ASHRAE Journal - April 2010 - 54

Frost Line

Figure 1: Map of Extreme Frost Penetration (from the U.S. Army Corps of Engineers).

Figure 2: Frost Expansion. Freezing of frost-susceptible soil behind walls causing thrust perpendicular to the freezing front (figure adapted from U.S. Department of Defense. 1984. “Technical Manual TM 5-852-4/AFM 88-19, Arctic and Subarctic Construction, Foundations for Structures.”).

Double Rubble Toil & Trouble
By Joseph W. Lstiburek, Ph.D., P .Eng., Fellow ASHRAE mostly because there is usually a tenuous link to some observed phenomenon that has a basis in fact. When water freezes, it experiences an increase in volume of “When you insulate your basement on the inside, the rubble foundation will freeze apart, and you will get swelling from the about 9%. Most folks know this; even Wikipedia manages to get this right. So, the thinking goes freezing soil collapsing the wall, and that if saturated soils freeze they will adfreezing will lift the wall so high expand and exert forces on foundayou will need a ladder to get down tions and collapse them. However, it from the sump pit.” does not work that way. A couple of That’s what builders in cold climates real-world observations rear their intell me when I tell them they need to teresting heads. Frost heave can be far insulate their foundation walls. The greater than 9%. It can be thousands fact that it’s not true, except if you do of percent, in fact tens of thousands something stupid and is pretty uncomof percent. More about that later. In mon to boot, even if you do something the real world, frost expansion is not stupid, is beside the point. uniform. It is in the direction of the In case it is not obvious yet, we are heat loss. Memo to everyone reading going to be dealing with ground frost penetration and basement founda- Photo 1: Slab Heaving in Unheated Basement. So why this—this is a huge insight. Let me tions. For all of you who live where didn’t the walls collapse inwards and only the slab repeat this, expansion is in the direclakes and ponds never freeze over, buckled upward? Easy, the ground beside the founda- tion of heat loss (Figure 2). Heat loss in the ground during the don’t worry about any of this. Insulate tion wall was not wet, only the ground under the slab your basement and be done. was wet. Another “ah ha” moment.You need wet soil for winter is upwards from the ground to I will go further and say nothing frost heave. Not only do you need wet soil, you actually the air. It is never inwards from the bad will ever happen to foundations need “frost-susceptible” wet soil. A miserable type of soil ground into a basement if the basein areas where lakes and ponds freeze with an annoying distribution of fines. The size of voids ment is heated. The heave is upward, over, but where extreme frost penetra- in a soil determines its frost-susceptibility. More than never inward. Key point—heated tion does not go deeper than 60 in. 3% grains finer than 0.02 mm in diameter by weight basements never, ever, experience inward frost heave, if they are warmer (1524 mm)† (see map in Figure 1). In usually are a problem. fact, even regions with extreme frost than the ground. This is true even if penetration deeper than 60 in. (1524 mm) rarely have problems the basement is insulated. But, if you don’t heat the basement, with foundations. But the stories persist, and rumors abound watch out (Photo 1).
54 ASHRAE Journal ashrae.org April 2010

ASHRAE Journal - April 2010

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