Winter and summer Strategies:
- 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.
- Minimize infiltration. “Infiltration” refers to uncontrolled air leakage around doors and windows and through joints, cracks, and faulty seals in the building envelope. 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.
- 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-lag effect of subsurface construction.
- 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 as a heat sink in summer.
Winter Strategies:
- 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 use 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 surfaces by conduction, and also by increasing infiltration (or more properly, exfiltration) losses. Siting and shaping a building to minimize wind exposure or providing windbreaks can reduce the impact of such winds.
Summer strategies:
- 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 (using photovoltaic for fan power) can be an efficient way 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 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 only marginally, 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 evaporative cooling the exterior envelope such as by a roof spray.) These simple and traditional techniques are most useful in hot-dry climates if water is available for controlled usage. Mechanically assisted evaporative cooling is achieved with an economizer-cycle evaporative cooling system, instead of, or in conjunction with, refrigerant air conditioning.
Design of Atriums:
Solar heating
If heating efficiency alone is the primary energy design goal of the atrium, the following design principles should be paramount:
- H1 To maximize winter solar heat gain, orient the atrium aperture (openings and glazing) to the equator. If possible, the glazing should be vertical or sloped not lower than a tilt angle equal to the local latitude.
- H2 For heat storage and radiant distribution, place interior masonry directly in the path of the winter sun. This is most useful if the heated wall or floor surface will in turn directly radiate to building occupants.
- H3 To prevent excessive nighttime heat loss, consider an insulating system for the glazing, such as insulating curtains or high performance multi-layered window systems.
- H4 To recover the heat that rises by natural convection to the top of the atrium, place a return air duct high in the space, possibly augmenting its temperature by placing it directly in the sun. Heat recovery can be accomplished if the warm air is redistributed either to the lower area of the atrium (a ceiling fan) or redirected (and cleaned) to the mechanical system, or through a heat exchanger if the air must be exhausted for health and air-quality reasons.
Why use it?
Because a large air volume must be heated, the high volume helps to make an overheated space acceptable, especially if the warmest air rises to the top. In cool climates, an atrium used as a solar heat collector would require as much winter sunlight as possible. In over bright conditions, dark finishes on surfaces where the sun strikes will help reduce glare and also to store heat. On surfaces not in direct sun, light finishes may be best to reflect light, especially welcomed under cloudy conditions.
Natural cooling
Several guidelines related to the use of an atrium design as an intermediary or buffer zone apply to both heating and cooling. If an unconditioned atrium is located in a building interior, the heat loss is from the warmer surrounding spaces into the atrium. In buildings with large internal gains due to occupants, lighting, and machines, the atrium may require cooling throughout the year. If one were to design exclusively for cooling, the following principles would predominate:
- C1 To minimize solar gain, provide shade for the summer sun. According to the particular building-use, the local climate and the resulting balance point (the outside temperature below which heating is required); the “overheated” season when sun shading is needed may extend well into the autumn months. While fixed shading devices suffice for much of the summer period, movable shading is the only exact means by which to match the seasonal shading requirements at all times. In buildings in warm climates, sun shading may be needed throughout the year.
- C2 Use the atrium as an air plenum in the mechanical system of the building. The great advantage is one of economy, but heat recovery options (discussed above) and ventilation become most effective when the natural airflow in the atrium is in the same direction and integrated with the mechanical system.
- C3 To facilitate natural ventilation, create a vertical “chimney” effect by placing ventilating outlets high (preferably in the free-flow air stream well above the roof) and by providing cool “replacement air” inlets at the atrium bottom, with attention that the air stream is clean, that is, free of car exhaust or other pollutants.
Daylighting
In all climates, an atrium can be used for daylighting. Electric lighting cost savings can be achieved, but only if the daylighting system works; that is, if it replaces the use of artificial lighting. Atriums serve a particularly useful function in daylighting design for an entire building by balancing light levels—thus reducing brightness ratios—across the interior floors of a building. If, for example, an open office floor has a window wall on only one side, typically more electric lighting is required than would be required without natural lighting to reduce the brightness ratio. An atrium light court at the building interior could provide such balanced “two source” lighting. An atrium designed as a “lighting fixture” that reflects, directs, or diffuses sunlight, can be one of the most pleasing means of controlling light.
The following principles apply to atrium design for daylighting:
- L1 To maximize daylight, an atrium cross-section should be stepped open to the entire sky dome in predominantly cloudy areas. In predominantly sunny sites, atrium geometry can by based upon heating and/or cooling solar orientation principles.
- L2 To maximize light, window or skylight apertures should be designed for the predominant sky condition. If the predominant sky condition is cloudy and maximum daylight is required (as in a northern climate winter garden), consider clear glazing oriented to the entire sky dome, with movable sun controls for sunny conditions. If the predominant sky condition is sunny, orient the glazing according to heating and/or cooling design requirements.
- L3 Provide sun-and-glare control by geometry of aperture, surface treatment, color, and adjustable shades or curtains.