A Case for Frost Protected Shallow Foundations
In colder climates like ours here in Maine, foundations traditionally required depth. Footings have to be down below the potential frost depth in order to protect your home from a heaving foundation.
Everyone is familiar with the traditional “full” basement foundation, a six to eight foot wall of concrete on which the house is built. It provides an amazing amount of useful space, effectively doubling a home’s square footage if it is built under the entire footprint.
It is not, however, without its drawbacks. Cost… there’s no getting around it. Putting in a full foundation is a significant, labor intensive, and time consuming process. You’ll generally have to coordinate with at least two different contractors: an excavator and a concrete guy. Right off the bat you’ve got two major bills…that’s two different companies billing you for labor, machine time, materials, overhead, and profit margin. With the skyrocketing cost of construction in recent years, it is not uncommon for future homeowners to be faced with six figures in the foundation before they even start building. It would be reasonable to think that one might save some money by considering a simple frost wall, not a full basement, but the cost savings is minimal as this still requires all of the same steps as your full foundation. Going slab to grade with that frost wall foundation could cost even more, since you are now accounting for significant amounts of backfilling material.
Then there’s energy efficiency… or the complete lack of it. Everyone knows that cold, wet feeling of a damp basement. Many basements are installed without any insulation or heat source to speak of, as they are not considered “living space”. They are also known for water problems. The only truly dry basements that I have ever seen have been the brand new ones. So in many cases, the basement becomes the ideal breeding ground for mold and other allergens which can have serious long term health consequences.
There is an alternative! The Frost Protected Shallow Foundation (FPSF) is an insulated slab design that was first developed in the Scandinavian countries in the mid-20th century. The method gained some traction in the United States in the early 90s when the U.S. Department of Housing and Urban Development (HUD) conducted long term studies and testing in the colder climates within the U.S. In 1994 the Builders Guide to Frost Protected Shallow Foundations was published by HUD. It was adopted into the International Residential Code (IRC) in the early 2000s and now occupies a permeant place as an explicitly “code approved” type of foundation. It can be found in the 2024 IRC Section 403.3. Despite the credentialed history, widespread use of FPSFs has been slow in the northern states. However, the price hikes of the 2020s have many people going back to the drawing board on their building plans and looking for just such solutions as a FPSF might offer.
So what is is? A Frost Protected Shallow Foundation is essentially a mono-poured, insulated slab. It has a deeper section of concrete around its perimeter which acts as the “footing”, and it is insulated with compression resistant styrofoam insulation. The insulation is installed underneath, up the sides, and critically…around the exterior perimeter. This “wing” if you will, of insulation extending out and away from the foundation keeps frost from being able to penetrate beneath the concrete.
Benefits? Well…cost again, that’s almost always a factor. Using a FPSF can easily cut 30%-50%, in some cases, of a new home’s foundation cost. Time frame is another. A huge portion of the cost difference is in how quickly and how much less labor intensive a FPSF is versus the full basement foundation. Weeks, in some cases only days, versus months. Once the site is prepped a single pour of concrete is all it takes. You can start building on it right away. The energy efficiency is unmatched. I could go into much more detail about the thermal mass of a slab and how it functions in a well insulated house, but that discussion is better saved for its own article. Simply put, if you want to lower your energy bills, considering a FPSF is a must. Another benefit, which I personally think doesn’t get enough consideration, is the hypoallergenic aspect. Eliminating that damp, moldy basement will drastically cut down the amount of toxins and allergens released into the air that you breathe everyday…to say nothing of rodents which love basements! Anyone with considerations such as allergies, breathing problems, or a general concern about the quality of air inside their home and its potential long term health affects, should strongly consider starting their new home on a FPSF.
You can find out more about FPSF and specific projects that we have done here. Or call Justin Spear directly at 207-751-5575 to see how Spear Head Excavation might be able to help you with your next project.