How the two differ in kind
Frost heave needs three things at once: a soil that is frost-susceptible, water available to feed ice growth, and temperatures that reach the soil beneath the footing. Remove any one and there is no heave. The two options here remove the third, by different means.
The traditional answer puts the footing BELOW the depth frost reaches, which in a mild climate is a spade's depth and in a severe one can be two metres or more. It is simple, it is accepted everywhere, and its cost is set by a number nobody controls: excavate, concrete and backfill a trench that deep around the whole building, for a structure that may be a single storey.
The alternative keeps the ground warm instead. Rigid insulation laid vertically against the foundation and extending horizontally outward from it — the WING insulation — slows heat loss from the ground so that the freezing front never reaches the footing. Under a heated building it has two heat sources to work with: the ground's own stored summer heat, and heat escaping from the building. Under an unheated one it has only the first, which is why an unheated design needs more insulation extending further, and why copying the house's detail onto a detached garage is the way these fail.
The factors that actually differ
Show
| Frost-protected shallow foundation | Footing below the frost line | |
|---|---|---|
| What stops the frost | Heat — retained in the ground by insulation, so the freezing isotherm is pushed away from the footing. | Depth. The footing is simply below where frost gets to. |
| Excavation and concrete | Shallow. A fraction of the digging, spoil, concrete and backfill, which is the entire economic case. | Proportional to frost depth, which in a severe climate is most of the foundation cost for a low building. |
| Heated or unheated building | A different design for each. Unheated buildings have no internal heat to contribute, so they need more insulation extending further out, and the two details are not interchangeable. | Indifferent. Depth works the same under a house and under a shed. |
| Acceptance | Recognised in codes and standards but less familiar, so it sometimes needs to be argued and evidenced with an inspector. | Universal. Nobody has ever had to explain a footing below the frost line. |
| What can go wrong afterwards | Anything that removes or damages the insulation — a service trench, a new path, a soakaway, landscaping. It is buried and invisible, and the consequence appears a winter or two later. | Very little. The footing does not depend on anything anyone might dig up. |
| Ground conditions | Still needs adequate bearing at shallow depth. Insulation solves frost, not soft ground. | Reaches deeper ground, which on a site with poor topsoil or fill is a bearing advantage as well as a frost one. |
| Thermal side-effect | Substantially reduces heat loss through the slab edge, which is one of the larger thermal bridges in a low building. The insulation is doing two jobs. | None. A deep footing is a structural element with no thermal benefit. |
| Groundwater and drainage | Depends on free-draining backfill and on water not accumulating against the insulation — drainage is part of the design rather than good practice. | Deeper excavation is more likely to meet groundwater, which is its own cost. |
Which one, and when
Choose frost-protected shallow foundation when…
- The climate has a real frost depth — the deeper it is, the larger the saving — and the building is heated.
- The building is low and light, so a deep perimeter trench would dominate the foundation cost.
- Slab-edge heat loss matters, and the insulation earns twice.
- Excavation is expensive or awkward: rock near the surface, a tight site, a high water table.
Choose footing below the frost line when…
- The building is unheated and the extra insulation an unheated design needs erodes the saving.
- There is a bearing reason to go deeper anyway — made ground, soft topsoil, fill of unknown depth.
- The site will be dug up: services, landscaping, future extensions, anything that would cut through wing insulation.
- The approval route has to be straightforward, or the local inspectorate is unfamiliar with the alternative.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Does a frost-protected shallow foundation work under an unheated building?
- Yes, and it is a different design rather than the same one with a note. A heated building leaks heat into the ground beneath it all winter, and the standard details rely on that. An unheated garage, store or porch has only the heat the ground itself stored over the summer, so the insulation has to be thicker, has to extend further horizontally, and usually has to continue under the slab as well as around the perimeter. Detailing an unheated structure with the heated detail is the commonest failure of the method, and it is invisible until the first hard winter lifts a corner. If the building might be unheated for a season — a holiday property, a building left empty through a winter — it should be designed as unheated.
- What happens if someone digs through the wing insulation?
- The frost protection stops working locally, and nobody finds out until the ground heaves. Wing insulation extends horizontally outward from the foundation, at a shallow depth, which is exactly where a cable trench, a drainage run, a new path's sub-base or a tree pit ends up. It is buried and unmarked, and the people who do that work later have no reason to know it is there. The practical mitigations are to record it on the as-built drawings, to say so in the property's documents, and to prefer the deep footing where a site is likely to be dug up repeatedly. Where insulation has been cut, it has to be reinstated properly — a patch of soil where insulation used to be is a cold path straight to the footing.
- Which soils actually heave?
- Frost-susceptible ones, and the property that matters is capillarity rather than moisture content. Silts and fine sandy silts are the worst, because they can draw water upward to a freezing front fast enough to feed growing ice lenses continuously — which is what lifts a foundation, rather than the freezing of water already present. Clean gravels and coarse sands barely heave at all, because they cannot pull water up. Clays are intermediate: they hold water but move it slowly. So one legitimate strategy is to remove the susceptible soil under and around the footing and replace it with free-draining granular fill, which attacks the first of the three conditions instead of the third. Drainage attacks the second. All three approaches are valid and the cheapest depends on the site.
- Is this accepted by building control?
- It is a recognised method with published design standards, and it is used at scale in the Nordic countries where the frost depths make deep footings punitive. Acceptance in practice varies with how familiar the local inspectorate is with it. The route that works is to design it to a named standard, submit the insulation layout and thicknesses with the application rather than after, and treat it as a designed system rather than a substitution. Where an inspector is unfamiliar, the evidence is a published standard and a design, not an assurance — and if that conversation looks likely to be difficult on a small project, the deep footing is sometimes worth its cost simply for being unarguable.
- Does it help with heat loss as well?
- Substantially, and it is an under-valued part of the case. The junction between a slab and its perimeter is one of the largest thermal bridges in a low building — a continuous path from the heated interior to the outside air around the entire perimeter — and the vertical and horizontal insulation an FPSF requires interrupts exactly that path. So the same material bought for frost protection is also doing work that would otherwise need separate edge insulation. On a building where the energy standard already demands perimeter insulation, a large share of the FPSF's material cost is being spent anyway, which changes the comparison against a deep footing considerably.
- Does either option deal with expansive clay?
- No, and conflating the two is a real mistake. Frost heave is water freezing and expanding beneath a foundation; expansive clay movement is a soil changing volume as its moisture content changes, seasonally, with no frost involved. A footing below the frost line is not below the zone of seasonal moisture movement in a shrinkable clay — that zone is deeper, and it is deeper still near a tree. The remedies are different: deeper foundations designed for the clay's movement, void formers beneath ground beams so heave has somewhere to go, or piles. A building on shrinkable clay in a cold climate has both problems and needs both answered, and an FPSF answers only one of them.