Eco-friendly ways to keep your home cool in summer

Sunset  – July 15, 2009

To keep a house cool, it’s all about (1) keeping the sun out, or (2) keeping the air cool/conditioned. The key to this efficiency is having a solid thermal envelope for your home. In a typical residence, you’ll see about 30 percent air leakage in the house duct system (this means hot air from the attic and from outdoors is leaking into the living spaces through electric plug sockets, recessed light fixtures, and the like). If your budget allows, have a local home-energy auditor do an infiltration test (or whole-house energy audit) to identify where leaks are located. Sustainable Spaces provides this service for homes in the greater San Francisco Bay Area; to find an auditor near you, visit

How to keep your home cool without overheating the planet: 

Add sealing and insulation

• Heat shield: Have dense insulation―such as foam or blown cellulose―installed in your attic, if you have one.

• Thermal envelope: Seal air leaks around electrical sockets, doors, and windows with weather-stripping.

Increase ventilation

• Hot air out, cool air in: Obvious, but worth repeating―on hot days, keep your doors, windows, and shades closed until it’s cooler outside.

• Fans don’t cool air, they move it: Fans cool you only when you’re in the room with them. So turn them off when you leave.

Take advantage of shade

• Sun block: Exterior awnings and trees keep solar heat from direct contact with windows and roofs.

• No more greenhouse: Install reflective film, sunscreen-fabric curtains, or roller shades. If you’re investing in new windows, choose panes with a low solar heat gain coefficient (SHGC).

Assess your house’s current situation with these three questions: 

1. Is there an attic that can be insulated? How much existing insulation is there? How well is it insulated? 

2. Are there eaves that are blocking some of the summer sun? (Overhangs should block the sun in the summer and allow it to come in during the winter.) 

3. Is there an existing cooling system? Is it properly sized?

Optimal solution:

For an average house with an attic in a temperate climate, we recommend reducing the cooling load as much as possible and making use of passive cooling, via the following steps. 

• Improve the sealing around all windows and doors 

• Ensure that there’s sufficient attic insulation to create a thermal barrier 

• Install a radiant barrier if there are ducts in your attic or your AC is in the attic―install it under the roof deck, not on the attic floor 
• Add low-E glazing to windows 

• Install a whole-house fan. Whole-house fans―which can use up to 90 percent less energy than AC units―draw fresh cool air into the house at night, exhausting hot air through vents. For more details, visit
• Install high-quality windows with a low solar heat gain co-efficient (SHGC)

The passive-cooling route is very cost effective from a long-term standpoint (i.e., you’re not paying to condition the air, you’re just paying for the equipment and installation). 

Things to keep in mind when you’re thinking of installing a whole house fan:

• It will need proper ventilation 

• You may need more than one whole house fan, depending on the size of your home 

• Noise may be a factor for older models; newer models are quieter 

• The economics of operation: A fan requires more upfront cost than an AC unit, but its operating costs can be as little as 1/20th that of air-conditioning)
Visit for more information.

Things to keep in mind when shopping for windows: 

• U-value: This is the measure of how well the panes keep heat in the house from escaping. 

• Solar heat gain coefficient (SHGC): This measures how much solar heat the window prevents from entering the house.

If you don’t have an attic and therefore can’t install insulation, you may consider reroofing and re-insulating the roof. 

• Use rigid foam insulation and closed-cell spray foam with a reflective coating (aka “cool roofs”)

Alternative solution:

Can’t bear to give up your air conditioner? Here’s how to reduce the load.

• Close your windows and blinds so the AC functions as efficiently as possible.
• Turn up your thermostat: You use 1 to 3 percent less energy for every degree that it’s set above 72.
• Use the AC only on the hottest days, and remember to turn off your system (or set it to “away” mode) when you’re traveling.
• Any exterior AC equipment should be installed on the north or shaded side of the house, with at least 18 inches of free space around it.
• If you’re buying a new AC system, choose the proper size for your space―a unit that’s too large will cycle on and off repeatedly, which wastes energy. Also, look for a high seasonal energy-efficiency ratio (SEER) rating―16 to 18, if possible. Be sure to take advantage of the stimulus plan to improve your home’s energy efficiency. Visit for details.
• If air handling equipment is located in the attic, we recommend encapsulating the attic with air-impermeable spray foam and bringing the attic into the building envelope. This prevents hot attic air (150° Fahrenheit or higher) from infiltrating the duct system

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