Insulation Vs. Frost Depth

Retrofitting Insulation

• Insulating a basement can have an entirely unexpected effect on the adjacent ground. As D.A. Figley and L.J. Snodgrass noted in an article for the "Journal of Thermal Insulation," “With a reduced heat flow to the surrounding soil, the soil temperatures are lowered and the depth of the frost penetration increases.” New behavior can be expected from the ground, which will directly impact the building.

Impact

• Heat loss to surrounding soil is reduced when sub-ground level walls are insulated. While this is desirable in terms of reduced heating costs and preventing damage to pipes and water-bearing appliances, the reduced transmission of heat to the ground can also have unfortunate side effects. Only in extremely well-insulated basements does frost typically extend below the footings, so a previously uninsulated basement may never have been surrounded by frozen soil. Frost depth is likely to increase after such a basement has been insulated. Soil that did not previously freeze may do so; and when the soil freezes, it expands. Building damage can occur.

Potential Effects

• The experts at Walters Forensic Engineering note that the movement which accompanies a change in soil volume, properly called “frost heave,” is responsible for “damage to buildings, roads, and equipment every winter.” Ground movement can be up to 30 percent of the volume of frozen soil, meaning that if frost depth is 3 feet, that soil can move almost 1 foot. Movement of approximately 4 inches is the norm, and this is sufficient to cause considerable damage to buildings. The volume change can cause damage as minor as small cracks in walls or as substantial as above-grade windows shattering and serious structural deterioration. Whole buildings can be lifted several inches.

Prediction

• Predicting the likely effect of retrofitted below-grade insulation on frost depth is an exact and demanding science. Soil density and drainage are major factors, as are ground cover -- vegetation and snow -- and the ambient temperature of the air. The proximity of other below-ground structures can have an effect, if they are radiating heat, as can the shadowing caused by their above-ground extensions; shadows block sunlight, which increases temperature.