Pre-existing foundation not meeting required frost depth 2

[amyrice72]

The foundation is fine for additional load, but we will have to somehow "get" the frost depth (18" more). Any way to achieve this without physically adding depth - for example, any way to insulate to an equivalent modified depth?

 

[steellion]

1) How old are we talking about when you say "pre-existing"? Can this foundation be grandfathered in code-wise as being acceptable, or is there a code that is mandating this change?

2) My understanding is that the frost depth requirement only applies to the perimeter. Therefore, if you protect the perimeter from frost to the required depth in some manner, the interior footings should be fine.

 

[mmillerpe]

ASCE publishes "Design and Construction of Frost-Protected Shallow Foundations" (SEI/ASCE 32-01) which will provide answers to your questions. Adding frost protection may be possible by excavating down to a certain depth and providing a layer of horizontal and/or vertical insulation. The amount and extents of insulation would depend on the required frost depth, number of degree days, whether the building is a heated or unheated building, etc.

 

[BAretired]

I have occasionally added insulation when a foundation is within the active frost layer. The rule of thumb I have used is that one inch of insulation is equivalent to one foot of soil. Use a good grade of insulation and ensure it is impervious to water.

The insulation must extend far enough so that the shortest path for frost to reach the underside of footing is equivalent to the required depth of footing.

BA