Summer is here, with a pattern of hot weather and (in our part of the country) high humidity. We can be glad in Vermont that we’re not dealing with temperatures approaching 120°F, as are Phoenix and Las Vegas. (Death Valley was predicted to hit 130°F the other day, just four degrees shy of the highest temperature ever recorded on earth — 100 years ago.)
Most new homes in Vermont today are being built with central air conditioning, and in much of the rest of the country, one wouldn’t consider a house without it. I’m not going to suggest that we should skip air conditioning systems with new houses (particularly with global warming), but through good design and smart operation of homes, we should at least be able to minimize the use of that equipment.
So, here are my top-10 strategies for home design to minimize air conditioning needs. Some apply only to new construction, others to renovations as well. They are not ranked by priority.
1. If you’re building a new house, orient the house wisely
If the site permits, the house should be oriented so that more of the windows face south than either east or west. During the summer months, far more sunlight enters a house through east- or west-facing windows than through south- or north-facing windows.
It may also be possible to orient the house to benefit from summer breezes.
2. Shade windows
South-facing windows can effectively be shaded with fixed overhangs, because in the summer the sun is much higher in the sky than in the winter — when that sunlight is beneficial for heating. With fixed overhangs, overheating in the fall months may still be a concern — because by then the sun is lower in the sky, while the outdoor temperatures may still be fairly high.
On east and west windows, fixed overhangs do not work well for shading, because the sun’s path through the sky is fairly low as it rises and falls during the day. For these orientations, vertical louvers, exterior roller screens, operable awnings, and plantings can provide effective shading.
In general, shading windows on the exterior is better than using interior blinds, as it keeps out more unwanted heat.
3. Tune window glazing by orientation
With or without shading, we can control unwanted solar gain fairly well by installing windows with different types of glass on the different orientations.
On the south side, glass with a high solar heat gain coefficient (SHGC) makes sense to bring in significant solar heat during the heating season, while on the east and west, low-SHGC glass makes more sense.
In most applications, look for SHGC values of 0.3 or lower for east and west orientations, while SHGCs over 0.5 (or even 0.6) usually make more sense for the south if passive solar heat is desired in the winter.
4. Insulate the house well
Unwanted heat gain enters a house not only through windows, but also through the walls and roof of a house. Installing lots of insulation in the building envelope will go a long way toward minimizing unwanted heat gain.
In a northern climate, R-40 in the walls and R-50 or more in the ceiling or roof makes sense. The house should also be airtight, so that when you want to close up the house during the day to keep heat out, you can do so.
5. Provide a reflective roof
A light-colored, reflective roof may help to keep unwanted heat out of your house. The best such roof should not only be reflective, but also have high emissivity. (A bright, galvanized-metal roof is quite reflective, but it also has low emissivity, so it can introduce a lot of unwanted heat.) Look for roof materials certified by the Cool Roof Rating Council or the Energy Star Roof program.
Note that this benefit is often over-sold. With more insulation in the attic or roof, the benefit of reflective roofing will be less — though it will still help in reducing the urban heat island effect (a general warming of urban areas, largely due to absorption of solar energy).
6. Consider thermal mass
In some climates, providing thermal mass in the house can help reduce air conditioning requirements. Use of thermal mass, such as a brick chimney, plaster walls, or slate floor, can help keep a house interior from getting too hot during the day.
Heat is absorbed by these high-mass materials during the daytime, and effective night flushing (see #8, below) can then get rid of that heat at night. This benefit is greatest in climates with large diurnal temperature swings.
7. Control moisture well
Humidity makes us uncomfortable and raises air conditioning requirements. House design for passive cooling should provide for absolutely no moisture seepage into basements, should have proper flashing to avoid rain entry (you’d be surprised at how often flashing is done incorrectly), and should have bathroom fans to exhaust moisture where much of it is generated. Bath fans should be quiet to increase the likelihood that they will actually be used.
8. Design the house to facilitate effective ventilation
When the outdoor temperature and humidity are low enough, homeowners should be able to ventilate the house, exchanging warmer indoor air with cooler outdoor air. It often makes sense to close up a house during the daytime and then carry out “night flushing.”
For this to be effective, enough operable windows must be provided. In hot regions where the night temperatures drop, such as the Southwest, an outlet vent high in the building, even a specialized cooling chimney, can make a great deal of sense. Whole-house fans can increase this night flushing significantly.
9. Install efficient lights and appliances
Lighting and appliances can introduce a lot of unwanted heat. A standard incandescent light bulb, for example, converts only about 10% of the electric current into light; the rest is emitted as heat. Fluorescent lights produce a lot less waste heat than incandescents, and the best LED lights today are even better.
More efficient appliances (refrigerators, dishwashers, clothes washers, etc.) also produce a lot less unwanted heat.
10. Provide ceiling fans
Ceiling fans keep us cool by increasing evaporation from our skin. With gently moving air from such a fan, we may be comfortable with air temperatures as much as five degrees Fahrenheit warmer than with still air. This may reduce the periods during which mechanical air conditioning is needed.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
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