Keeping cool indoors when it is hot outdoors is a problem. The sun
beating down on our homes causes indoor temperatures to rise to
uncomfortable levels. Air conditioning provides some relief. But the
initial costs of installing an air conditioner and the electricity costs
to run it can be high. In addition, conventional air conditioners use
refrigerants made of chlorine compounds, suspected contributors to the
depletion of the ozone layer and global warming. But there are
alternatives to air conditioning. This information provides some common
sense suggestions and low-cost retrofit options to help you "keep your
cool"- and save electricity.
An alternative way to maintain a cool house or reduce air conditioning
use is natural (or passive) cooling. Passive cooling uses non-mechanical
methods to maintain a comfortable indoor temperature.
The most effective method to cool your home is to keep the heat from
building up in the first place. The primary source of heat buildup
(i.e., gain) is sunlight absorbed by your house through the roof, walls,
and windows. Secondary sources are heat generating appliances in the
home and air leakage. Specific methods to prevent heat gain include
reflecting heat (i.e., sunlight) away from your house, blocking the
heat, removing built up heat, and reducing or eliminating heat generating sources in your home.
Reflecting Heat Away
The most effective method to cool your home is to keep the heat from
building up in the first place. Dull, dark colored home exteriors absorb 70% to 90% of the radiant
energy from the sun that strikes the home's surfaces. Some of this
absorbed energy is then transferred into your home by way of conduction,
resulting in heat gain. In contrast, light colored surfaces effectively
reflect most of the heat away from your home.
Installing a radiant barrier
Radiant barriers are easy to install. It does not matter which way the
shiny surface faces - up or down. But you must install it on the
underside of your roof - not horizontally over the ceiling, and the
barrier must face an airspace.
For your own comfort while in the attic, install the radiant barrier on
a cool, cloudy day. Use plywood walk boards or wooden planks over the
ceiling joists for support. Caution: Do not step between the ceiling
joists, or you may fall through the ceiling.
Staple the foil to the bottom or side of the rafters, draping it from
rafter to rafter. Do not worry about a tight fit or small tears in the
fabric; radiant transfer is not affected by air movement. The staples
should be no more than 2 to 3 inches (5 to 8 centimeters) apart to
prevent air circulation from loosening or detaching the radiant barrier.
Use a caulking gun to apply a thin bead of construction adhesive to the
rafters along the seams of the foil barrier. This will make the
About a third of the unwanted heat that builds up in your home comes in
through the roof. This is hard to control with traditional roofing
materials. For example, unlike most light colored surfaces, even white
asphalt and fiberglass shingles absorb 70% of the solar radiation. One
good solution is to apply a reflective coating to your existing roof.
Two standard roofing coatings are available at your local hardware store
or lumberyard. They have both waterproof and reflective properties and
are marketed primarily for mobile homes and recreational vehicles. One
coating is white latex that you can apply over many common roofing
materials, such as asphalt and fiberglass shingles, tar paper, and metal.
A second coating is asphalt based and contains glass fibers and aluminum
particles. You can apply it to most metal and asphalt roofs. Because it
has a tacky surface, it attracts dust, which reduces its reflective
Another way to reflect heat is to install a radiant barrier on
the underside of your roof. A radiant barrier is simply a sheet of aluminum
foil with a paper backing. When installed correctly, a radiant barrier can
reduce heat gains through your ceiling by about 25%. (see box for
information on installing a radiant barrier.)
Radiant barrier materials cost between $0.13 per square foot ($1.44 per
square meter) for a single-layer product with a kraft-paper backing and
$0.30 per square foot ($3.33 per square meter) for a vented multiflora
product with a fiber-reinforced backing. The latter product doubles as
Wall color is not as important as roof color, but does affect heat gain
somewhat. White exterior walls absorb less heat than dark walls, and
light, bright walls increase the longevity of siding, particularly on
the east, west, and south sides of the house.
Roughly 40% of the unwanted heat that builds up in your home comes in
through windows. Reflective window coatings are one way to reflect heat
away from your home. These coatings are plastic sheets treated with dyes
or thin layers of metal. Besides keeping your house cooler, these
reflective coatings cut glare and reduce fading of furniture, draperies,
Two main types of coatings include sun-control films and combination
films. Sun-control films are best for warmer climates because they can
reflect as much as 80% of the incoming sunlight. Many of these films are
tinted, however, and tend to reduce light transmission as much as they
reduce heat, thereby darkening the room.
Combination films allow some light into a room but they also let some
heat in and prevent interior heat from escaping. These films are best
for climates that have both hot and cold seasons. Investigate the
different film options carefully to select the film that best meets your
needs. Note: do not place reflective coatings on south facing windows if
you want to take advantage of heat gain during the winter. The coatings
are applied to the interior surface of the window. Although you can
apply the films yourself, it is a good idea to have a professional
install the coatings, particularly if you have several large windows.
This will ensure a more durable installation and a more aesthetically
Blocking the Heat
Two excellent methods to block heat are insulation and shading.
Insulation helps keep your home comfortable and saves money on
mechanical cooling systems such as air conditioners and electric fans.
Shading devices block the sun's rays and absorb or reflect the solar
Weatherization measures - such as insulating, weather stripping, and
caulking - help seal and protect your house against the summer heat in
addition to keeping out the winter cold. The attic is a
good place to start insulating because it is a major source of heat
gain. Adequately insulating the attic protects the upper floors of a
house. Recommended attic insulation levels depend on where you live and
the type of heating system you use. For most climates, you want a minimum
of R-30. In climates with extremely cold winters, you may want as much as
Wall insulation is not as important for cooling as attic insulation
because outdoor temperatures are not as hot as attic temperatures. Also,
floor insulation has little or no effect on cooling.
Although unintentional infiltration of outside air is not a major
contributor to inside temperature, it is still a good idea to keep it
out. Outside air can infiltrate your home around poorly sealed doors,
windows, electrical outlets, and through openings in foundations and
exterior walls. Thorough caulking and weather stripping will control
most of these air leaks.
Shading your home can reduce indoor temperatures by as much as 20°F
(11°C). Effective shading can be provided by trees and other vegetation
and exterior or interior shades.
Landscaping is a natural and beautiful way to shade your home and block
the sun. A well placed tree, bush, or vine can deliver effective shade
and add to the aesthetic value of your property. When designing your
landscaping, use plants native to your area that survive with minimal
care. Trees that lose their leaves in the fall (i.e., deciduous) help
cut cooling energy costs the most. When selectively placed around a
house, they provide excellent protection from the summer sun and permit
winter sunlight to reach and warm your house. The height, growth rate,
branch spread, and shape are all factors to consider in choosing a tree.
Vines are a quick way to provide shading and cooling. Grown on
trellises, vines can shade windows or the whole side of a house. Ask
your local nursery which vine is best suited to your climate and needs.
Besides providing shade, trees and vines create a cool microclimate that
dramatically reduces the temperature (by as much as (9°F [5°C]) in the
surrounding area. During photosynthesis, large amounts of water vapor
escape through the leaves, cooling the passing air. and the generally dark
and coarse leaves absorb solar radiation. You might also consider low
ground cover such as grass, small plants, and bushes. A grass-covered lawn
is usually 10°F (6°C) cooler than bare ground in the summer. If you are in
an arid or semiarid climate, consider native ground covers that require
Planning Your Planting
Placement of vegetation is important when landscaping your home. The
following are suggestions to help you gain the most from vegetation.
Plant trees on the northeast-southeast and the northwest-southwest
sides of your house. Unless you live in a climate where it is hot
year round, do not plant trees directly to the south. Even the bare
branches of mature deciduous trees can significantly reduce the
amount of sun reaching your house in the winter.
Plant trees and shrubs so they can direct breezes. Do not place a
dense line of evergreen trees where they will block the flow of
air around or through them.
- Set trellises away from your house to allow air to circulate and
the vines from attaching to your house's facade and damaging its
exterior. Placing vegetation too close to your house can trap heat
and make the air around your house even warmer.
Do not plant trees or large bushes where their roots can damage
septic tanks, sewer lines, underground wires, or your house's foundation.
Make sure the plants you choose can withstand local weather
Both exterior and interior shades control heat gain. Exterior shades are
generally more effective than interior shades because they block
sunlight before it enters windows. When deciding which devices to use
and where to use them, consider whether you are willing to open and
close them daily or just put them up for the hottest season. You also
want to know how they will affect ventilation.
Exterior shading devices include awnings, lovers, shutters, rolling
shutters and shades, and solar screens. Awnings are very effective
because the block direct sunlight. They are usually made of fabric or
metal and are attached above the window and extend down and out. A
properly installed awning can reduce heat gain up to 65% on southern
windows and 77% on eastern windows. A light colored awning does double
duty by also reflecting sunlight.
Maintaining a gap between the top of the awning and the side of the
house helps vent accumulated heat from under a solid- surface awning. If
you live in a climate with cold winters, you will want to remove awnings
for winter storage, or by retractable ones, to take advantage of winter
The amount of drop (how far down the awing comes) depends on which side
of your house the window is on. An east or west window needs a drop of
65% to 75% of the window height. A south-facing window only needs a drop
of 45% to 60% for the same amount of shade. A pleasing angle to the eye
for mounting and awning is 45°. Make sure the awning does not project
into the path of foot traffic unless it is at least 6 feet 8 inches (2
meters) from the ground.
One disadvantage of awnings is that they can block views, particularly
on the east and west sides. However, slatted awnings do allow limited
viewing through the top parts of windows.
Louvers are attractive because their adjustable slats control the level
of sunlight slats control the level of sunlight entering your home and,
depending on the design, can be adjusted from inside or outside your
house. The slats can be vertical or horizontal. Louvers remain fixed and
are attached to the exteriors of window frames.
Shutters are movable wooden or metal covering that, when closed, keep
sunlight out. Shutters are either solid or slatted with fixed or
adjustable slats. Besides reducing heat gain, they can provide privacy
and security. Some shutters help insulate windows when it is cold
Rolling shutters have a series of horizontal slats that run down along a
track. Rolling shades use a fabric. These are the most expensive shading
options, but the work well and can provide security. Many exterior
rolling shutters or shades can be conveniently controlled from the
inside. One disadvantage is that when fully extended, the block all
Solar screens resemble standard window screens except they keep direct
sunlight from entering the window, cut glare, and block light without
blocking the view or elimination air flow. They also provide privacy by
restricting the view of the interior from outside your house. Solar
screens come in a variety of colors and screening materials to
compliment any home. Although do-it-yourself kits are available, these
screens will not last as long as professionally built screens.
Although interior shading is not as effective as exterior shading, it is
worthwhile if none of the previously mentioned techniques are possible.
There are several ways to block the sun's heat from inside your house.
Draperies and curtains made of tightly woven, light-colored, opaque
fabrics reflect more of the sun's rays than they let through. The
tighter the curtain is against the wall around the window, the better it
will prevent heat gain. Two layers of draperies improve the
effectiveness of the draperies' insulation when it is either hot or cold
Venetian blinds, although not as effective as draperies, can be adjusted
to let in some light and air while reflecting the sun's heat. Some newer
blinds are coated with reflective finishes. To be effective, the
reflective surfaces must face the outdoors. Some interior cellular
(honeycombed) shades also come with reflective mylar coatings. But they
block natural light and restrict air flow.
Opaque roller shades are effective when fully drawn but also block light
and restrict air flow.
Removing Built-Up Heat
Nothing feels better on a hot day than a cool breeze. Encouraging cool
air to enter your house forces warm air out, keeping your house
comfortably cool. However, this strategy only works when the inside
temperature is higher than the outside temperature.
Natural ventilation maintains indoor temperatures close to outdoor
temperatures close to outdoor temperatures and helps remove heat from
your home. But only ventilated during the coolest parts of the day or
night, and seal off your house from the hot sun and air during the
hottest parts of the day. The climate you live in determines the best
ventilation strategy. In areas with cool nights and very hot days, let
the night air in to cool your house. By the time the interior heats up, and the outside air
should be cooler and can be allowed indoors.
In climates with day time breezes, open windows on the side from where
the breeze is coming and on the opposite side of the house. Keep
interior doors open to encourage whole house ventilation. If your
location lacks consistent breezes, create them by opening the windows at
the lowest and highest points in your house. This natural
"thermo siphoning," or "chimney," effect can be taken a step further by
adding a clerestory or a vented skylight.
In hot, humid climates where temperature swings between day and night
are mall, ventilate when humidity is not excessive. Ventilating your
attic greatly reduces the amount of accumulated heat, which eventually
works its way into the main part of your house. Ventilated attics are
about 30°F (16°C) cooler than unventilated attics. Properly sized and
placed louvers and roof vents help prevent moisture buildup and overheating
in your attic.
Reducing Heat-Generating Sources
Often overlooked sources of interior heat gain are lights and household
appliances, such as ovens, dishwashers, and dryers. Because most of the
energy that incandescent lamps use is given off as heat, use them only
when necessary. Take advantage of daylight to illuminate your house, and
consider switching to compact fluorescent lamps. These use about 75% less
energy than incandescent lamps, and emit 90% less heat for the same amount
New, energy efficient appliances generate less heat and use less energy.
Many household appliances generate a lot of heat. When possible, use
them in the morning or late evening when you can better tolerate the
extra heat. Consider cooking on an outside barbecue grill or use a
microwave oven, which does not generate as much heat and uses less
energy than a gas or electric range.
Washers, dryers, dishwashers, and water heaters also generate large
amounts of heat and humidity. To gain the most benefit, seal off your
laundry room and water heater from the rest of the house.
New, energy efficient appliances generate less heat and use less energy.
When it is time to purchase new appliances, make sure the are energy
efficient. All refrigerators, dishwashers, and dryers display an energy
guide label indicating the annual estimated cost for operating the
appliance or a standardized energy efficiency ratio. Compare appliances and
buy the most efficient models for your needs.
Using any or all of these strategies will help keep you cool. Even if
you use air conditioning, many of these strategies, may not be enough.
Sometimes you need to supplement natural cooling with mechanical
devices. Fans and evaporative coolers can supplement your cooling
strategies and cost less to install and run than air conditioners.
Ceiling fans make you feel cooler. Their effect is equivalent to
lowering the air temperature by about 4°F (2°C). Evaporative coolers use
about one-fourth the energy of conventional air conditioners.
Many utility companies offer rebates and other cost incentives when you
purchase or install energy saving products, such as insulation and
energy efficient lighting and appliances. Contact your local utility
company to see what it offers in the way of incentives.
Cooling Strategies Checklist
Cooling strategies to consider:
lighten roof and exterior wall color
- replace/coat roof with bright white or shiny material
- install a radiant barrier
- add reflective coatings to windows
- insulate attic and walls
- caulk and weather strip to seal air leaks
- add shade trees, bushes, or vines
- add exterior awnings and shades
- add interior drapes and shades
- ventilate attic
- increase natural ventilation
- isolate heat-generating appliances
- replace heat-generating appliances
- replace light bulbs with energy-efficient fluorescent's