Sheet 01 · Background

What causes frost heave under a freezer floor

Frost heave is the upward movement of a slab caused by ice forming in the soil below it. Under a freezer the cause is steady: the room is a heat sink that never shuts off.

01Heat flow through an insulated slab

Heat moves from the warmer ground up through the base, insulation and slab into the room. For a slab-on-grade the rate per square foot is roughly

q = ΔT ÷ R_total [Btu/h·ft²]

where ΔT is the difference between the ground and room temperatures and R_total is the combined thermal resistance of the insulation, slab and surface films.

Worked example

A 10,000 ft² freezer at −10 °F sits on ground that starts at 50 °F. With 6 in. of foam insulation at about R-5 per inch, plus the slab, R_total is about R-31.

q = 60 ÷ 31 ≈ 1.9 Btu/h·ft² (≈ 0.57 W/ft²)
Q = 1.9 × 10,000 ≈ 19,000 Btu/h, or about 5.7 kW, drawn out of the ground around the clock

Natural heat flow up from deep in the earth is on the order of 0.006 W/ft², about one percent of that loss. The ground cannot resupply the heat, so its temperature falls year after year until the soil freezes.

The example is for understanding only. The design tables on the System Design sheet add allowance for edge losses, cable spacing, derating in conduit and a safety margin.

Rule of thumb

Doubling the insulation halves the heat loss, and roughly doubles the time before the ground freezes. It delays the outcome; it does not change it.

Fig. 1Illustrative temperature profile. With heat in the base, the drop from ground to room temperature happens inside the insulation. Without it, the soil eventually falls below 32 °F. Not to scale.

02Three conditions for frost heave

Damaging heave needs all three of these at the same time:

Condition	Under a freezer
Freezing temperature in the soil	Supplied by the room itself, all year. Insulation only delays it.
Frost-susceptible soil	Silts, silty sands and lean clays are the most susceptible. Their fine pores pull water toward the freezing front by capillary action. Clean sand and gravel are much less susceptible.
A supply of water	A water table, perched groundwater, poor site drainage or wet fill. The soil does not need to be saturated when the building is new; water keeps migrating to the cold zone.

Water expands about 9 % when it freezes, but that alone would cause little movement. Heave comes mostly from ice lenses. As the freezing front stalls at a depth, water moves up to it and freezes in growing horizontal layers of clear ice. Stacked lenses can add inches or feet to the soil thickness.

Design note

Removing one condition helps but is hard to guarantee over the life of a building. Non-frost-susceptible fill and good drainage lower the risk. Keeping the soil above freezing removes the cause.

03Why the damage shows up years later

A new freezer sits on warm ground that has a large store of heat. The insulation slows the loss, so it can take one to several years, depending on room temperature, insulation, soil and building size, before the 32 °F line reaches the soil. By then there is a large mass of cold ground, and ice lenses keep growing even if the problem is caught.

Large floors are at the most risk in the center. Near the walls, heat from outside the building reaches the soil. The middle of a large freezer gets no such help, which is why heave usually shows as a hump away from the walls and why sensors are placed toward the center.

Common symptoms

Cracks radiating from the center of the floor, or a raised area you can see with a straightedge or laser level
Doors that drag, bind or no longer seal
Rack columns and walls out of plumb, damaged wall panels at the floor line
Frost or ice at slab joints and the floor–wall junction

Exploded view of freezer floor layers: concrete slab, two insulation layers, vapor barrier, heated aggregate and prepared subgrade — **Fig. 2**Floor build-up with a heated base. Heat is added below the vapor barrier and insulation so the soil, not the room, is kept warm.

04Ways to prevent it

Method	How it works	Notes
Electric heat cable	Self-regulating or constant-wattage cable in conduit in the base, switched by a controller.	No fluids or moving parts in the floor. Each circuit can be measured and alarmed. Design details
Hydronic (glycol)	Warm glycol pumped through plastic tubing in the base.	Can use waste heat from the refrigeration system. Needs pump, heater, manifold and fluid maintenance. Comparison
Ventilated base	Pipes or ducts in the base carry outside or building air, naturally or with fans.	Depends on the air being warmer than freezing. Not suited to cold climates without heated air.
Elevated floor	The freezer floor is built above a ventilated crawl space.	Separates the floor from the soil. High structural cost; common only in special cases.

05Existing freezers

Floors built with no subfloor heat, or where a glycol or air system has failed, can develop cold spots. When monitoring or a survey finds soil approaching freezing in a limited area, heaters can be installed from above by drilling through the slab and insulation and lowering heaters into the base at the trouble spot. This reclaim approach works best on small areas and must be evaluated for each floor, including the structural effect of the penetrations. Contact us with the floor history and any temperature data.