Bioclimatic design strategies

In winter (or underheated periods), the objectives of bioclimatic design are to resist loss of heat from the building envelope and to promote gain of solar heat. In summer (or overheated periods), these objectives are the reverse, to resist solar gain and to promote loss of heat from the building interior. The strategies can be set forth as:


• Minimize conductive heat flow. This strategy is achieved by using insulation. It is effective when the outdoor temperature is significantly different either lower or higher than the interior comfort range. 
In summer, this strategy should be considered whenever ambient temperatures are within or above the comfort range and where natural cooling strategies cannot be relied upon to achieve comfort (that is, mechanical air conditioning is necessary).


• Delay periodic heat flow. While the insulation value of building materials is well understood, it is not as widely appreciated that building envelope materials also can delay heat flows that can be used to improve comfort and to lower energy costs.
Time-lag through masonry walls, for example, can delay the day’s thermal impact until evening and is a particularly valuable technique in hot arid climates with wide day-night temperature variations. Techniques of earth-sheltering and berming also exploit the long-term heat flow effect of subsurface construction.


• Minimize infiltration. “Infiltration” refers to uncontrolled air leakage through joints, cracks, and faulty seals in construction and around doors and windows. Infiltration (and the resulting “exfiltration” of heated or cooled air) is considered the largest and potentially the most intractable source of energy loss in a building,
once other practical insulation measures have been taken.


• Provide thermal storage. Thermal mass inside of the insulated envelope is critical to dampening the swings in air temperature and in storing heat in winter and “coolth” in summer. (The term “coolth,” coined by John Yellott, describes the heat storage capacity of a cooled thermal mass, that is, its capacity to serve as a heat sink for cooling).


• Promote solar gain. The sun can provide a substantial portion of winter heating energy through elements such as equatorial-facing windows and greenhouses, and other passive solar techniques which utilize spaces to collect, store, and transfer solar heat.


• Minimize external air flow. Winter winds increase the rate of heat loss from a building by “washing away” heat and thus accelerating the cooling of the exterior envelope and also by increasing infiltration (or more properly, exfiltration) losses. Siting and shaping a building to minimize wind exposure or providing wind-breaks can reduce the impact of such winds.


• Promote ventilation. Cooling by air flow through an interior may be propelled by two natural processes, cross-ventilation (wind driven) and stack-effect ventilation (driven by the buoyancy of heated air even in the absence of external wind pressure). A fan can be used to augment natural ventilation cooling in the absence of sufficient wind or stack-pressure differential.


• Minimize solar gain. The best means for ensuring comfort from the heat of summer is to minimize the effects of the direct sun, the primary source of overheating, by shading windows from the sun, or otherwise minimizing the building surfaces exposed to summer sun, by use of radiant barriers, and by insulation.


• Promote radiant cooling. A building can lose heat effectively if the mean radiant temperature of the materials at its outer surface is greater than that of its surroundings, principally the night sky.
The mean radiant temperature of the building surface is determined by the intensity of solar irradiation, the material surface (film coefficient) and by the emissivity of its exterior surface (its ability to “emit” or re-radiate heat). This contributes little, however, if the building envelope is well insulated.
• Promote evaporative cooling. Sensible cooling of a building interior can be achieved by evaporating moisture into the incoming air stream (or, if an existing roof has little insulation, by evaporatively cooling the exterior envelope, such as by a roof spray.)
 These are simple and traditional techniques and most useful in hot-dry climates if water is available for controlled usage. Modern evaporative cooling is achieved with an economizer-cycle evaporative cooling system, instead of, or in conjunction with, refrigerant air conditioning.