Insulation does not stop the flow of heat, it slows it. The ground under a −20 °C room cools over time; the 0 °C boundary reaches the soil after months or a few years. As the water in the soil freezes it draws up new water from below, and lens-shaped layers of ice grow. This ice pushes the floor up by centimetres, in bad cases by 20–30 cm: the concrete cracks, the racking leans, the door will not close, the columns lift. The phenomenon is called frost heave. Because repair requires the store to be emptied and the floor taken up, the precaution is taken during construction.
There are three common methods. Electric heating: heating cables in protective conduit are laid in the concrete under the insulation; the installed power is typically 10–20 W/m², and it is the simplest solution for small and medium rooms. Glycol system: a glycol–water mixture at 10–20 °C is circulated through plastic pipes embedded under the floor; because the heat is recovered from the condenser side of the refrigeration system it costs almost nothing to run, and it is the standard for large stores. Ventilation: pipes of 100–150 mm diameter are laid under the floor and outside air is passed through them naturally or by fan.
Control is simple: sensors embedded under the floor switch the heating on and off so as to keep the temperature between +5 and +10 °C. Keeping it warmer is unnecessary, because part of the heat supplied passes through the insulation and becomes a load on the room. Sensors and cables are run in conduit so that they can be replaced later, and are laid with redundancy; before the concrete is poured the resistance and insulation of every circuit are measured and recorded. A fault alarm is essential: a heating failure may go unnoticed for months, and by the time it is noticed the ground will have frozen.
When is it needed? In every room whose floor sits on the ground and which runs continuously below zero: frozen storage, blast freezers, ice cream stores. In rooms on an intermediate floor, over a basement or on a steel platform open underneath it is not needed, since there is no contact with the ground; small rooms of a few square metres are mostly built on skids ventilated underneath. The risk is greatest where groundwater is high and the soil is fine-grained (clayey, silty). The small extra heat that underfloor heating brings to the room is included in the floor conduction item of the cooling load calculation.