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The idea that metal roofing would make your house hot seems like a sensible one at first thought. Most people have likely touched a piece of metal that has been sitting in the blazing sun all afternoon to find that it scalded their hand. There's no denying that a material left out in the summer sun is going to heat up, but this applies to any kind of material.
The short answer to the question of, "will a metal roof make your house hot?", is no, it won't -- but the sun will. The question that we should be asking is how efficient is your roof at shielding the structure beneath it from solar heat gain? Let's talk a little science.
Emittance
Simply put, Thermal Emittance is the relative ability of a roof surface to radiate the heat that it absorbs. Thermal emittance is a dimensionless quantity that is measured on a scale of zero to one -- zero being a perfect reflector and one being a perfect emitter. You may remember the three methods of heat transfer from back in school: conduction, convection, and radiation. Emittance refers to a substance's ability to release heat through radiation. The specific characteristics of a roof's surface has a significant impact on radiation heat transfer. For example, a shiny material doesn't radiate heat as well as rougher "matte" surfaces.
Reflectance
Recall when solar radiation strikes a surface it will be transmitted, reflected, or absorbed. Reflectance refers specifically to visible light and represents the ratio of reflected light to incident light (light that falls on a subject). This is referred to as Light Reflectance Value or LRV. There is also another term called albedo that refers to overall solar radiation reflectance. When referencing roofing materials albedo is usually referred to as solar reflectance (SR). An example of a nearly perfect surface reflection would be a silvered mirror. Conversely, a flat black surface would be an example of near perfect absorption. (Think of the way wearing a black t-shirt in the summer makes you much hotter than wearing a white t-shirt.)
When we're dealing with roofing materials the color isn't the only thing that is dictating it's energy efficiency. A combination of high solar reflectance (albedo) and emittance resists solar heat gain most effectively.
There is now a concept known as the Solar Reflectance Index (SRI) that is used to compare energy efficiency between roofing types.
How does SRI work?
SRI ratings are calculated according to ASTM E 1980 and by the Cool Roof Rating Council standard CRRC-1. The Solar Reflectance Index (SRI) measures how effective your roof is at staying cool in the sun by emitting thermal radiation and reflecting solar radiation. The higher the SRI number the better your roof will be at staying cool and is thus more energy efficient for the structure it is attached to. Conversely, the lower the SRI number the less ability the roof has to stay cool in the sun. For example, a standard black surface would have an initial SRI of 0. A standard white surface has an initial SRI of 100.
How does this help us answer the question?
We've gone into all of this detail to answer the basic question of, "will metal roofing make my house hot?", and now we can definitively say no, especially when it is compared to more traditional counterparts like asphalt shingles. For example:
The typical black asphalt shingle has low solar reflectance (albedo) and high emittance and will have an SRI number of around 1.
A metal roof with a silicone-modified polyester black coating (our standard black coating) will have a slightly lower emittance than an asphalt shingle but a significantly higher solar reflectance (albedo) and will have an SRI number of around 31.
This shows a significant gap in the energy efficiency of a black metal roof versus a black asphalt shingle. One of the biggest advantages of metal roofing is the wide selection of available colors, each one with a unique SRI. A white metal roof from True Metal Supply would carry an initial SRI number of 84! Every color available for metal roofing will be substantially more energy efficient than an asphalt shingle, including black. See our color chart with the corresponding SRI number for each available color here.
In conclusion, we see that not only does metal roofing NOT make your house hot, it is actually one of the most energy efficient materials you could put on your roof to help keep your house cooler. In fact, independent studies by Oak Ridge National Laboratories in Oak Ridge, TN have found that homeowners in the southeast could see annual savings as high as 40% by putting a cool metal roof on their home!
If you have questions about your roofing project we would love to speak with you. You can email us at info@truemetalsupply.com or call our office at (865) 224-3055.