Checklist for Reducing Cooling Loads Reducing Solar Gain
1. Site the building carefully. Orient the building so as to minimize heat gain through east- and west-facing windows and all skylights, yet provide for passive-solar heating during the winter and year-round daylighting. Try to take advantage of existing trees on the building site. 2. Use trees and other vegetation to shade the house. Design plantings (or house location) to provide shade on the east and west sides of the building and the roof, where heat gain is greatest (see Figure 2). Deciduous trees are effective because they lose their leaves in the winter and provide more light. Trellises with vines or tall annuals are effective at providing shade on the east and west side of a house. Vegetation around a house can also serve to cool the exterior through evapotranspiration. (Keeping the air around the house cooler will reduce conductive gains.) Consideration should be given to air flow through trees if natural ventilation is a part of the cooling strategy. 3. Minimize size of windows on east and west; limit size on south. Unless well shaded, the east- and west-facing window area should be small to minimize summer heat gain. Keep the south window below 8% of the floor area if special measures have not been taken for heat storage (slab floor, brick wall facing, etc.). If thermal storage and overhangs are provided, the area of south-facing windows can be up to about 12% of the floor area. 4. Use low-solar-transmittance glazings to reduce solar gain. For east- and west-facing windows and all skylights, use low-solar-heat-gain-coefficient or low-shading-coefficient glass to reduce solar heat gain. The solar heat gain coefficient is a newly defined property of window glazings that accounts for both solar gain and conductive gain; it is being adopted by the National Fenestration Rating Council as a replacement for the confusing shading coefficient. The low-E2 glass used by many of the largest window manufacturers has a solar heat gain coefficient of less than 50%, compared with conventional insulated glass at 89%, but unlike tinted glass, it has very good visible light transmittance properties. Note that high-solar-heat-gain-coefficient glazing may be preferred on south-facing glass to benefit from passive-solar heating. In existing buildings it may make sense to retrofit reflective films onto windows. 5. Shade windows with architectural features. South-facing windows can be shaded from the summer sun using overhangs or awnings, but the benefits are sometimes overemphasized by passive-solar designers. Make sure overhangs or awnings extend on both sides of the window to provide effective shading throughout the day. For east and west windows, consider wing walls, porches, ells, and attached garages to provide shading. 6. Use window treatments to reduce solar gain. Exterior shades provide the most effective shading. Moveable exterior shades are uncommon and have durability concerns, but sun-blocking screens, such as those produced by Pfiferwire of Tuscaloosa, Alabama, are very satisfactory. Interior shades and blinds are less effective but more common. They should have a light outer surface to reflect light back out through the window before the sunlight is absorbed and converted into heat.
Reducing Conductive Heat Gain
1. Provide adequate insulation levels. In most of North America, wall and ceiling insulation levels optimized to reduce winter heat loss will be adequate for reducing summertime heat gain. In some southern areas, more insulation is justified for cooling load avoidance than for winter heat loss. To reduce conductive heat gain, insulation in the roof or ceiling is most important. 2. Install radiant barrier in attics. Radiant barriers reflect heat radiation and can reduce cooling loads. A radiant barrier should be installed so as to provide an air space between it and the roof sheathing. It can be draped loosely over the top chord of the trusses or rafters before sheathing is installed, or it can be stapled to the underside of rafters or top chord of trusses. Radiant barriers are generally not cost-effective in northern climates, and even in the south, the costs and benefits should be weighed against those of additional insulation or a reflective roof surface. Two inches of insulation is roughly comparable to a radiant barrier in blocking heat gain. A reflective roof surface will keep out more heat gain than a radiant barrier. 3. Provide light-colored roof and wall surfaces. Conductive heat gain through the building envelope can be significantly reduced by making outer surfaces more reflective. Light-colored wall siding is beneficial, but most effective is reflective roofing. Choose a roofing material that is reflective (light-colored asphalt shingles are not—they still absorb nearly all of the sunlight striking them), or apply special reflective elastomeric coatings to less-reflective roof surfaces (see EBN Vol. 2, No. 5). Reflective roofing will provide the greatest savings in houses with air conditioning ducts running through the attic. 4. Provide attic or roof ventilation. Install continuous soffit and ridge vents to prevent high temperatures from building up in unheated attics—which will increase heat flow through the insulation. Operable windows in unheated attic spaces should also be considered. With cathedral ceilings (insulated roofs), provide soffit and ridge vents and a continuous air space under the roof sheathing for ventilation.
Reducing Infiltration and Ventilation Heat Gain
1. Design a tight envelope. Make sure the envelope is tight to reduce both sensible and latent infiltrative heat gain. When an air conditioner is operating, air leakage increases due to pressure imbalances in the building. 2. Suggest that building owners keep windows and doors closed during hot, humid weather. When outdoor temperature and humidity levels are above the comfort range, doors and windows should be kept closed. This will reduce demand on mechanical air conditioning or, without air conditioning, maintain more comfortable conditions. This strategy works well with nighttime ventilation in many areas. 3. Avoid inappropriate use of ventilation fans. Suggest using whole-house ventilation systems or window fans only when outside air is more comfortable than indoor air—typically during evening hours. With good air flow, whole-house ventilation can be done effectively with air temperatures as high as 82°F. Ventilation for indoor air quality maintenance will also increase cooling loads, but the air exchange rate is far lower than ventilation for cooling. Ceiling fans should be used only when the rooms are occupied to reduce heat gains from fan motors. 4. Keep moisture-laden air from the basement or attached greenhouse out of the living space. Situations in which moisture evaporates in one location (such as a basement or attached greenhouse) and the air from that space enters the house can also be sources of significant latent heat gain. The living space will not benefit from the evaporative cooling but will get the latent heat in the moisture-laden air. Moisture sources in these spaces (basements leaks, drying firewood, excessive plant watering) should be controlled, or the spaces should be vented to the outdoors.
Reducing Internal Heat Gain
1. Improve electrical efficiency. Install energy-efficient lighting, refrigerators, office equipment, and other electrical loads. Doubling the energy efficiency of lighting, for example, will reduce heat gain from lighting by 50%. Improving the motor and fan efficiency of HVAC equipment is an important way to reduce heat gain. 2. Insulate cooling system ducts. Seal and insulate any cooling system ducts that run outside of the insulated building envelope. Heat gain into these ducts can effectively increase the cooling load by 15%. When possible, cooling ducts should be located within the conditioned space. 3. Reduce losses from water heater and pipes. Insulate the water heater and hot-water pipes, and turn down the temperature setting on the water heater. In southern climates it may make sense to locate the water heater in an unconditioned garage or utility room. 4. Spot-ventilate heat sources. Vent kitchen ranges to the outside for indoor air quality reasons as well as for cooling load avoidance. In commercial buildings, it makes sense to vent refrigeration equipment, computer rooms, vending machine rooms, mechanical equipment rooms, and other locations of significant heat generation. 5. Minimize or vent water vapor sources.
Use spot fans or central ventilation systems to eliminate moisture sources from bathrooms and kitchen ranges. Use quiet fans that occupants will be likely to use. Clothes dryers should never be vented into the house.