As children, everyone learns that a white shirt will keep the body cooler than a black shirt. Reinforced by experimental evidence, the concept proves to be true not just for shirt and body temperatures but roofs and buildings as well.
How a white metal roof saves energy depends on a number of factors beyond the basic color and material, but energy savings begin with the color white.
A white-painted metal roof has the highest reflectance value of any roofing product available.
- Whirlwind’s Cool White metal panels have a solar reflectance index (SRI) rating of 85
- Whirlwind's Black metal panel has a solar reflectance index (SRI) of 0
A white painted metal roof can save up to 40% on annual energy bills depending on the geographic location of the structure, and one rated as an Energy Star cool roof can keep interior building temperatures 50 to 60 degrees F cooler than a dark asphalt shingle roof.
- Standard or dark roofs can reach surface temperatures of 150 degrees F or more in the summer.
- Cool roofs, under the same conditions, remain more than 50 degrees F cooler on the surface.
The Solar Reflectance Index (SRI) is a measure of the solar reflectance and emissivity of materials that can be used as an indicator of how hot they are likely to become when solar radiation is incident on their surface. The lower the SRI, the hotter a material is likely to become in the sunshine
The Science Behind the Cool Roof
Cool roofs rely on solar reflectance and thermal emissivity as well as other roofing and building techniques to provide optimal energy savings.
Solar reflectance (SR) is the ability, stated as a ratio, of a particular surface to reflect solar radiation away from itself. The higher the reflectance, the more solar radiation is reflected away from the roof. White and other light colors have higher solar reflectance than darker colors.
Thermal Emissivity (TE) is the ability, stated as a ratio, of a particular surface to re-emit or release absorbed heat. Emissivity depends more on pigments and chemicals than the hue. Special pigments can be added to any color to increase the thermal emissivity of the material. A highly emissive painted or granular coated metal roof can re-emit up to 90% of absorbed solar radiation.
If a roof can reflect the majority of solar radiation away and quickly release a significant portion of absorbed heat, then the structure beneath will remain cooler using less energy.
However, solar reflectance and thermal emissivity are only part of a cool roof solution.
Ventilation, creating dead air space between the cladding and the deck, allows air to move freely under the roofing material and provides a little insulation into the bargain. Free air flow allows warm air to escape while pulling cooler air in to replace it, maintaining a convection cycle to protect the structure below from excess heat.
Metal roofing materials have a reduced thermal mass in comparison to asphalt shingles, which are heavy and dense. Materials with a high thermal mass take a long time to heat but are also very slow in releasing it. Metal has a low thermal mass and heats up less quickly while releasing absorbed heat quickly.
Interior Reflectance is a benefit in cooler months. A heated building with a white roof can reflect heat, not just away from the exterior surface, but back into the interior, allowing less heat to escape through the roof and saving heating costs and energy. The same principle has been used to develop insulation and radiant barrier coatings for attics, but a white metal roof has always had these characteristics.
Other techniques can minimize heat gain.
- Spacers placed between the metal and the building deters heat transfer by separating the materials.
- Metal roofs integrate easily with insulated roofing systems.
- Colors with a reflective surface have higher solar reflectance values than a matte surface.
Asphalt shingles, by comparison, are usually laid directly onto a structure and allow built-up heat to be transferred to the attic or loft.
Benefits of a White Steel Roof
Steel is one of the most environmentally friendly materials on the market. It is both durable and recyclable. Most steel in use today is made up of recycled steel as well as virgin steel. Using a white steel roof increases the benefits through energy conservation and lower energy costs.
The Cool Roof Coalition and Oak Ridge National Laboratory find the installation of a reflective metal roof can save a home up to 40% in summer cooling energy costs. The installation of a highly emissive metal roof can reduce urban air temperatures as much as 12 degrees, providing an environmental benefit beyond the structure itself.
- Less energy use translates into reduced peak energy demand, preventing outages.
- Power plant emissions are reduced, including CO2, sulfur dioxide, nitrous oxides and mercury.
- Quickly re-emitted heat reduces the urban heat island effect, reducing energy use within a neighborhood.
- When replacing a metal roof, no materials end in a landfill; the steel is recycled, whereas asphalt shingles must be removed from the roof and hauled to a landfill.
- One benefit of a white roof that a few building and homeowners may appreciate is the building appears taller.
A standing seam metal roof is ideal for installing photovoltaic panels to harvest solar energy. Also, a metal roof can be installed directly on top of asphalt shingles, further reducing landfill waste and scrap transportation costs.
A white metal roof saves energy through high solar reflectance. Additional coatings and pigments can also increase the thermal emissivity of a white roof, further increasing the energy savings.
A roof design with white metal panels and the appropriate ventilation, plus photovoltaic cells can not only save energy costs but produce energy as well. White metal roofs can be installed on both low and steeply sloped roofs with the same benefits.
Metal roof panels come in multiple colors, textures and profiles, providing a wide variety from which to select a roof that matches neighborhood standards while keeping the energy bill low.
All of this is how a white metal roof can save energy.