WHAT IS A COOL ROOF?

Approximately $40 billion is spent annually in the United States to cool buildings, one-sixth of all energy consumed annually. Black and dark-colored roofing materials can dramatically increase a building’s cooling load.

Energy-efficient roofing systems, also called “cool roofs”, can reduce roof temperature by as much as 100°F during the summer, and thereby reduce the building’s energy requirements for air conditioning. Cool roofs reflect the sun’s radiant energy before it penetrates into the interior of the building. In fact, reflective materials also help keep the building’s neighborhood cool.

There are many types of roofing materials with a varying range of reflectance and emittance levels. Asphalt shingles, the most common type of roofing material, are the least efficient at reflecting the sun’s heat energy. The asphalt is composed of asphalt-saturated mats made from organic felts or fiberglass. Roofing granules, one-millimeter sized stones coated with an inorganic silicate material, protect the roof from the sun’s ultraviolet light. The coating contains microscopic pigment particles, similar to those used in paint, to provide color.

Asphalt’s low solar reflectance can be attributed to several factors. First, there is a limited amount of pigment in the granule coating. Second, the roughness of the shingle contributes to multiple scattering of light and thus to increased absorption. Third, the black asphalt substrate is not 100% covered by the granules, and reflects only about 5% of the light that strikes it.

Solor Reflectance

Solar Reflectance is the fraction of the solar energy that is reflected by a roof, expressed as a number between zero and one. The higher the value, the better the roof reflects solar energy. For example, white reflective coating or membrane has a reflectance value of 0.85 (reflects 85% of solar energy hitting it and absorbs the remaining 15%), while asphalt has a value of 0.09 (reflects 9%).

Emittance is the amount of absorbed heat that is radiated from a roof, expressed as a number between zero and one. The higher the value, the better the roof radiates heat.

Solar Reflectance Index (SRI) indicates the roof’s ability to reject solar heat, and is the combined value of reflectivity and emittance. It is defined so that a standard black is zero (reflectance 0.05, emittance 0.90) and a standard white is 100 (reflectance 0.80, emittance 0.90). Because of the way SRI is defined, very hot materials can have slightly negative SRI values, and very cool materials can have SRI values exceeding 100.

There are four broad categories of roofing materials that can be used to upgrade a roof’s reflectivity to [PSOrg: ‘ORG139161’, ‘ENERGY STAR’, -encodenone] levels of efficiency: metal, tile, roofing membranes and reflective coatings.

QUESTION:

What buildings and roofing projects are subject to the Cool Roof Requirements of the Title 24 2005 building Energy Standards?

ANSWER:

The 2005 Standards Cool Roof Requirements apply to roofs on conditioned (cooled or heated) non-residential buildings that have low-sloped roofs (2:12 or less). The Requirements apply to roofs on newly constructed buildings and to most re-roofs on existing buildings.

QUESTION:

What happens to sunlight reflected from a Cool Roof?

ANSWER:

On a clear day about 80% of sunlight reflected from a horizontal Cool Roof will pass into space without warming the atmosphere or returning to Earth.

TYPES OF BUILDINGS SUBJECT TO COOL ROOF REQUIREMENTS:

Group A – Assembly
Building or structure, or portion thereof, for the gathering of 50 or more persons for purposes such as civic, scial, or religious functions, recreation, instruction, food or drink consumption, or awaiting transportation. Examples: restaurants, arenas, churches, theaters.

Group B – Business
Building or structure, or portioned thereof, for office, professional or service-type transactions; includes storage of records and accounts and restaurants with occupant load less than 50. Examples: animal hospitals, kennels, automobile showrooms, banks, barber shops, outpatient clinic and medical offices, educational occupancies above the 12th grade, fire stations, florists and nurseries, testing and research labs, print shops, radio and TV stations.

Group E – Educational (through 12th grade)
Building or structure, or portion thereof, for educational purposes through 12th grade for more than 12 hours per week or 4 hours in any one day. Examples: schools, nonresidential buildings used for daycare for more than six children, residential buildings used as daycare for more than 14 persons.

Group F – Factory (low- and moderate-hazard)
Building or structure, or portion thereof, for fabricating, manufacturing, packaging, processing, etc. Examples: furniture manufacturing, bakeries, food processing plants, paper mills, printing or publishing facilities, refuse incineration, shoe factories, dry cleaning facilities.

Group H – Hazardous facilities
Building or structure, or portion thereof, that involves the manufacturing, processing, generation or storage of materials that constitute a high fire, explosion, or health hazard. Examples: manufacturing plants for explosives, blasting agents, fireworks, flammable gases; storage facilities for such products.

Group M – Mercantile (sale of merchandise)
Building or structure, or portion thereof, for the display and sale of merchandise. Examples: department stores, shopping centers, wholesale and retail stores, markets.

Group S – Storage facilities
Building or structure, or portion thereof, for storage not classified as a hazardous occupancy. Examples: storage of beer or wine in metal, glass, or ceramic containers, of cement in bags, of foods in noncombustible containers, of gypsum board, of stoves, washers, and dryers.

Group U – Utility facilities
Private garages, carports, sheds, agricultural buildings, and towers.