GREEN ROOFING KEEPING COOL TOPSIDE

GREEN ROOFING KEEPING COOL TOPSIDE

KEEPING COOL TOPSIDE

T ravel 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)

KEEPING COOL TOPSIDE

T ravel 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."

GREEN ROOFING KEEPING COOL TOPSIDE

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)


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