A frost protected shallow foundation (FPSF) is simply a more practical construction method used in cold climates as an alternative to standard footer foundations. Traditional foundations require a footer to be placed at the frost line with a foundation wall and eventually slab on top of that. FPSF’s use additional insulation on the outside of the foundation walls that actually change the frost line underneath and around the perimeter of the structure by bringing it up to shallower ground. This allows foundations to be poured as little as 12″ deep in areas that would normally require 3′ or more for a traditional foundation to reach the frost line for that area.
Frost protected shallow foundations can also be poured in one monolithic concrete pour once the insulated forms are created underneath and around the perimeter as required. Most FPSF’s will save time, money and material costs while yeilding a better insulated foundation. This sounds like something we might be interested in.
Lastly, the shallow foundation is able to withstand poor soil conditions due to the fact that the weight of the building is spread over the entire structural slab. This is muy bueno for us as almost every vacant lot in Philly has poor soil conditions.
History of the FPSF
Frost protected shallow foundations have been used for over 50 years in some of the most extreme climates. Most got their origin in the Scandinavian countries like Sweden, where hundreds of thousands have been built. Many have also been built in Canada and Frank Lloyd Wright was using them in the 40′s and 50′s on his Usonian homes in the US.
Benefits of the FPSF for the 100K House Model
The diagram above is a good illustration of what our detail would most likely look like. In our climate, it is recommended to use only 1″ (R-5) of rigid on the exterior of the foundation wall to a depth of only 12″. We will probably beef up this insulation level for thermal performance reasons, but it is good to know that the minimum requirements are very reasonable for our climate.
Below are the basic reasons why the team is excited about the possibility of FPSF’s in all of our future 100K House inspired homes:
- Less labor for foundation work
- One concrete pour vs. three for traditional foundation
- Less concrete used in foundation
- Superior insulation value compared to traditional
- Less excavation and hauling of existing soil
- Able to withstand poor soil conditions
- Potential for prefab insulation forms (Legalett)