WHAT IS A "COOL ROOF"?

Today it seems that the hottest trend (pun intended!!) in the Commercial Roofing market are "Cool Roofs". So what the heck is a "Cool Roof" and why do you need one? First, let's talk about what qualifies a flat roof to be called "Cool".
For a flat roof to be considered "cool" it must meet two criteria: it must have high solar reflectance and high emissivity. So what the heck does that mean?

Solar Reflectance is a pretty simple concept to understand.

Solar reflectance is a measure of the ability of a surface material to reflect sunlight—including the visible, infrared, and ultraviolet wavelengths—on a scale of 0 to 1. Solar reflectance is also called "albedo".

"Cool Roofs" generally reflect at least 70 percent of the Sun's rays away from the building. Many products have 85% or higher. White roofs usually have high reflectivity, while black roofs have low reflectivity. With me so far? OK! Lets tackle emissivity.

Emissivity is a little trickier concept. Here is the definition:

The emissivity of a material (usually written ε or e) is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. It is a measure of a material's ability to radiate absorbed energy.

Here is a way to envision emissivity: Imagine a chrome bumper out in the sun on a hot day.

We know that it has very high reflectance, it is like a mirror! But if you picked up this piece of chrome it would be very hot. That means it has very low emissivity; it holds heat! This would not be good combination for a "cool roof". It would reflect the suns rays but still allow too much of the heat to enter the building.

Bottom line? A cool roof reflects and emits the sun's heat back to the sky instead of transferring it to the building below. "Coolness" is measured by two properties, solar reflectance and thermal emittance. Both properties are measured from 0 to 1 and the higher the value, the "cooler" the roof.

Wow! This stuff is easy. So why do you need a cool roof? That will be the subject of my next article. 



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