Understanding Freezer Floor Heave
By: Robert D Judson, PE
We receive several calls a month from clients wanting to know why their freezer floors are rising in the middle and heaving. Freezer floor slab design is assumed to be simple. Design solutions are shared and misapplied. Freezers are built and floors heave. Why? The concept of preventing floor heave has been misunderstood and misapplied for years resulting in freezer floor failure which causes facility shutdowns and litigation.
One of the most misunderstood concepts is the relationship of vapor pressure and temperature and their affects on freezers. Cold air has a lower vapor pressure than warm air. Warmer air will move toward cooler air because of the driving forces of vapor differences between cold and warm air. Warm air has a higher vapor pressure which will force its way through a pin hole or move through earth and soil until the vapor reaches equilibrium or adheres to the underside of a frozen slab. Warm moist air in the soil will migrate to the bottom of the freezer slab if a vapor barrier is not installed. In order to keep the vapor from freezing beneath the slab, the earth must be warmed above the freezing point. The freezer slab must be insulated to retain the loss of heat from the slab and balance the vapor pressure with the warm ground below the freezer floor.
What are the key design elements of a freezer floor?
Concrete Slab Design – The concrete slab must be designed to resist the punching shear and weight of the pallet racks.
Under Floor Insulation – Under floor insulation must provide sufficient compressive strength to support the loads from the overlaying floor slab without crushing it. EPS is recommended since it provides a compressive strength of 30 PSI. The insulation should be placed directly below the freezer slab.
Vapor Barrier – A vapor barrier must be placed directly below the insulation. The vapor barrier keeps vapor from migrating from the soil and through the insulation to the bottom of the freezer slab.
Mud Slab – A mud slab is to be placed below the vapor barrier. The mud slab is made up of a rich concrete sand mix with no aggregate and contains the under floor heat transfer media.
Under Floor Heat – Beneath the mud slab, under floor heat is required to keep the earth from freezing. The insulation R value is used in conjunction with the difference in soil and freezer temperatures to calculate the heat required to avoid ice buildup. Good design practice calls for a value of 1.5 to 2.5 BTU/SF/Hr. Heat applied using gravity or forced air ducts is not recommended due to a history of failure. Recommended options are:
Electric Under Floor Heat – Use of under floor heat tape in conjunction with a temperature sensor placed below the mud slab can be an economical solution to provide heat. It can be used as a corrective measure to repair a heaved floor slab. Repair is only an option if the heat tapes can be placed inside 1” galvanized conduits located in the sealed end of failed air ducts.
Glycol Tubing Under Floor Heat – One of the most economical methods of applying heat to a freezer floor is to pump glycol through tubing that has been heated from a waste heat source. Waste heat sources can be via an ammonia compressor, air compressor, or even through solar collection.
Summary – If you have a freezer floor that is heaving, the under floor heating system has most likely failed. Correction of floor heave can possibly be achieved with the supply of adequate floor heat. Specific design solutions for correcting floor heave will depend on the amount of insulation installed, the presence of a vapor barrier, and if heat can be reapplied to bring the floor back. Corrections should be addressed as quickly as possible to minimize freezer floor damage.