7. Heat Loss from Basement Walls and Floors

The floors and the underground portion of the walls of a basement are in direct contact with the ground, which is usually at a different temperature than the basement, and thus there is heat transfer between the basement and the ground. This is conduction heat transfer because of the direct contact between the walls and the floor, and it depends on the temperature difference between the basement and the ground, the construction of the walls and the floor, and the thermal conductivity of the surrounding earth. There is considerable uncertainty in the ground heat loss calculations, and they probably constitute the least accurate part of heat load estimates of a building because of the large thermal mass of the ground and the large variation of the thermal conductivity of the soil [it varies between 0.5 and 2.5 W/m · ºC (or 0.3 to 1.4 Btu/h · ft · ºF), depending on the composition and moisture content]. However, ground heat losses are a small fraction of total heat load of a large building, and thus it has little effect on the overall heat load.

Temperature measurements of uninsulated basements indicate that heat conduction through the ground is not one-dimensional, and thus it cannot be estimated by a simple one-dimensional heat conduction analysis. Instead, heat conduction is observed to be two-dimensional with nearly circular concentric heat flow lines centered at the intersection of the wall and the earth (Fig. 36). When partial insulation is applied to the walls, the heat flow lines tend to be straight lines rather than being circular. Also, a basement wall whose top portion is exposed to ambient air may act as a thermal bridge, conducting heat upward and dissipating it to the ambient from its top part. This vertical heat flow may be significant in some cases.

Despite its complexity, heat loss through the below-grade section of basement walls can be determined easily from

where

U_{wall, avg} = Average overall heat transfer coefficient between the basement wall and the surface of the ground
A_{wall, avg} = Wall surface area of the basement (underground portion)
T_basement = Interior air temperature of the basement
T_{ground surface} = Mean ground surface temperature in winter

The overall heat transfer coefficients at different depths are given in Table 14a for depth increments of 0.3 m (or 1 ft) for uninsulated and insulated concrete walls. These values are based on a soil thermal conductivity of 1.38 W/m · ºC (0.8 Btu/h · ft · ºF). Note that the heat transfer coefficient values decrease with increasing depth since the heat at a lower section must pass through a longer path to reach the ground surface. For a specified wall, U_{wall, avg} is simply the arithmetic average of the U_wall values corresponding to the different sections of the wall. Also note that heat loss through a depth incrementis equal to the U_wall value of the increment multiplied by the perimeter of the building, the depth increment, and the temperature difference.

The interior air temperature of the basement can vary considerably, depending on whether it is being heated or not. In the absence of reliable data, the basement temperature can be taken to be 10ºC since the heating system, water heater, and heating ducts are often located in the basement. Also, the ground surface temperature fluctuates about the mean winter ambient temperature by an amplitude A that varies with geographic location and the condition of the surface, as shown in Fig. 37. Therefore, a reasonable value for the design temperature of ground surface can be obtained by subtracting A for the specified location from the mean winter air temperature. That is, 

Heat loss through the basement floor is much smaller since the heat flow path to the ground surface is much longer in this case. It is calculated in a similar manner from

where Ufloor is the overall heat transfer coefficient at the basement floor whose values are listed in Table 14b, Afloor is the floor area, and the temperature difference is the same as the one used for the basement wall.

The temperature of an unheated below-grade basement is between the temperatures of the rooms above and the ground temperature. Heat losses from the water heater and the space heater located in the basement usually keep the air near the basement ceiling sufficiently warm. Heat losses from the rooms above to the basement can be neglected in such cases. This will not be the case, however, if the basement has windows.