Our current building project is exploring how we can create a structure using local materials. We’re currently building the foundation and thinking a lot about how to create a firm, long-lasting platform on which we can build the rest of the structure while using as little time, effort, and materials as possible.
In Wisconsin and other colder regions, foundations are built to reach below the frost line. The frost line is the maximum depth at which groundwater freezes. Even though the map says that we’re in the 60 in zone here in southern Wisconsin, a look at data over the last twenty years says we haven’t gotten below 32 in.
Our construction is designed to comply with the Wisconsin Uniform Dwelling Code (UDC), but we are also able to make use of equivalent International Residential Code (IRC) when needed. The UDC states that we must build foundations at least 4 ft below the surface for frost protection.
Note the exception for shallow foundations designed with ASCE-32. A shallow foundation allows us to use significantly less materials. The American Society of Civil Engineers (ASCE) publication can be purchased here (it is also available in various locations on the web, such as Sci-Hub). You can find a more step-by-step guide from the National Association of Home Builders.
Why Is Frost Dangerous?
Frost, which is just frozen water, causes problems with poorly built foundations in the same way that it destroys miles of roads up here each winter. When water freezes, it expands with enough force to break stone, concrete, mortar, asphalt, and any other brittle material. In roads, a pothole full of water will expand each winter as the freeze-thaw cycle makes it wider. In foundations, water trapped near, under, or inside can cause failure when the ground freezes. Water in the ground can cause frost heave, pushing the foundation sideways and causing cracks due to the pressure. Moisture in the foundation itself can cause failures in concrete, stone, or mortar when it freezes. To avoid frost damage, foundations need to be kept from freezing, getting wet, or — preferably — both.
What is a Frost-Protected, Shallow Foundation?
At its most basic, a frost-protected, shallow foundation uses insulation and design to keep it dry and from freezing. The ASCE guide describes options for heated and unheated buildings in environments from the mild south to the frozen north (all the following images come from that guide). They lay out designs for slab-on-ground, unvented crawlspace, walk-out basement, continuous wall, and column foundations.
The design process depends on the type of foundation selected, but in most cases, the builder uses tables and formulas to specify how much insulation is need and how it should be configured. Let’s walk through our design process.
This works by creating an artificially shallow frost depth around the house. The frost line varies by what is on top of the ground. Forests and structures protect the ground and frost penetrates less deeply. In roads, fields, and other open areas, heat is easily lost and frost is deeper.
Around a house with a frost-protected foundation, the frost depth is raised by forcing the heat of the house down and out, into the soil surrounding it. This pushes the frost back and creates a “heat bulb” under the structure.
To have an effective frost-protected shallow foundation, cold bridges must be avoided. A cold bridge is any route by which exterior cold can directly sap heat out of the foundation or ground below the structure. The following design shows a few examples. In the first, veneer bricks pull cold air below the foundation. In the second, a cement wall does the same. In the last, the foundation itself is exposed. In all cases, the solution is to create a contiguous line of insulation from the bottom of the foundation to the superstructure.
We chose an unvented crawlspace stem-wall foundation. Since we’re building with an eye to local materials, concrete is out for us, so we cannot do a slab foundation. We also are digging by hand, so a shallow stem wall would be ideal.
The design calls for a structure floor with an R-value of less than 28 (R-value is a standardized insulation rating — the higher the rating, the better the insulation). This allows heat to seep into the crawlspace and help heat the foundation. The crawlspace cannot be more than 24 in below the floor joists.
Now we must get a few site details. The site’s Air Freezing Index is the “cumulative degree days above and below 32°F (0°C)” (ASCE, 1). This is drawn from a topographic-style map, shown below. Our location is 2,000.
The height of the cavity is also a factor, as a larger crawlspace requires more heat to keep the foundation from freezing. Ours is 6-12 in.
Then we must determine the R-value of the floor. In our case, this is 1 1/2 in of floorboards and then R-25 insulation, adding up to an R-value of about 26.5.
We then use the following table to determine how much insulation the outside of our foundation requires for F-100 days (2000) and floor (h=12 in, R-26.5). Without interpolation, we need foundation insulation of R-8.5, so we went with R-10 insulation (Owens Corning Foamular 250 R-10, 2 in).
Using the same information and the following table, we determined that we do not need “wing” insulation to further isolate the foundation from freezing. We also found that our foundation depth (below grade) must be 14 in or more.
Taken all together, this has led us to the following foundation design.
From the outside moving in, the foundation is protected by flashing to keep water and critters out. The flashing goes at least 6 in below the ground and then extends up into the structure exterior. Below that is R-10 exterior, subsoil-rated insulation (the pink foam board stuff). The exterior of the foundation is parged, which means it is covered with a mortar specially mixed to be water resistant. The foundation is next. Inside a vapor barrier separates the ground from the underside of the structure. The foundation is also topped by another layer of flashing to separate the porous stone and mortar from the wooden frame — without this, moisture could wick into the wood and cause rot. Below the foundation is 6 in of 3/4-in gravel without fines to provide a solid footing and drainage.