Chapter 9 - Building Heat Gain

Heat gain from walls

The total heat gain through the building wall includes heating from solar radiation as well as conduction heating from the outside air. The wall heat transmission coefficient is used to calculate heat gain through a wall based on the temperature difference across the wall. In order to calculate the total building heat gain, the appropriate heat transmission coefficient for each wall and roof surface must be estimated.

Floor contribution to summer heat gain or loss is usually ignored. With a litter floor the floor heat gain (or heat loss if the floor is cooler than the indoor air) is so small that it can be neglected. With a concrete floor there is likely to be some cooling effect by the floor in the hottest part of the day and some warming effect during the cooler evening and night periods, since the mass of the floor takes time to heat up and then cool down.

A practical method of accounting for solar heat gain uses the sol-air temperature concept. The sol-air temperature is the equivalent air temperature that causes the same heat gain through a wall as the conduction plus solar radiation heat gains. Thus it is a measure of the solar heating effect added to the effect of the outside air temperature.

The solar heat gain through a particular roof or wall surface is affected by that wall’s orientation; the outside solar heating for the location and time of year; cloudiness; wind; and the reflectiveness of the surface. Walls and roof surfaces facing the sum will obviously collect more solar heating than those facing away. Solar radiation is affected by the latitude of the location (with the North Pole receiving less radiation than the equator). Also, light-colored, shiny (metallic) surfaces reflect more and absorb less solar radiation than dull, dark surfaces. For example, at 40° north latitude, the difference between sol-air temperature and ambient temperature is 36°F for a light roof but 79°F for a dark roof. Note that the difference in the solar radiation loads between 32 and 40 degrees latitude is only 1%. So it is reasonable to use the above temperature differences for a wide range of locations.

Broiler heat production

Broiler sensible heat production is affected by air temperature and humidity; air speed; and the type, age and stocking density of the broilers. For summer design purposes, broilers are generally assumed to be market weight. Broiler sensible heat loss contributes to building air temperature rise and must be estimated to determine required ventilation rates in hot weather. The latent heat loss is affected by humidity, and it contributes to the indoor humidity level, which is more of a problem for winter environment control.