Designing Houses That Keep Their Cool
Follow these strategies to reduce the need for air conditioning
Posted on Jul 4 2013 by Alex Wilson
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.
