Concrete Floors on Grade (at Ground Level)

Many residential and commercial buildings do not have a basement, and the floor sits directly on the ground at or slightly above the ground level. Research indicates that heat loss from such floors is mostly through the perimeter to the outside air rather than through the floor into the ground, as shown in Fig. 39. Therefore, total heat loss from a concrete slab floor is proportional to the perimeter of the slab instead of the area of the floor and is expressed as

where U_grade represents the rate of heat transfer from the slab per unit temperature difference between the indoor temperature T_indoor and the outdoor temperature T_outdoor and per unit length of the perimeter p_floor of the building.

Typical values of U_grade are listed in Table 14c for four common types of slab-on-grade construction for mild, moderate, and severe weather conditions. The ground temperature is not involved in the formulation since the slab is located above the ground level and heat loss to the ground is negligible. Note from the table that perimeter insulation of slab-on-grade reduces heat losses considerably, and thus it saves energy while enhancing comfort. Insulation is a must for radiating floors that contain heated pipes or ducts through which hot water or air is circulated since heat loss in the uninsulated case is about three times that of the insulated case. This is also the case when base board heaters are used on the floor near the exterior walls. Heat transfer through the floors and the basement is usually ignored in cooling load calculations.