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.