Frost-Protected Shallow Foundations – subslab insulation?
When we make a FPSF (aka ‘Swedish Foundations’, since they rapidly adopted them upon discovering obscure US Army research into the topic, before US builders rediscovering it abroad), it’s been explained to me we are implicitly attempting to thermally couple the soil closely with the slab. We do this with lots of perimeter vertical insulation and even some perimeter wing insulation, but no extra center-slab insulation – the soil is in direct thermal contact with the slab. Soil is not a great insulator, but it’s good enough if you lay extra foam in deep vertical layers around the perimeter, and then a thick blanket of foam on top of it extending four feet away from the building, so the heat has to go through all that material to reach sky. The building leaks heat out through the slab downwards and into the soil underneath the slab, which becomes a much higher temperature than the exterior air – too hot to freeze. A 3D thermal gradient is created from the center of the slab outwards and downwards, and sharp step changes occur at foam insulation in perimeter and the horizontal foam ‘wings’, which combined with the R-value of the soil ultimately make the fact that you are heating ‘The Earth’ in order to prevent phase change, a manageable energy expenditure.
By this understanding, a FPSF building *cannot* be left unheated over winter, because one time reaching thermal equilibrium with the outside would cause the dreaded frost heave.
Good so far. Now let’s see how we’re actually doing it.
What the?
We’re throwing down huge amounts of subslab foam (one person explained to me it was R-50 (“passivehaus levels”) because… uhh… FPSF), first flat over large amounts of aggregate, and then in cavity blocks, and then mechanically reinforcing the foundation with concrete ‘ribs’ to avoid cracking in the negative space between the blocks. We’re still, for some reason, doing the perimeter insulation and wings, despite the fact that very little heat will be transferred from the building to the soil at all, so we would expect large amounts of frost heave. It seems like instead this is designed to *mechanically* decouple a slab from heaving subsoil. Foam centers and ‘ribbed’ concrete is used instead of a solid ultra-thick concrete slab… presumably because concrete is more expensive than foam (note: they’re about the same here) per cubic foot.
Am I confusing two entirely different technologies? Why do we lump them together?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Burn,
A building with a FPSF doesn't have to be heated. There are FPSF details for unheated buildings like garages, sheds, and barns. Most FPSF manuals include the necessary details.
You may be confusing a FPSF with a raft slab. For more on raft slabs, see "Foam Under Footings."
You do seem to be lumping a few different technologies together. The principles you describe (relying on heat transfer from the building to prevent frost heaving) applies only to one specific type of frost protected shallow foundation, and that type is generally not popular with builders that value energy efficiency, for obvious reasons.
There is a second type of frost protected shallow foundation designed for unheated buildings, which relies on using perimeter and sub-slab insulation to prevent heat from transferring from the soil to the air. In other words, you trap the natural heat of the earth to prevent the ground from freezing. While it is designed for unheated buildings like sheds or garages, it works just as well with any heated building.
There is a separate category of slab foundation known as insulated raft slabs, which generally eschews wing insulation in favor of using aggregate to keep the area below the slab dry and relatively free from frost (it is possible the mechanical decoupling you describe plays some role).
In general, a thick layer of sub-slab insulation is a waste of money. The ground is generally warmer than the air, especially in the case of frost-protected shallow foundations. It makes marginally more sense for a raft slab, but even then the money is probably better placed elsewhere. The reason passive house designers often spec so much is because they have strict energy guidelines to meet which do not take climate into account, and so designers often run out of other places to put insulation.
Well put!
Aedi, I'd like to know more about this second type of frost protected shallow foundation designed for unheated buildings. I can find no reference to it in IRC 2015 or 2018. Would it work, for example, on a complete house with attached unheated garage?
Here is the building code language from the 2018 IRC that may be creating some confusion:
R403.3 Frost-protected shallow foundations.
For buildings where the monthly mean temperature of the building is maintained at not less than 64°F (18°C), footings are not required to extend below the frost
line where protected from frost by insulation in accordance with Figure R403.3(1) and
Table R403.3(1). Foundations protected from frost in accordance with Figure R403.3(1)
and Table R403.3(1) shall not be used for unheated spaces such as porches, utility
rooms, garages and carports, and shall not be attached to basements or crawl spaces
that are not maintained at a minimum monthly mean temperature of 64°F (18°C).
I interpret this language to mean that you can't eliminate footings that go below the frost line unless you maintain the designated interior monthly average minimum.
On the issue of ground-coupled heat loss: conductive heat loss to an "infinite sink" such as the soil below a slab is really complicated and means that the net loss or gain over the fours seasons is also complex. Some criticize Passive House for the extreme efforts they go to decouple concrete slabs from the Earth while others say the more ground coupling the better. I think it is a complex climate-, soil-, site-dependent answer.
Peter
Burninate: you are missing the fact that the earth is warm and it's legitimate to use foam to prevent this warmth from escaping to the atmosphere. No building heat is needed to avoid frost.