Many of these energy conservation measures are taken to reduce the flow of energy into your home through your roof in the hotter months, to lower your air conditioning costs. A light-colored roof will reflect more solar radiation than a dark roof and also allow less heat to enter when the air is hotter than the roof. In fact, a white roof will reflect more than half of the solar radiation that strikes it and a black roof will absorb almost all of it. A white roof can reduce annual air-conditioning energy use of a single-story building up to 15%, relative to a black roof in a very hot climate. Air conditioning use is reduced by a lower percentage in two- and three-story houses because their roofs are smaller relative to their total floor space. The ability of a surface to reflect solar radiation is called its “solar reflectance” or “reflectance”. The ability of a surface to resist heat flowing through it due to temperature difference is called, “thermal emittance” or “emittance”, and the lower the thermal emittance, the greater is its ability to resist heat flow. Thus, a white roof has the highest solar reflectance and the lowest thermal emittance.
Roofing products that are designed for high solar reflectance and low thermal emittance are called “cool roof” products. In cooperation with state and local governments, many utility companies offer rebates for using cool roof systems and products. These rebate programs are offered nationwide, not just in southern climates.
- If your air conditioning costs are much higher than your heating costs and your roof is black or very dark and the sun shines brightly on it, it may be cost effective to paint it with roof paint to change it to white. Painting it white will lower your air conditioning costs, but raise your heating costs. It will extend the life of asphalt shingles and some other roofing materials because it will make the roof cooler. Painting the roof could increase the value of your home by lowering your air conditioning costs, unless a white roof makes your home less attractive. See Appendix A, “Painting Your Roof to Lower Your Air Conditioning Costs”.
- If you must replace asphalt shingles, use the lightest color that is attractive if your air conditioning costs are higher than your heating costs. Light-colored asphalt shingles last longer that black ones, especially in hot, sunny regions, because they stay cooler. If your heating costs are higher than your air conditioning costs, black shingles are best because they will slightly lower your heating costs.
White, cedar, and tan asphalt shingles
- If your roof is flat and the roof coating must be replaced, use fibered aluminum roof coating rather than conventional black asphalt roof coating. This will lower your air conditioning costs and will last years longer. These are the most widely-used roof coatings for flat roofs. Fibered aluminum roof coating is very similar to black asphalt roof coating, but has aluminum particles mixed in to give it a shiny silvery surface. The silvery color helps to reflect the sun’s rays and also reduces the heat energy that flows into the roof when the air temperature is high. It costs more per gallon, but it lasts longer because it stays cooler. Some brands cover a larger area because they spread thinner. As all other roof coatings, its main purpose is to waterproof the roof by sealing cracks and very small holes, and to prevent metal roofs from rusting. Fibered aluminum roof coating is easily applied, using the long handled brush that is available where roof coatings are sold. As with other roof coatings, you must seal larger cracks with a fabric called “roofing mesh”. You bury this between two coats of coating. Some brands of fibered aluminum roof coating are not very thick, so even very small cracks and holes require using roofing mesh. Fibered aluminum roof coating is sold at roofing stores, home centers and some hardware stores for about $60 to $80 for a 5 gallon can.
If you have an air conditioner with a compressor the roof, changing the roof coating from black asphalt to fibered aluminum will improve the air conditioner’s efficiency. The compressor releases heat taken from the house. If the roof coating is black, the air close to the roof is hotter. A compressor releases heat from the house much less efficiently in hotter air.
- If your roof is flat and the coating must be replaced, use white elastomeric roof coating. It will lower your air conditioning costs more than fibered aluminum roof coating or conventional black asphalt roof coating, and it will provide greater roof protection. It is a thick, rubber-like coating which expands like rubber without cracking. The better brands are made of acrylic with silicone, as are the most popular brands of exterior caulk. These are labeled as “siliconized elastomeric roof coating”. Some brands cost more than fibered aluminum roof coating or black asphalt roof coating. It reduces air conditioning costs more than them because it is white. It lasts longer than these two other types of coatings because it does not crack. It is sometimes used because the roof is in poor condition. If yours is, ask at a roofing store if elastomeric coating would be a good investment. This reason alone my be reason to use it. It is easily applied using a long handled brush. As with other roof coatings, you must seal larger cracks with a fabric called “roofing mesh”. You bury the roofing mesh between two coats of coating. It is sold at roofing stores and home centers, but may have to be special ordered from some home centers. It should be washed at least once each year to keep it white, for its insulating ability to remain high, so if you are unable to wash your roof, it would probably be a poor investment. The cost of a 5-gallon can may be similar to fibered aluminum roof coating, but some brands are applied in a thicker coat so they cover a smaller area. One product, which has insulating ceramic beads, costs $135 for 5 gallons, and covers 250 sq. ft. A possible additional expense is that it elastomeric roof coating may only be available in 5-gallon cans, so if you need, for example, 12 gallons, you must buy 15 gallons. Fibered aluminum roof coating is always sold in one-gallon and 5-gallon cans. There are other pros and cons to using it instead of aluminum roof coating, so it is best to ask at a roofing store, after describing the condition of your roof.
If you have a rooftop air conditioner on a flat roof, changing the roof coating from black asphalt to white elastomeric will improve its efficiency more than changing the roof coating to fibered aluminum roof coating. The machine on the roof is the compressor, and it releases heat taken from the house. The air around it will be hottest if the roof coating is black and least hot if it is white, and a compressor releases heat less efficiently in hotter air.
- If you have a flat roof and no attic, you could add much insulation by building a new roof over your present roof. The contractor may prefer to use 2×4’s on edge as rafters and lay 3 ½” fiberglass insulation batts between them. He may prefer using 2×2’s, with boards of high R-value rigid insulation between them, because this would be more attractive. Using 2×6’s as rafters would probably be too attractive. The project may not save you enough in lowered utility costs alone, but if your roofing needs to be replaced, you will save paying for this. As with most large projects, it will increase the home’s property value if it is well constructed and it does not harm the home’s appearance.
- If your air conditioning costs are much greater than your heating costs, you could mount a wind turbine (air vent wind turbine, or turbine vent) on your roof. This reduces your air conditioning costs by drawing hot air out of your attic in the summer. It uses no electricity because it is rotated by the wind, and as it rotates it draws air out of the attic. In locations where there is much strong wind it is very effective, but it will probably raise your heating costs.
Galvanized wind turbine with two piece base for adjustable angle
Your savings in air conditioning costs, and the increase in heating costs are not predictable. Experience has shown that they are usually cost effective in areas where winds of over 5 mile per hour are frequent, if the home’s air conditioning costs are much higher than the heating costs. Much heated air is lost through them in the heating season because it escapes due to the high pressure in the attic. Every home has attic vents which air escapes through in the heating season due to its higher pressure. If you install a wind turbine, you may check with a qualified contractor on whether to close one of your attic vents, to compensate, unless you will cover it with a wind turbine cover each heating season.
They cost about $40. They are normally installed by a roofer or handyman, but some do-it-yourselfers install them with the help of the instructions included or a do-it-yourself book or web site. The least expensive models can make noise inside the house if not oiled periodically. They are available in several colors to match your roof.
- If you have a wind turbine on your roof, buy or make a wind turbine cover to install each winter. A wind turbine is a spherical device that draws air out of the attic when rotated by the wind. Its purpose is to cool the house in summer. In winter, it draws warm air out of the attic, raising the heating costs. If the house has central air conditioning, the cover should be removed each summer because a wind turbine will improve its efficiency.
If your wind turbine is necessary for adequate attic ventilation, do not cover it. One way to determine this is to check if there are similar houses on your street that do not have wind Wind turbine cover
turbines and appear to have the same number of eave vents. Another way to check is to use the building code to compute the required area of attic ventilation. Ask at a library for the applicable building code. Wind turbine covers are available in the weatherization section of some home center stores, but if they are plastic, cover the wind turbine with a piece of canvas instead.
Appendix A Painting Your Roof to Lower Your Air Conditioning Costs
As explained on the first page, a white surface best reflects solar radiation and resists heat flowing through it due to temperature difference. If you would like to paint your roof white to lower your air conditioning costs, roof paint is available for almost every type of roof. There are paints formulated for only one type of roof, and there are “multi-surface roof paints” which are for formulated for several types of roof material. There are also roof paints that contain ingredients such as microscopic ceramic balls to increase solar reflection.
To be cost effective in lowering your air conditioning costs, you must paint it yourself, and even then it may not be worth the cost. This will depend on how much direct sunlight you have, whether you keep the paint clean and bright, how high are your air conditioning costs, and how many years the paint lasts. If you paint asphalt shingles your payback will depend on how many more years your shingles last. There are other advantages to painting your roof a very light color. If you have asphalt shingles it will help them last longer because they will stay cooler; it may also beautify your home and thus increase its value. Your savings will depend on how much you change the roof’s color, so if you change it from black to tan or brown your savings will not pay for the paint. Most asphalt shingles are designed to last only 25 years, so painting them solely to lower your air conditioning costs is probably a poor investment unless they are almost new and your climate is very hot.
In the hottest and sunniest regions of the country, you may find the roof paint you want at a home center or paint store. In other regions you will probably be able to “special order” it
You can paint almost any type of roof. The label of one brand of multi-surface roof paint says it is designed to cover asphalt shingles, unglazed tile, masonry shingles, anchored gravel, flat metal roofs, composite sheet, urethane foam, and shake roofs of slate or wood. This product is “intended to brighten roofs and reflect heat”, but “not intended to waterproof or repair roof leaks”. It covers 250-450 sq. ft. per gallon. Its price per gallon is in range with other exterior latex pants, but it is thicker so it covers much less. The coverage depends entirely on the roof’s material, it would cover the most on a very smooth material. Its label says, “a second coat is optional for improved durability”. Another brand is designed to cover, “metal roof, asphalt shingle, concrete tile, and fiber cement shingle”. Another brand, which offers “heat reflective technology”, is formulated for metal, asphalt shingles, concrete tile, and fiber cement shingle. It has a recommended application rate on metal roofs of 1,000 sq. ft. per 5-gallons, per coat, and two coats are needed. It has a recommended application rate on tile and asphalt shingle roofs of 825 sq. ft. per 5-gallons, per coat, and two coats are needed. Experience has shown that when used on asphalt shingle roofs, it covers a much smaller area on the first coat than the second, so 825 sq. ft. is probably closer to what it covers on the second coat. It is better to use a roof paint that was designed only for your type of roof because it will probably adhere better. This is especially important if you have a metal roof.
There are primers designed for only one type of roof material: one for cement roof tiles, one for asphalt shingles, one for metal roofs, and one for composite tiles. It is best to use a primer and roof paint of the same brand. Primers are especially important for metal roofs.
The cost of most roof paints is about $200 for a 5-gallon bucket. The area covered by this varies greatly, depending on how smooth the surface is and on how thick the paint is designed to be applied.
It is difficult to estimate how much paint you will need. The area covered per gallon that is given on the can may be a wide range, or may be an area that is actually at the top of the range. To buy the correct amount, buy a 5 gallon can, use it up, and estimate from that how many more gallons to buy. If, for example, your roof is 2000 sq. ft. and has asphalt shingles, one popular brand of roof paint would take about 14 gallons for the first coat and only about 7 gallons for the second coat because shingles absorb much paint.
Asphalt shingles should be painted twice because the first coat absorbs much paint. Either use the same paint for each coat or use an asphalt primer for the first coat.
If you have a clay tile roof you could paint it with rented paint spraying equipment to avoid walking on it. However, clay tile roofs are most often red, so changing the color from red to white would not lower your air conditioning costs enough to be cost effective, except possibly in hottest climates. Also, changing the color from red to white would make your home less attractive.
If your roof is steel or aluminum, painting it will protect it for many years, and should be done for this purpose alone. It is better to coat a flat metal roof with asphalt based coating than with roof paint because it costs less, protects better, and is easy to apply. If your roof is steel or aluminum and not flat, use a roof paint formulated only for metal roofs, and a metal roof primer. If the roof has begun to rust you should scrape off the rust with a wire brush and paint this area with a rust converter before applying the primer. This should be explained on the can of the primer. It is best if the primer and top coat are the same brand, to better adhere to each other. Some metal roof paints contain “cool roof” pigments, such as ceramic beads. They increase the solar reflectance, even of dark-colored roofs.
Before painting, wash the roof using a long-handled brush, and allow it to completely dry. If this is not done the paint may not adhere well. If it is an asphalt shingle roof, you could clean it with a product designed for cleaning asphalt shingles. A power washer should only be used for asphalt shingles with extreme care because it may loosen some of the shingles. A power washer is good for washing a metal roof, but use the low setting to prevent denting the roof. There is much to know about cleaning a roof, which can be found in do-it-yourself books and web sites. Apply the paint with a 4” paint brush or with a roller. Paint stores sell a 4” paint brush mounted to a pole, which is used to paint porches. Rolling it on can cause paint to build up between the shingles, wasting paint. This buildup can also eliminate patterns that have been created by the designs of the shingles. If you brush it on, the paint can be worked into small crevices and perhaps give a better seal to the roof.