A frost protected shallow foundation (FPSF) is an interesting and well-proven alternative to deeper, more-costly foundations in cold regions with seasonal ground freezing and the potential for frost heave - and the simplest way to build a solid and dependable foundation is to use frost protected shallow foundation (FPSF) forms - which come at a certain cost, but also with many advantages in terms of convenience, longevity and thermal performance - leading to a comfortable home with excellent energy efficiency and an efficient use of carbon intensive materials - like concrete.
The soil conditions of some building sites are simply not suitable for a slab-on-grade foundation with a thickened-edge footing without first beginning with extensive and costly soil remediation. Opting for a raft slab in such conditions can be a much cheaper option.
A raft slab is a type of frost-protected shallow foundation, or slab-on-grade foundation, that does not include the standard thickened-edge footing on which the entire load of the house bears. It is engineered to distribute the load evenly over the entire building surface.
Building sites with poor soil conditions (disturbed soil, expansive soils, low-bearing capacity, high water tables, etc.) traditionally require significant investment in drainage, soil replacement and compaction before an engineer will approve a project. Add the potential for frost heave in cold climates into the equation, and it's easy to understand why so many homebuilders and custom home construction companies tend to think they have to dig deep and install strip foundations below the frost line then deep frost walls. During a recent concept home build in Quebec, on expansive clay and with significant localized potential for frost heave - Ecohome chose instead to install a pre-engineered frost protected shallow foundation forms system.
The photos below are images of drainage measures that were required to support a thickened-edge footing slab-on-grade floor on a site with a high water table. Images courtesy of Yanni Milon.
How do frost protected shallow foundations (FPSF) work ?
A FPSF raft slab acts a bit like a snowshoe in the way it distributes weight evenly over a larger surface. For this reason, they can often be built on soil that could not support other types of structures or ones subject to differential expansion and contraction, like expansive clay soils, or also frost heave in cold climates. In the case of our s1600 concept home, it was all of the above, so about as bad as it gets for designing, engineering and installing a succesful foundation system!
The typical soil bearing requirements for a thickened edge footing are 150 kpa (3,000psf), where a raft slab may be able to sit on soil with one third of that bearing capacity, or even less with additional engineering measures. This will often be the most affordable (and possibly only) option for building on sites with particularly unsuitable soil conditions. Even with an averaged-sized house, such costs can sometimes climb into the tens of thousands of dollars and possibly stop a building project in its tracks.
By choosing pre-engineered frost protected shallow foundations instead of going the traditional route for building foundations in a cold climate or on expansive clay soils, the risk of encountering inconsistently poor soil quality at the depth of traditional foundations is also avoided, so a raft slab on any site can avoid possible costly surprises once excavation has begun. Given its more robust design, it is also less prone to movement and cracking than buildings that bear on a footing as the whole foundation takes the load evenly.
You may or may not have success in finding an engineer experienced at this level of complexity in raft slab design, alternatively you can look into companies that specialize in custom designed prefabricated ICF slab on grade form kits, who also carry out all the engineering and can supply stamped drawings like we did.
Excavation and slab placement for cold climates
Our first recommendation, if at all possible, in constructing a frost protected shallow foundation at a reasonable cost is to choose a building lot that has a flat area sufficiently large to build the home you want to live in. Any leveling of the site, or digging in, creating a partially subterranean rear wall to the property adds a lot of additional complication, potential failure points, and most importantly time and expense. In general we find that the public or those who only build once or infrequently, underestimate the additional costs of building on a site that has changes in levels. After that, and having decided the optimal siting for insolation - especially if building to passive solar principles, here's a general timeline for building an insulated and frost protected slab-on-grade foundation.
- Remove the approximately 6 inches of organic material, two feet beyond where the building footprint will be.
- Build a retaining wall if necessary to create a level building surface.
- If there are large holes where tree roots have been removed, they can be filled with aggregate and compacted.
- If the site is sloped at all, bring it up to level with 0-2.5 inch compactable fill, being sure to compact it with a plate packer at the required intervals.
- Lay down 6 inches of levelled clean stone, two feet out from the building perimeter.
- Install pins where the corners of the building will be.
- Install all sub-slab plumbing, electrical conduits and radon gas evacuation tubes.
Note: We would strongly recommend seeking a plumber with experience in slab-on-grade foundation construction. As all plumbing work will be embedded in concrete, the accuracy of location, drain height and proper sloping of drains is essential.
- Install raft slab frost protected shallow foundation forms, interior floor insulation and reinforcement mesh as per engineering specifications.
- If you are installing in-floor radiant heating, be sure to have it designed by engineer to properly locate the heating delivery system so as not to affect the structural integrity of the slab.
Frost protected shallow foundation forms installation
First the formed edge pieces are laid in place, the corners need to be squared and secured in their location. following that is the installation of interior board insulation, a radon gas/vapour membrane, reinforcement mesh and any heating systems, all done according to the engineered building plans and directions. All slab images courtesy of Legalett.
Slab-on-grade floors are often heated, which provides a very comfortable and even distribution of heat throughout a home. A large volume of heated concrete inside a building envelope will act as a thermal battery by storing and releasing heat, which helps balance temperatures in both summer and winter.
Such quantities of heated thermal mass inside the building envelope also provides heat security in the event of a power outage, by slowly releasing its heat over the course of days. Raft slabs can be heated with hydronic systems (liquid) or air-heated tubes as seen below.
A better basement design - starting with an insulated foundation slab:
Traditional basements begin with a poured footing, then a foundation wall, and finally the slab floor. Rarely are footings insulated, and depending on how the walls are insulated, the result can be a thermal bridge between the footing and walls or floor. This brings unwanted heat loss, as well as a greater risk of condensation forming on those colder portions of concrete.
Alternatively, a basement can be constructed by beginning with a frost protected shallow foundation slab, followed by an ICF foundation wall. This provides a continuous layer of insulation separating the concrete from the ground. The result is a very comfortable and energy efficient basement, with no thermal bridging and reduced risk of mold.