Frost Protected Shallow Footing (FPSF)

Not many sustainable design choices are as easy to make as this one. As long as this detail aligns with the criteria of the existing design, benefits of a frost protected shallow footing can be realized. This includes reduced materials resulting in lower embodied carbon and lower initial cost when compared to a stem wall foundation.

Construction Detail of Frost Protected Shallow Footing

3D Model Cut-Away of Frost Protected Shallow Footing

Foundations and water.

Water is the main enemy for buildings. Water intrusion typically means rot of organic materials or mold issues. For foundations, water can cause damage in a couple ways. Depending on soil conditions, the presence of water can cause expansion resulting in a hydraulic force that can heave and crack slabs and basement walls. Buildings are always trying to divert water down and away. The same principle applies even at the lowest point of the building for the foundation footing. This is why a common detail, often overlooked, is the inclusion of a good trench drain. This is the 4” PVC pipe wrapped in silt fabric and washed gravel at the lowest point in the foundation system and routed either to day light or a sump basin with pump.

Frost protection strategies.

Designers and builders can do an amazing job with diverting water down and away. Even with large roof overhangs, gutters with proper downspouts, and landscape drainage, water can still find a way to get under the footing and foundation system. If water does get under the foundation system another way it can cause damage is from expansion from freezing regardless of soil types. Frost protection strategies vary based on region and code authority. In the Midwest United States the most common detail for residential houses is the stem wall with the footing below the frost depth, typically 48” deep. This is the reason most houses in the Midwest have basements. After excavating and pouring the concrete stem wall you might as well go 48” deeper and get more living space for the same building footprint. However, this is an expensive build out. Not all buildings need full basements and it might be cost restrictive to do so. Luckily the frost line can be controlled through strategic use of insulation and heat from the building.

Construction Detail of Insulated Stem Wall with a Slab on Grade

Controlling the frost line.

Frost Protected Shallow Footings control the frost line relative to the building with skirt insulation and the buildings’ heat to prevent water from freezing. It’s not often that high performance homes want to transfer heat outside the building envelope. However, for the purpose of reducing initial cost and materials the slight heat loss may be justified. The amount of heat loss is not known as empirical testing is not standardized for energy modeling for this construction detail so heat transfer intuition and judgement must be used. The simplest way to reduce the amount of heat transfer from the building is to increase the amount of skirt and side insulation. The further the skirt insulation extrudes from the building perimeter, the further the slab is able to be separated from the cold soil. Of course the thickness of the insulation is another large influence on the performance of the slab. The side insulation that is exposed to the air is of more concern so resources should be allocated to increasing the side insulation thickness before worrying about the skirt insulation thickness.

Thermal benefits of a slab on grade.

There are many design advantages of using a slab on grade opposed to an elevated floor with crawlspace or a basement. For thermal performance, the large thermal mass within the control layers of the building envelope greatly increase the comfort for occupants and make using passive solar heating through windows a huge performance bonus. Ancient architecture is able to exhibit thermal mass at scale. Many of the old catholic churches, as one example, have large megalithic structures made of stone and no insulation. If you get the chance to visit a structure such as this during the blistering heat of the summer you will find a noticeable difference in the cool temperature inside the building. If you don’t have the luxury of visiting such a site, you may be able to visit another natural occurrence that displays thermal mass even better, a cave. Large dense masses are able to retain heat very well and not fluctuate in temperature as drastically as frame construction with light weight materials. In summary, you will notice a large difference of thermal comfort when you have large dense masses in the controlled environment.

When to design a slab on grade.

Not all circumstances call for a slab on grade. For homes in a flood plain, finished basements might not be allowed by the code official. Often the basement will require break away or open flood walls or windows that can be a significant expense and opening in your exterior envelope. This is a great use of a slab on grade to prevent additional headache of dealing with floodways.

If you’re looking to build the cheapest home possible, look no further. Slab on grade construction is as simple as it gets with constructability. Of course you increase initial slab cost when you consider improving the skirt insulation above code requirements, but if you wish to be comfortable in your home and not have to pay for it in energy then additional skirt insulation will save you in the long run. It may even save you on initial costs with reducing the size of the HVAC equipment.

Slab on grade construction is not just for the pocket cautious. Radiant floors provide superior comfort often displayed in luxury homes. Unfortunately not all luxury homes are high performance. Often luxury home builders are more concerned with the finishes and appeal to polished concrete with radiant floors but fail to understand thermal bridging and build an uninsulated stem wall. Code is the bare minimum for construction and it does not mandate insulating the concrete slab. Even if you are not using a FPSF make sure the stem wall is well insulated to prevent creating a massive thermal bridge that will chill your feet in the winter.

More questions about thoughtful construction details?

Send us a message and we can schedule a quick phone call to answer your questions. We have no issue with just having a chat at no cost to you. The goal of Crafted Performance Homes is to promote sustainable construction and design. If you would like to pay us for services then we will be glad to arrange an agreement.