Cool roofs are one of the biggest buzzwords in roof energy efficiency today. They work by reflecting a large fraction of incident solar radiation, dramatically reducing the amount of heat absorbed by your home. They also quickly release whatever amount of heat they end up absorbing.
But is reflectivity and emissivity all there is to an energy-efficient roof?
The short answer is NO. According to former California Energy Commissioner Art Rosenfeld, the ideal energy-efficient roof is:
The thing is, such a material doesn’t exist (yet). Modern cool roof materials are around 65% reflective (meaning they absorb around 35% of the sun’s energy). And unfortunately, the emissive and reflective properties of roofs don’t really change to suit the changing seasons. The ideal 100% reflective/100% absorbent roof is—for now—an impossibility.
But Rosenfeld’s definition of an ideal roof emphasizes an interesting point: focusing on a roof’s ability to absorb heat can also make it more energy efficient.
About Thermal Mass
Thermal mass refers to a material’s ability to absorb and store heat (the term is also used to refer to the material itself). A material with high thermal mass can hold a larger amount of heat and takes longer to fully absorb and release that heat compared to a material with low thermal mass.
You can think of a thermal mass as a high-capacity “battery” that gets charged and then releases, not electrical energy, but heat.
Roofing materials with high thermal mass can reduce the heating and cooling needs of homes in most climates. They are flexible enough to have the ability to:
Roofing Materials with High Thermal Mass
Two things make a material a good thermal mass: a high specific heat capacity and a high density.
Roofing materials that meet these requirements include clay tile, concrete tile and slate. (Bonus: clay and concrete tile roofs can be modified with reflective coatings to make them even more energy efficient.)
A Quick Note
Roofing materials with high thermal mass tend to be heavy, so your roof deck and the underlying structures must be able to handle the weight. If you’re switching from a lighter material, modifications will have to be made to ensure that the structural integrity of your home won’t be compromised.
What if a roof replacement isn’t in the offing? Find out how you can harness the cool benefits of cool roofs without a complete reroof in Part Three of this series.