I am planning to build an ICF house next spring. What I am wondering about is the foundation and freezing. While the foundation is under the frost line the dirt under the slab is above it and can freeze if it isn't warmed by the house. So my question is why doesn't this cause a problem?

We built a 30x60 foot garage for my dad years ago and we did it mostly on our own. We only called someone in to do the walls and put on a metal roof. What we did was pour 6 foot deep piers that were 8 foot long by 4 feet wide at the four corners and several places in between them around the edge. Then along the 60 foot direction we poured two footings that were 2 foot wide and 2 foot deep and ran the 60 foot length of the garage. After that we put in gravel to support the slab and then poured a 14" slab that was tied into the footings/piers with rebar. So along the sides of the garage there are places in between the piers where the supporting dirt is exposed and presumably freezes. Though in all these years it has never cracked or noticeably shifted. Does the freezing raise the whole building equally or does it just raise it a small enough of an amount that you wouldn't ever notice? The walls are keyed into and sitting on top of the slab.

So would I be safe to use the same type of footings/piers for my house since it seems to have worked fine for that large garage?

Thank you in advance.

You can use foam in a "frost protected shallow foundation" design to eliminate the risks caused by freezing under concrete. And avoid some thermal bridging (which isn't good for energy use).

OP - frost only works on moisture. Building heave when the moisture in soils expand due to freezing. Inside a garage the soil may freeze but since it is protected from rain by the floor and roof, it should remain quite stable.

Jonr - Is it not true to say that FPSF work because the heatloss from the building prevents the frost from penetrating?? Hence from an energy efficient point of few this might not be the best system?

FBBP, heat loss through a slab is the primary reason I want to avoid a slab. If a monlithic slab is not poured, how should the ICF's be supported? Rather than a slab, could dense foam SIPs specifically for flooring be held in place by ICFs?

There are two types of FPSF designs. One depends on heat from the building and the other has insulation everywhere, which keeps heat in the building and uses the natural heat from the ground to keep it from freezing (it blocks the cold from above). IMO, the second type is the most energy efficient and the safest, since it works even if the building is left unheated.

We talk about frost "getting under" the slab. Unless you build on permafrost, the freezing comes from the ground outside the building cooling and then working its way under the slab. It does not come up from below. The warmer the building is, the less likely you are to get freezing temperatures working their way underneath it. In general, even unheated buildings retain enough heat in their walls and floor to ward off freezing temperatures directly underneath. However, in more extreme climates, the weather is cold enough long enough to threaten unheated and sometimes, even heated buildings.

The second line of defense against "frost heave" is preventing moisture. Frost heave depends on the expansive nature of water when it freezes and the reason we use gravel under slabs and foundations is to keep that water down. If you look carefully at engineering specs for gravel, they require that the gravel not be able to retain or wick water up from below. Without the water, it may be freezing under the slab, but there will be no frost to heave it.

If you use piers, they are what the building is supported on. Sending them deep put them below the frost heave line.

there are places in between the piers where the supporting dirt is exposed and presumably freezes.

A small amount of freezing under the edges will probably not overcome the intrinsic strength of the concrete. The farther the freezing can get under the slab, the greater the chance of movement.

Edit: change that last "unheated" to "heated".

Posted By ICFHybrid on 13 Oct 2012 12:20 PM
We talk about frost "getting under" the slab. Unless you build on permafrost, the freezing comes from the ground outside the building cooling and then working its way under the slab. It does not come up from below. The warmer the building is, the less likely you are to get freezing temperatures working their way underneath it. In general, even unheated buildings retain enough heat in their walls and floor to ward off freezing temperatures directly underneath. However, in more extreme climates, the weather is cold enough long enough to threaten unheated and sometimes, even unheated buildings.

The second line of defense against "frost heave" is preventing moisture. Frost heave depends on the expansive nature of water when it freezes and the reason we use gravel under slabs and foundations is to keep that water down. If you look carefully at engineering specs for gravel, they require that the gravel not be able to retain or wick water up from below. Without the water, it may be freezing under the slab, but there will be no frost to heave it.

If you use piers, they are what the building is supported on. Sending them deep put them below the frost heave line.

there are places in between the piers where the supporting dirt is exposed and presumably freezes.

A small amount of freezing under the edges will probably not overcome the intrinsic strength of the concrete. The farther the freezing can get under the slab, the greater the chance of movement.

Thank you. That makes sense. You mention though that the piers are what support the building, but the slab was poured on both the piers and on the gravel/dirt where there are no piers. So I would think that if the dirt under the floor froze and the dirt under the piers, which are below the frost line didn't, then it would still lift the floor and try to pull the piers up with it.

You still haven't mentioned your location. Frost will have to get a ways under a 14" reinforced slab to heave it.

There is heaving, which isn't so bad and cracking. Make it strong enough and you only get the former.

Posted By Gary Olsen on 13 Oct 2012 09:18 AM
FBBP, heat loss through a slab is the primary reason I want to avoid a slab. If a monlithic slab is not poured, how should the ICF's be supported? Rather than a slab, could dense foam SIPs specifically for flooring be held in place by ICFs?

Gary - The ICF needs a footing unless you are building on rock. It can be strip footing, piles or as you mentioned a monolithic slab. If you don't have a floor slab, there will still be transfer of heat between the house and the ground which it sits over.

If you are in an area with deep winter frost (i.e. code calls for 4' of protection,) than you have to do something to stop the frost from crawling under the support structure or you have to let sufficient heat escape to counteract the frost drive. This is one of the advantages of ICF. If you put it below the frost line it will slow the frost drive into the house or under the house to the point where you need very little heat loss to over come it. Therefore you can put foam under the floor slab and limit the heat loss to the soils. If you are building in this climate and attempt to totally insulate the footings and floor as by way of a FPSF you will still get frost jacking of the whole affair, unless you can be sure there is no moisture under the package. An remember, under the right (wrong) circumstance even a poorly place downspout can contribute to the moisture. The only thing worse than the whole building lifting is having only one corner lifting.

Posted By kdw75 on 13 Oct 2012 12:47 PM

Posted By ICFHybrid on 13 Oct 2012 12:20 PM
We talk about frost "getting under" the slab. Unless you build on permafrost, the freezing comes from the ground outside the building cooling and then working its way under the slab. It does not come up from below. The warmer the building is, the less likely you are to get freezing temperatures working their way underneath it. In general, even unheated buildings retain enough heat in their walls and floor to ward off freezing temperatures directly underneath. However, in more extreme climates, the weather is cold enough long enough to threaten unheated and sometimes, even unheated buildings.

The second line of defense against "frost heave" is preventing moisture. Frost heave depends on the expansive nature of water when it freezes and the reason we use gravel under slabs and foundations is to keep that water down. If you look carefully at engineering specs for gravel, they require that the gravel not be able to retain or wick water up from below. Without the water, it may be freezing under the slab, but there will be no frost to heave it.

If you use piers, they are what the building is supported on. Sending them deep put them below the frost heave line.

there are places in between the piers where the supporting dirt is exposed and presumably freezes.

A small amount of freezing under the edges will probably not overcome the intrinsic strength of the concrete. The farther the freezing can get under the slab, the greater the chance of movement.

Thank you. That makes sense. You mention though that the piers are what support the building, but the slab was poured on both the piers and on the gravel/dirt where there are no piers. So I would think that if the dirt under the floor froze and the dirt under the piers, which are below the frost line didn't, then it would still lift the floor and try to pull the piers up with it.

It would IF it was wet under the floor. The floor probably stopped any moisture therefore it could freeze with out damage. BTW this is why we put frost cushion (void form) under gradebeam or as you said the grade beams would lift the pilings.

Posted By ICFHybrid on 13 Oct 2012 08:33 PM
You still haven't mentioned your location. Frost will have to get a ways under a 14" reinforced slab to heave it.

Northwest corner of Missouri.

Posted By jonr on 13 Oct 2012 09:52 PM
There is heaving, which isn't so bad and cracking. Make it strong enough and you only get the former.

I don't know how much it would heave, but I seriously doubt if anyone would even notice an inch or two of tilt. So maybe it does lean a little during the winter.

So basically if the footing go down below the frost line all around the edges of the slab then it protects it from freezing by trapping warmth from the structure and from the soil and also prevent moisture from getting into the dirt in the first place. Therefore the slab itself can sit up above the frost line without any problems.

The floor probably stopped any moisture therefore it could freeze with out damage.

Moisture comes from the sides and below. That's why the gravel is important; it helps drain, but it also prevents drawing up moisture from below.

So would I be safe to use the same type of footings/piers for my house since it seems to have worked fine for that large garage?

Is there some reason you want to cut corners and avoid a proper foundation?

Thanks FBBP.
and KDW75 for starting this thread.

My site is dug into a southern sloping granite mountain. The north edge of the site is 6ft into granite. At 35ft south from the north edge it is about flush with the mountain surface. Excavation leveled the site another 20 ft southward from the 35ft line. Frost line is about 2'. My well is 200ft deep. The terrain becomes a torrent in a heavy rain. Almost all of the "neighborhoods" end with the word "gulch".

Control of ground water runoff is important, including roof run-off.

One thing I recalled while reading the replies above is PAHS (passive annual heating system). Engineer John Hait did it in Missoula MT. He covered a cement dome with dirt, an umbrella of insulation, and 400ft of 4" plastic tubing. Air out runs parallel and close to air in through the tubing. It took a year for the soil beneath the umbrella to stabilize. Once stable the dome stayed at 68F year around with only a fan.

An architect in WA, Don Stephens, adapted the system to more typical house structures. IIRC, he put a skirt around the foundation and covered it with dirt.

So, if I lay a skirt of insulation along the ICF on the outside and inside, and slope a foot of road base on top, I would still need to go a foot deep for the grade beam.

I did not understand the comment regarding piles. Are piles holes in the ground to bedrock and filled with cement?

Frost line is a term that applies to building on soil or dirt. Building on bedrock is a whole different thing. Maybe an engineer would be in order.

While you are finding an engineer, you might look into building the entire house above any unstable soil. Ie, piers/piles down to granite and then beams connecting them. Nothing sitting on frost, erosion susceptible or disturbed soil.

Posted By ICFHybrid on 13 Oct 2012 11:50 PM

The floor probably stopped any moisture therefore it could freeze with out damage.

Moisture comes from the sides and below. That's why the gravel is important; it helps drain, but it also prevents drawing up moisture from below.

So would I be safe to use the same type of footings/piers for my house since it seems to have worked fine for that large garage?

Is there some reason you want to cut corners and avoid a proper foundation?

I am not trying to cut corners I am just trying to understand why the way we did the garage isn't acceptable since it hasn't ever had any problems. I also don't care for some of the conventional foundations that only go down a total of say 3 feet because it doesn't seem like that is enough anchoring to keep the structure in position. I had always looked at the foundation as a device to keep the house from sliding down a hill or tipping over, but it seems it is more about spreading the load out. I suppose we have just been lucky that we never had any problems with our pours.

I am just trying to understand why the way we did the garage isn't acceptable since it hasn't ever had any problems

Just because you haven't seen any problems doesn't mean there won't be any. Code requirements have to address a wide variety of possible scenarios, some of which may never occur.

it seems it is more about spreading the load out.

In general, yes.

I had always looked at the foundation as a device to keep the house from sliding down a hill

SOmetimes they need to be designed to do that.

or tipping over

Under some building circumstances, yes.

I also don't care for some of the conventional foundations that only go down a total of say 3 feet because it doesn't seem like that is enough anchoring to keep the structure in position.

AN engineer can answer that for you.