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.

FIGURE 36
Radial isotherms and circular heat flow lines during heat flow from uninsulated basement.

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

Uwall, avg = Average overall heat transfer coefficient between the basement wall and the surface of the ground
Awall, avg = Wall surface area of the basement (underground portion)
Tbasement = Interior air temperature of the basement
Tground 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, Uwall, avg is simply the arithmetic average of the Uwall values corresponding to the different sections of the wall. Also note that heat loss through a depth incrementis equal to the Uwall value of the increment multiplied by the perimeter of the building, the depth increment, and the temperature difference.

FIGURE 37
Lines of constant amplitude of annual soil temperature swings.

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.

FIGURE 39
An on-grade concrete floor with insulated foundation wall.

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 Ugrade represents the rate of heat transfer from the slab per unit temperature difference between the indoor temperature Tindoor and the outdoor temperature Toutdoor and per unit length of the perimeter pfloor of the building.

Typical values of Ugrade 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. 

Heat Loss from Crawl Spaces

A crawl space can be considered to be a small basement except that it may be vented year round to prevent the accumulation of moisture and radioactive gases such as radon. Venting the crawl space during the heating season creates a low temperature region underneath the house and causes considerable heat loss through the floor. The ceiling of the crawl space (i.e., the floor of the building) in such cases must be insulated. If the vents are closed during the heating season, then the walls of the crawl space can be insulated instead.

The temperature of the crawl space will be very close to the ambient air temperature when it is well ventilated. The heating ducts and hot water pipes passing through the crawl space must be adequately insulated in this case. In severe climates, it may even be necessary to insulate the cold water pipes to prevent freezing. The temperature of the crawl space will approach the indoor temperature when the vents are closed for the heating season. The air infiltration in this case is estimated to be 0.67 air change per hour.

When the crawl space temperature is known, heat loss through the floor of the building is determined from

where Ubuilding floor is the overall heat transfer coefficient for the floor, Afloor is the floor area, and Tindoor and Tcrawl are the indoor and crawl space temperatures, respectively.

Overall heat transfer coefficients associated with the walls, floors, and ceilings of typical crawl spaces are given in Table 15. Note that heat loss through the uninsulated floor to the crawl space is three times that of the  insulated floor. The ground temperature can be taken to be 10ºC when calculating heat loss from the crawl space to the ground. Also, the infiltration heat loss from the crawl space can be determined from 

where ACH is the air changes per hour, Vcrawl is the volume of the crawl space, and Tcrawl and Tambient are the crawl space and ambient temperatures, respectively.

In the case of closed vents, the steady state temperature of the crawl space will be between the indoors and outdoors temperatures and can be determined from the energy balance expressed as

and assuming all heat transfer to be toward the crawl space for convenience in formulation.

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