Q: We are building a family home in Toronto, Canada using a local builder.
Due to soil conditions, we had to raise our founding level. The code requires the underside of footings to be 4 ft. below grade for frost protection. In our case with a sloping grade, the footing depth is 4 inches less than the required 4 ft in some locations.
The structural engineer’s opinion is that no additional frost protection measures are required since we have an 8 in. concrete mud slab below the footings. So there is no soil within 4 ft. of the bottom of the mud slab.
As an additional measure, the builder is recommending putting rigid foam that is 2 ft. vertical and 2 ft. horizontal above the footings and foundation. where it’s at a shallower depth. He is saying the cost is minimal and there are no real cons.
Please let me know your opinion. Are there are any cons to this approach and does it provide any long-term value? Not sure how effective rigid insulation would be around 2-4 ft. below grade. with wet soil conditions and other factors. Wouldn’t the foam just rot away over some time? Would it pollute the soil? Any other considerations? Not sure if its common practice and whether it’s effective in preventing frost risk. –Rick M.
A: There are two separate issues here:
1) Does your foundation need additional protection from frost damage?
2) Is this the best approach to achieve that?
Your engineer, who has been on site and knows the local soil conditions does not believe additional protection is required. If I understand you correctly, the footings sit on top of the mud slab, so the bottom of the concrete is really more than 4 ft. below grade so is not at risk. That makes sense to me.
However, if you want extra protection from frost due to extreme cold temperatures combined with wet soil, you may want to consider your builder’s approach as an upgrade. His approach is commonly called the Frost-Protected Shallow Foundation and is backed by extensive research in the US and Europe.
The approach of building shallower footings and protecting them with insulation was pioneered in Scandinavian countries after WWII, but now is accepted by the IRC (International Residential Code) and widely endorsed by the building industry, which has done a great deal of research on the topic.
The approach, referred to a Frost-Protected Shallow Foundations (FPSF) is mainly used with monolithic (thickened edge) slabs that can sit as little as 16 in. below grade in most of U.S. and southern Canada.
The general concept is that a combination of vertical and horizontal rigid foam placed around the foundation will create a warmer zone under and around the footings, protecting them from frost. The area below the insulation is warmed by the deep earth temperatures, which range from 50-60° F. throughout much of the U.S. and southern Canada.
The soil is also heated by heat loss from the building itself. In an unheated building, the same approach can work, but these generally require thicker insulation and wider horizontal “wings”.
How Much Insulation
There are many different configurations, depending on the foundation type, but the key is to have a continuous layer of vertical and horizontal insulation that isolates the foundation wall and footings from the exterior air.
The minimum thickness and width of insulation to prevent frost heaves is determined by the Air Freezing Index, compiled from heating -degree-day data. For the two most common slab configurations, a thickened-edge slab and a stem wall, use Table R403.3 below. Similar insulation details can be used around posts and piers to limit their depth in cold climates.
I prefer to use extruded polystyrene insulation (XPS), which is the most durable foam insulation buried in the ground. However, higher densities of expanded polystyrene (EPS) can also work well. The foam R-value is reduced somewhat for losses over time. For example, XPS is lowered from R-5 to R 4.5 per inch.
It is important to install the insulation with care so that it is not damaged when placed or backfilled. Because concrete is highly conductive, even modest gaps can cause thermal short-circuits or “cold bridges” that lead to cold spots in the soil or slab edge. In the worse cases, this could undermine the frost protection and lead to frost heaves in the area of the cold bridge (illustration below).
Foam Durability and Green Credentials
To what extent will foam be damaged by insects, frost, and general wear and tear over the years? Probably to some extent, but most studies have found only minor degradation after 20-30 year or more – for both XPS and higher densities of EPS.
In areas subject to subterranean termites, it would be wise to treat the soil or use treated foam as termites seem to use the soft foam for housing and protection, although they do not eat the foam. (Above grade., carpenter ants also find foam board an attractive home.)
Since it is inert, foam should not affect the soil. On the other hand, it’s not environmentally benign because
- It is made from petroleum
- It is manufactured using CFCs and HCFCs, which contribute to global warming
- Because of its durability, it is hard to recycle
EPS foam uses pentane as a blowing agent, which is not considered a significant contributor to global warming.
A final point to consider is landscaping. Deep-rooted plants may not do well planted in soil only two feet deep. But holding plants away from the foundation by a few feet is not a bad thing – it’s usually better for drainage away from the house. — Steve Bliss, Editor, BuildingAdvisor.com
For more information:
Read more on Foundation Insulation