylene gas, need to be stored differently than potatoes.  As ethylene basically has the same density as air, you can't rely on it concentrating on the floor or ceiling.  So, if you have apples in the same building, they probably need to be stored in an old freezer (just the shell).  They can at least be sealed against the release of the ethylene gas outside of the freezer.  You want an air inlet on one side and an exhaust on the other side, and you might want a small fan to "stir" the air inside the container.

A flat roof would be the easiest to construct, and the x12 joists leave ample room for insulation, and will allow for under sheathing ventilation as well.  You still need to deal with extra water (aka rain, snow, ...) on the roof.  You need some overhang, if nothing else to cover the ends of exterior insulation on the walls.

I would put a solar chimney on the south side, about midway, to try and assist with ventilation in winter, and to drive ventilation in summer (which would tend to cool the root cellar)..

But, to have this flat roof on a farm, my thoughts are to put a living roof on the root cellar.  You still need to do something about excess water, which is mostly concerned with the edges of the roof.  So no soil there, that area is reserved for the water works, insulation and then maybe covered with solar cells to store energy for a few 12V fans inside the root cellar.  Maybe a few LED lights to illuminate things when the room is occupied?

The soil is somewhat of an insulator in winter (it is probably dry then, at least here) and is more like a lot of thermal mass in summer.  But, a living roof wants there to be less insulation in the ceiling, so that the roots in the soil don't get too cold.  Is passive heating going to provide enough heat for this?  If you do put a living roof on the root cellar, I think you need to put a fence (people building decks would call them railings) on the edges to keep Bambi and friends off the roof.  A deck wants railings that can take a 200 pound load in any direction with a safety factor of 2.5 (so a 500 pound load).  A mule deer is heavier than most adult humans (here); moose and elk are way beyond humans.  So you probably need to make your fence (sorry, I mean railings) quite a bit more robust than for a deck.

It may be, that part of the floor inside the root cellar needs to be raised up (concrete pad), which would make it drier, possibly partitioned off, and positioned such that the "winter heating air" entered this partition first?  Would that constitute conditions amenable for squash?  The walls at the ceiling could have a small gap, so that this "warm" air could then spill into the rest of the room, and then we would need to find some way to mix the energy.  Perhaps one pumps warm air towards the floor with fans (in tubes), perhaps one paints things inside so that radiative transfer is more effective?

If one is using dimensional lumber, thermal bridging is a concern.  So you could put a layer of styrofoam immediately next to the joists on the ceiling, and then cover with something like moisture resistant drywall.