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GREEN ROOFING
KEEPING COOL TOPSIDE

Travel posters for the Mediterranean often feature white buildings under a brilliant sun, overlooking a blue sea. These buildings have been constructed and perfected over hundreds of years to stay cool under the hot sun, keeping their occupants comfortable -- without the benefit of mechanical air-conditioning.

Old-world builders were able to keep their dwellings cooler under the hot sun using tried and true methods. Their secret was a simple one: lighter colors stay cooler. These lighter-colored structures were able to stay cooler because they reflected more heat. When combined with adequate ventilation, these houses are much more comfortable to live in than darker-colored, less reflective dwellings.

Today's builders can adopt these old-world construction techniques to build houses that stay cooler in hot weather, often at no additional cost, using available materials and methods. Doing so helps reduce the energy needed for cooling, keeps the occupants of the home comfortable, and saves energy and money. Less energy consumption means less fossil fuels are consumed and fewer pollutants are released. Reducing the consumption of fuels also means a reduction in emissions of greenhouse gases into our atmosphere, including carbon dioxide. Elevated levels of carbon dioxide in the atmosphere could lead to an increase in the earth's surface temperature, creating catastrophic environmental changes.

This report discusses ways to increase a roof's ability to reflect the sun's heat. It also identifies criteria for material selection, and recommends a number of available roofing systems and materials to help homes stay cooler. Building cooler houses not only makes them more attractive to buyers, it can also save builders money. This is due to the fact that smaller cooling loads allow builders to specify smaller HVAC equipment. While the savings are greatest in hot and sunny climates, builders can use the concepts discussed in this report to keep homes cooler anywhere in the US.

Roofing Basics
Nothing symbolizes a home more than a roof. For a properly constructed structure, weather protection begins at the ridge of the roof, continuing down to form a unbroken barrier that keeps out the elements -- rain, snow, and the sun's light and heat. As the most visible component of a house, the roof style and materials used can also play a large part in determining a home's "look."

In the south and certain parts of the southwest, where roofs do not usually have to endure the weight of heavy snowfalls, flatter roofs are more common. Elsewhere, most are sloped and are variations of gable, hip, gambrel, or shed. These styles can be varied with the use of covering materials and textures. Currently, there is a multitude of roofing materials available, ranging from asphalt shingles, wood shingles and shakes, to roll-roofing and plastic membranes, to slate and tiles (clay and concrete), and finally to aluminum, copper and steel panels. Each roofing system is briefly discussed below. Note that nearly all of the available traditional roofing materials can be made to reflect more sunlight in order to help a house reduce its cooling load.

Asphalt Shingles These shingles are composed of an inner core, usually of fiberglass or cellulose fiber, coated with asphalt on both sides and topped with a protective mineral aggregate. Asphalt shingles are fairly durable, can be treated with fire-retardants, and come in different thicknesses. It is estimated that asphalt shingles cover over 70% of the houses in the US.

Roll Roofing Roll roofing is essentially a continuous sheet of asphalt shingle material. The major difference between roll roofing and asphalt shingles is that roll roofing does not last quite as long, and variations in its appearance are limited.

Plastic (Membrane) Roofing Also known as burnt-on roofing because it is flame sealed, this type of roofing resembles a large, shrinkwrap sheet covering the roof frame and underlayment. The membrane itself is made from a variety of materials, ranging from rubber to thermoplastic. Like built-up roofing, membrane roofing is fairly durable and is more common in commercial structures. Some common membrane types are EPDM (ethylene-propylene-diene-terpolymer membrane), CPE (chloro-polyethylene) and CSPE (chloro-sulfonated-polyethylene).

Built-up Roofing Also known as "tar and gravel" for the materials used, this type of roofing is suitable for flat and slightly sloped roofs. Also called hot-mopped roof, it consists of alternating layers of felt and hot tar, with a final layer of fine gravel. This type of roof is most commonly used on commercial buildings.

Wood Roofing Wood roofing usually comes in two distinct forms: shakes and shingles. Shingles are smooth and uniformly shaped while shakes have a rough, uneven texture. Today's wood shingles and shakes are also more sophisticated than those of the past, which came directly from the tree with little enhancement. For example, they can be factory treated with chemicals to boost their fire rating from unrated to Class A. They can also be pressure treated to resist rot. A composite wood shingle is also available.

Cementitious Roofing Cementitious roof tiles are made from steam-cured cellulose fiber-reinforced portland cement or concrete. Cementitious roofing tiles are very durable, resisting weathering, insects, fire, and fungus. Their weight also helps them to resist wind uplift.

Clay Roofing Clay tile, made out of kiln-fired clay, is one of the oldest and most durable roofing materials. It is extremely weather-resistant, fire-proof and insect-proof. Clay tiles usually require structurally strong roof decks and are fastened with a variety of clips and fasteners.

Metal Roofing Metal roofing is available in a number of materials and configurations, including aluminum, copper and steel. It can come unpainted or factory finished with various coating systems. Steel panels have been traditionally used on agricultural and utility buildings, but they can be found frequently on houses and light commercial structures too. Like cement and clay tiles, metal roofing is durable and insect and mold resistant.

Slate Roofing Slate tile, like clay tile, has been in use for centuries. Properly installed, a slate roof has remarkable durability, and it doesn't require much maintenance (used slate is available in some areas). When a slate roof fails, it is usually the fasteners or the flashing that are defective, and not the slates themselves.

Table 1. Selected Roofing Materials: A ComparisonMaterial Life span
Material Life Span (years) Fire Rating Weight (lbs/100 ft2) Cost ($/100 ft2) Installation ($/100 ft2) Total Cost per 100 ft2
Asphalt Shingles 12-40 A-C 225-385 30-60 70-125 100-185
Plastic (membrane) 10-12+ A-C 150-250 Varies Varies 110-200
Built-up 10-15 B 250-400 N/A N/A 125-215
Wood 10-35 A-C 300-400 150-200 130-160 280-360
Cement or Clay Tiles 20-25+ A 375-1100 125-500+ 100-300 225-800+
Metal 15-40+ A 50-270 35-250 35-400 70-650
Slate 30-100 A 500-1000 350-700 250-450 600-1150

Cooler Roofs
The discussion on cooler roofs is approached with the assumption that the decision to use a cool roof can be independent of the choice of roof system (shingles, tiles or metal). With certain roofing choices, however, there may be only one color option for a cool roof: a white (or light-colored) variety of the roofing material. The exception is built-up roofs, because a variety of surfacing materials may be used.

The option of a lighter colored, cooler roof may or may not increase the overall cost of the roof, depending on the choice of roof systems (or materials). The exception is clay tile, which can command up to one third more for the cool roof (white tile) option. Earth-tones (light brown, green, etc) can also be somewhat reflective in certain materials, such as clay tiles. Clay tiles also tend to cool more quickly when the temperature drops.

Cost and Energy Savings with Cool Roofs
A number of studies have begun to quantify possible energy savings with reflective roofing materials (alone and in combination with other measures). These studies compare cooling energy use before and after the reflectance is increased, for a number of roofing materials. One study by the Florida Solar Energy Center found that air conditioning energy use was reduced by an average of 23% in houses with increased roof reflectance. The Florida study also tracked attic temperatures before and after the reflectance change and found significant differences, especially for poorly insulated attics.

A minimum energy savings of 10 to 20% is expected from an energy-efficient roof which incorporates some type of radiant barrier and a white or light color roofing material. While cooling savings are certain with reflective roofing systems, it should be noted that light-colored and reflective roofing systems may incur a 1% to 3% heating penalty in areas with longer heating seasons. However, the cooling savings more than make up for this slight penalty. Where reflection cannot be maximized, the use of a radiant barrier– a light foil or foil and insulation combination – can be a key factor (along with good attic insulation and ventilation) in the reduction of heat buildup in attics on hot, sunny days.

For home owners, however, the savings can be more than just a 10% savings on their energy bill during the cooling season. In regions where there is a "peak" or "demand" charge set up (where users are charged more for electricity during peak consumption periods), cool roofs can reduce cooling needs during peak use periods, thereby lessening both electricity consumption and demand charges. Thus, additional expenditures associated with a cool roofing system can often be paid for in a few years with the reductions in cooling costs.

Table 2. Cool Roof Materials, Reflectance Increase and Cost Difference
Traditional option Normal reflectance Cool roof option Cool roof reflectance Reflectance increase Cost increase
Asphalt shingle (composition, fiberglass or organic) 5 - 15% White asphalt shingle with "premium" white granules *31 - 35% 15 - 30% < 1%
Clay tile 25 - 35% White clay or tile 70 - 80% 35 - 55% ~ 35%
Concrete tile 10 - 30% White concrete tile 70 - 80% 40 - 70% ~ 20%
Cementitious shingle 10 - 30% White cementitious/ fiber cement shingle 60 - 80% 30 - 70% none
Metal & metal shingle 70% White painted metal 70 - 80% 0 - 10% none
Built-up roof or coal tar with dark gravel 5 - 10% Built-up roof with white gravel 40% 30 - 35% slight to none
Coal-tar (or mopped-on) roof, smooth surface 5 -10% Built-up roof with gravel coating 60% 50 - 55% ~20%
Black single-ply membrane (EPDM, CPE, CSPE) 5 - 10% White or reflective single-ply membrane 70 - 80% 60 - 75% ~20%
Source: S. Bretz, LBNL
* Within the next three years asphalt shingles will be available with a reflectivity increase of 50% or higher.

For builders, a cooler house could mean lower costs. This is due to the fact that a smaller cooling load would permit a smaller AC or HVAC system, especially when ducts are located in the attic. For houses that are constructed to meet or exceed the Model Energy Code (MEC), like Energy Star Homes, the combination of tight construction and insulation, along with a reflective roof and or radiant barrier, should allow builders to more accurately estimate the smaller heating and cooling loads. By not over-sizing HVAC equipment, builders save money. These money-saving features can also help to sell a home.

Selecting the Right Roofing Materials
While the selection of a light color roof is an essential start, laboratory tests have determined that some materials are more reflective than others. Therefore, choosing the roofing materials with highest reflectance (see Table 2) that are within your budget, should be an important consideration. In most cases, an effective cooling strategy combines a light color and a highly reflective material or roofing system, such as, metal panels or cementitious tiles. For less reflective roofing systems, such as white asphalt shingles, a combination of a radiant barrier and good attic insulation is the best strategy. Cool roof options do exist for most of the available roofing materials, and we have identified a number of criteria below that you can use to choose the most environmentally suitable material for your needs.

  • Longevity: Long lasting products are environmentally more efficient in the long run because they create little or no additional manufacturing, shipping, installation, disposal, or environmental impacts. They also can increase the value of the property. We recommend that you select products that last for 20 or more years.

  • Reflectance: Look for inherently reflective products with estimated reflectance of 50% or more (see Table 2), and white or light-colored products. In cases where selecting a lighter color is not possible, look for products that still offer some degree of solar reflectance. Note also that while bare/unfinished metal roofs can be reflective, the metal can get very hot.

  • Fire Resistance: Products should be rated for class B fire resistance, at a minimum. Select products that have been rated by an independent, third party organization, such as Underwriters Laboratories.

  • Wind Resistance: In windy areas, look for products that have been rated for wind resistance, or have been tested by an independent laboratory under simulated high-wind conditions.

  • Recycled/Recyclable products: A number of products on the market are made with recycled materials. Choose these if they fit your needs. Recycled materials can reduce manufacturing energy consumption and reduce pressure on our landfills. Examples of these include aluminum panels made out of recycled soda cans, or tiles that use fly-ash (a by-product of coal combustion) as fillers.

    Radiant Barriers
    Where reflective roofing materials cannot be maximized, the use of a radiant barrier can be a key factor (along with good attic insulation and ventilation) in the reduction of heat buildup in attics on hot, sunny days. Radiant barriers are designed to block heat transfer between a heat radiating surface (a hot roof, for example) and a heat absorbing surface (such as attic space or attic insulation). Radiant barriers are a new energy-conserving concept that have proven effective in southern and western states. These barriers can provide an additional 5% or more in energy savings.

    A radiant barrier consists of a reflective surface, usually a type of aluminum foil, with some type of backing such as kraft paper or polypropylene. The aluminum foil layer helps to prevent heat that is absorbed by the roof from radiating into the attic and the rest of the house. To prevent tearing, manufacturers may also add a layer of reinforcement, such as fiber webbing, to the foil "sandwich." For attics with light insulation, "R" rated radiant barriers are often used. These are barriers that combine the radiant barrier with an additional insulation layer.

    There are many types of radiant barriers on the market, with costs ranging from $0.10 to $1.00 per square foot. Installation costs also range from $0.10 to $1.00 per square foot. There are three basic types of radiant barrier systems: blanket, draped and laminated. Blanket radiant barriers were the original type of barrier and were laid directly on top of attic insulation. Unfortunately, these type of barriers frequently had problems with dust accumulation and moisture. Draped radiant barriers are very effective and do not have problems with dust or moisture. These barriers are stapled to the bottom of the rafters during retrofitting of existing homes or draped over the top of the rafters for new construction. Laminated radiant barriers are also now available. With this type of barrier, an aluminum film is laminated directly to the sheets of decking material and is quite popular with new home builders.

    Regardless of when barriers are installed, they need to face an air space to work. The foil side should face down into the air space to avoid dust build-up on the barrier's reflective surface, since this can reduce its effectiveness.

    Available Products
    The number of available products is vast, therefore, the focus is on products with longer life spans and high reflectance that are typically found in new construction. Tables 3 and 4 contain standard lists of products that are typically available that have met longevity, reflectance, and fire resistance criteria. Several have also met the criteria for wind resistance and recycled/recyclable products.

    Table 3. Selected Metal Roofing Products
    MFR Product Type Fire Wind Weight Warranty Comments
    Atas International
    800-879-8382
    ScanRoof, Techo Tile, PermaShake Aluminum, Steel & Copper panels A x 140 to 160 50 year limited Light color premium
    AEP-Span
    800-527-2503
    Snap-Seam Steel panels A x Varies 20 year limited Silver color premium
    Atlanta Metal Products
    404-691-8500
    T-Rib, High Seam Steel & Aluminum panels A x Varies 20 year limited Metallic colors premium
    Berridge
    800-669-0009
    Tee-Panel, Tiles, Shingles Steel panels A x Varies NA Metallic colors premium
    Classic Products
    800-453-8938
    Rustic Shingle Aluminum shakes A x NA 50 year 98% post-consumer Al
    Englert
    908-826-8614
    A 1000 - A 1300 Steel panels A x Varies 30 year limited Mechanically formed seams
    Gerard
    800-237-6637
    Tiles, Shakes Steel panels A x 140 Lifetime limited Coated metal panels
    Metal Sales
    800-289-7663
    Stile Steel panels A x NA 20 year limited Offer panels with simulated tile look
    Met-Tile
    909-947-0311
    Met-Tile Steel panels A x 125 20 year limited Tile panels

    © 1997, Green Seal, Inc. Use of this chart for commercial purposes is prohibited. Information in this table was confirmed by the manufacturer.

    NA: the information is not available
    Fire: the highest fire rating obtained by product in independent testing. Certain products require proper underlayment for class "A" fire rating.
    Wind: An "x" indicates the product has been tested in laboratories under simulated high-wind conditions. Certain products also report the highest simulated wind speed that their products have been subjected to. Most products listed were tested under 70 to 80 mph wind conditions.
    Weight (Wgt.): average weight of 1 square (100 square feet) of materials.
    Warranty (Warr.): most products carry a limited warranty, dependent upon a set of conditions. Some allow transfer of ownership, others do not.
    Comments: some manufacturers offer light color as part of their line. Others offer light color as an optional or special custom selection. Some manufacturers use recycled materials or materials otherwise discarded, such as fly-ash.

    Table 4. Selected Clay and Cementitious Products
    Manufacturer Products Type Fire Wind Wgt. Warr. Comments
    AltUSA
    800-548-4531
    Spanish "S" Clay tiles A x 1000 NA Light clay color only
    American Cemwood
    800-367-3471
    Cascade Pacific Slate Cementitious shakes & slates A x NA 30 years Light & white color available
    Boral Industries
    941-676-9405
    Lifetile Concrete tiles A x 950 Lifetime warranty Light colors
    Comma Cast
    305-665-3664
    Santafe Clay tiles A NA 690 NA Light colors available
    Eternit
    800-233-3155
    Eternit Slates Cementitious slates A x 380- 467 50 year limited Light colors available
    GAF Corp.
    201-628-3000
    Ultra Slate Shakes Hexagonal Cementitious tiles & shakes A x 270 - 500 40 years limited White and light colors available
    Ludowici
    800-945-8453
    Various Clay tiles A NA 750 -1800 Up to 75 years Light colors available
    Louisiana - Pacific
    800-579-8421
    Nature Guard Shakes Cementitious shakes A x 580 25 year limited Uses fly-ash as filler. Light colors available
    MaxiTile
    800-338-8453
    MaxiTile MaxiShake MaxiSlate Cementitious tiles A x 340 - 500 50 year limited Light colors available
    Pioneer
    800-411-8453
    Concrete Tile Concrete tile products A x NA Limited Light colors available
    Monier Tile
    800-692-3733
    Cedar Lite Home Stead Split Shake Cementitious shakes/tiles A x 300 - 600 50 year limited White & light colors available
    Re-Con Building Products
    800-347-3373
    Quantum FireFree Cementitious shakes/tiles A x NA 30 to 60 years limited Light color available
    © 1997, Green Seal, Inc. Use of this chart for commercial purposes is prohibited. Information in this table was confirmed by the manufacturer.

    NA: the information is not available
    Fire: the highest fire rating obtained by product in independent testing. Certain products require proper underlayment for class "A" fire rating.
    Wind: An "x" indicates the product has been tested in laboratories under simulated high-wind conditions. Certain products also report the highest simulated wind speed that their products have been subjected to. Most products listed were tested under 70 to 80 mph wind conditions.
    Weight (Wgt.): average weight of 1 square (100 square feet) of materials.
    Warranty (Warr.): most products carry a limited warranty, dependent upon on a set of conditions. Some allow transfer of ownership, others do not.
    Comments: some manufacturers offer light color as part of their line. Others offer light color as an optional or special custom selection. Some manufacturers use recycled materials or materials otherwise discarded, such as fly-ash.

    Table 5. Membrane Roofing Products
    Below are selected manufacturers of membrane products. Because of the wide variety available, we have not attempted to list them all here.
    Manufacturer Products Notes
    Burkeline Roofing Systems
    800-669-7010
    Single-Ply Hypalon Light products available
    JP Stevens Co.
    800-621-7663
    Hi-Tuff/EP, Hi-Tuff/Hypalon, Cool Black © EPDM, Available in white, Cool Black®; also offer reflective properties. Company claims Cool Black® is up to 50% more efficient than comparable black EPDM products
    Mule-Hide Products
    608-365-3111
    Hypalon Roofing Systems Light & reflective products available
    Schuller Roofing Systems
    800-654-3103
    Single-Ply Roofing Systems Light & reflective products available
    Tri-R Products
    800-588-8747
    Membrane Roofing Systems Light color available

    © 1997, Green Seal, Inc. Use of this chart for commercial purposes is prohibited. Information in this table was confirmed by the manufacturer.

    Coatings
    Builders can also make roofs (and walls) cooler by using a number of available coatings. Depending on the roof system and the selected type of coating, a reflective, white coating can increase the roof's reflectivity and reduce energy consumption. This is a less permanent approach and may not offer the savings that a light color, reflective roof system can provide. However, coatings which can be used with a variety of existing roof materials can be attractive options for builders who want to offer a cool roof as an added feature.

    If you choose to offer a light or white color coating as an option in new construction, important factors to consider include:

  • A white or light coating by itself cannot achieve maximum energy saving. For a cool roof system to work, everything else must also be in place – insulation and/or radiant barrier, and a tightly constructed house.

  • In general, a white coating may not last as long as the roof system, and may require periodic re-coating. Therefore, it may not be possible for builders to realize savings from "right-sizing" the HVAC equipment.

    Selecting the Right Coating
    There is a large number of coatings available for a variety of surfaces. These products range from white paint to elastomeric, ceramic and aluminized coatings. Below are some general guidelines for what to look for in coatings.

  • Select a reflective product that maximizes reflectance. Not all white or light-colored products provide good reflectance.

  • Select products that last. Long lasting products are not only good for the owner, they are also good for the environment.

  • Select products that are suitable for your intended application. Metals roofs, for example, require different types of coatings, depending on how they are finished.

  • Select products that are lower in volatile organic compounds (VOCs). VOCs are present in most types of coatings. Products with high VOC levels (usually listed on labels or container top) release more of these pollutants when they are drying and over their lifetime, contributing to air pollution and smog formation.

    Roof Construction
    Regardless of the type of roofing material, proper installation is essential. This is an important factor to consider because without quality installation, a roof will last only as long as the weakest nail or tile clip. Manufacturers' instructions should be followed closely, and experienced, knowledgeable contractors should be selected. Doing so will reduce call backs as well as maximize the roof's life span.

    Written by: Green Seal


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