I am worried about potential frost problems in my basement. I recently completed some landscaping changes, and in the process exposed the footing of a concrete block wall about 4.5 m (15 ft) in length. The foundation was originally 1.2 m (4 ft) below grade, and it is now about 90 cm (3 ft) in depth. I have often heard that 1.2 m (4 ft) is the absolute minimum to prevent frost. Am I going to encounter problems, and if so, what can I do to prevent them?
The reason that foundations are usually 1.2 m (4 ft) below grade is because 1.2 m (4 ft) of earth on the outside of a foundation is just enough to insulate the foundation against frost in most parts of Canada. There is a method that you can use that will allow you to have a shallow foundation without encountering any frost problems. Dig 60 cm (2 ft) down all around the problem area. Cover the wall with 5 cm (2 in.) of rigid foam insulation and then skirt out 60 cm (2 ft) from the wall. A little soil and rigid foam insulation together will insulate the footings as if they were 1.2 m (4 ft) in the ground, and will keep them frost-free.