Many buildings in colder climates have basements built below grade. This floor is often used for storage and other activities that don’t need a conditioned space such as mechanical equipment, parking, and janitorial functions. The widespread perception is that it wouldn’t be economical to use these spaces for offices or other business functions. In fact, entirely the opposite is true: below-grade zones cost much less to heat and cool than the rest of the building.
The only problem with such areas is the lack of windows. Many spaces in a building, though, are customarily constructed without any wall openings, such as laboratories, fabrication and assembly areas, and kitchens. Indeed, in many buildings, the majority of offices are interior and have no outside windows anyway.
The advantage of below-grade construction is subtle. Soil does not experience the wide temperature extremes of the ambient air: It remains colder in the heat of the summer and warmer in the cold of the winter. This reduces both the total annual load and the peak daily load upon the building air-conditioning system.
To understand the dynamics of this situation, it’s necessary to consider the effect of the building temperature upon the soil near the building shell. This material is warmed (or cooled, as the case may be) and the heat-transfer process is slowed. The reason for this is that the rate of heat transfer is in direct proportion to the difference in temperature between the heat source and the heat sink, the conditioned space and the ground mass, respectively. Decreasing the temperature variance between the building space and the ground outside reduces the flow of heat, out of the building in the winter and into it in the summer. This insulating effect doesn’t happen on the above-grade construction because air is not a stagnant quantity.
The air moves away as soon as it is tempered, and it’s replaced by more air at the ambient temperature. This means that the soil temperature, which remains at a nearly constant temperature through the day, is kinder to the building exoskeleton than the atmosphere. It doesn’t cause a high peak load to skew the equipment design and operating efficiency, and there’s no solar heat gain to cope with in the summer. Finally, there’s no unwanted infiltration through below-grade walls to impose a high moisture burden on the equipment. All this combines to reduce the energy needed to fully condition below-ground spaces.
The only drawback to below-grade construction is the absence of sunlight. This can be alleviated by the use of special fluorescent lamps that approximate the visual spectrum of natural light. Another possibility is the use of the new optic-fiber technology. It collects daylight at a parabolic collector and transmits the concentrated light to a remote location. From there, the light dissipates through a special fixture into the occupied space.
__________
Excerpt from Energy Conservation Projects.