Thursday, February 18, 2010

Super insulated Frost Protected Shallow Foundations, a proposed building technique by Andrew Johnson

Disclaimer: I am not an engineer. Consult and engineer and get proper approvals before beginning any non-traditional construction technique.

In cold climates, a traditional foundation requires footings below the frost line so the foundation does not move and crack with frost heave as the ground freezes. A basement makes this easy, but for buildings above grade, this issue must be addressed.

Traditional foundations require what is called a stem wall or frost wall, typically 42" deep in Fairbanks, to rest on footings below grade under the frost line, and support the building above. Here's an example from the City of Fairbanks:

This is relatively expensive in both labor and materials, requiring as many as three concrete pours, additional types of labor, and additional types of heavy equipment.

The basic outline of construction is:
1. grading
2. excavation of trenches for the wall
3. forming and then pouring the footings
4. building the wall out of concrete blocks, formed concrete, or treated wood
5. applying insulation to the perimeter
6. backfilling
7. preparing a subslab substrate of compacted sand or gravel
8. pouring the slab

The Frost Protected Shallow Foundation is an alternative foundation that eliminates the frost wall. It is being adopted in the USA, and has long been used in Norwegian countries.

The basic outline of construction is much simpler:
1. grading
2. spreading and compacting Non-Frost Susceptible (NFS) gravel
3. digging, often by hand, a slightly deeper perimeter for added strength, since this part of the slab is the footing.
4. forming and pouring the monolithic slab
5. applying insulation to the perimeter

To avoid frost heave problems, the foundation is protected from frost in three ways:
1. Final grading is sloped away from the building to keep runoff water away from the foundation, so there is no water in the soil to freeze.
2. Non-frost susceptible (NFS) soils, usually granular gravel without fines, is used under the perimeter down to the frost line, so if it does freeze, it doesn't move.
3. Perimeter horizontal insulation beyond the footprint of the building is placed to keep the ground above freezing. In Fairbanks, this can require as much as 4 inches thick extending 60 inches away from the building at the corners. Additionally, heat from the slab perimeter escapes and heats the ground to keep it thawed.

The astute among you may see a pattern in that list: any one item would prevent frost heave.

This is true, frost heave only occurs if all three conditions exist:
1. Water saturated soils
2. Frost susceptible soils
3. Frozen soil temperatures.

Why attempt to prevent all three? Primarily because it’s a good safety factor. It's hard to predict what might happen over the life of a building. Clogged gutters, erosion, etc, can cause soils to become water saturated.

In today’s environment, striving to be as energy efficient as possible, we must seek additional ways to increase energy efficiency. At the same time, perhaps we can improve construction techniques to save time and money. We can do this with a slightly modified FPSF design:

The Super-Insulated Frost Protected Shallow Foundation design modifies the FPSF to reduce the heat-loss to the ground by fully encapsulating the slab (and thus the entire building envelope) in insulation.

This is done by adding insulation under the perimeter footings. Higher density XPS foam with a rating of 60 PSI is available. This is 8640 PSF, greater than most local soil bearing specs. Perimeter insulation is reduced to a single layer 24” out, primarily to aid drainage.

Additionally, extra care is taken to ensure the soil under the building is non frost susceptible and will remain dry. This is accomplished by ensuring a full 48” of excavation, lined with geotextile filtering fabric, with a drain system installed and filled with NFS soil, compacted in appropriate lifts, and graded carefully to ensure drainage.

You will note that there is a lot of foam under the slab, which could become costly. In addition to extra insulation in the hottest part of a building heated with in-floor heating, this also reduces labor. By building the thickness at the center of the slab, the slab can be poured on an easy to prepare, flat compacted gravel pad.

Due to the cost of foam, there are modifications that can be made to reduce cost. Concrete is cheaper than foam, so the slab can be thickened. Additionally, A very shallow trench can be carefully dug to accommodate one thickness of foam.

Previous design, real world application:
Experience has shown that digging an 8” trench to accommodate a 4” slab with a 12” parameter, and equal foam underneath, is difficult. Compaction in the trench causes the trench walls to erode, and fitting foam to the base of the trench and the trench walls is time consuming. There will be a slight void, which the concrete will have to span, so extra rebar at this area may be considered. This is how my garage was built, and two winters have shown no settling, but I think the flat pad or 2” trench is the way to go.