In the roofing industry, we don’t just say a roof is “cool” based on how it looks. We measure it using precise data. The metric we use is the Solar Reflectance Index (SRI), a calculated value that combines solar reflectance and thermal emittance into a single number. Standard black asphalt shingles might have an SRI of 5 or less. A pristine white TPO roof can have an SRI of over 100. At DISCOUNTED ROOFING LLC, we use these metrics to specify the right system for your building’s thermal profile.
Reflectance is straightforward—it’s how much sun bounces off the surface. Emittance is trickier—it’s how efficiently the material sheds the heat it does absorb. You want both. A metal roof, for example, might reflect light well but hold onto heat (low emittance), making it hot to the touch at night. A modern silicone membrane excels at both, releasing absorbed heat quickly so the heat load doesn’t transfer into the building structure.
The challenge in an urban environment like ours is maintaining that SRI over time. City air contains particulates—soot, dust, and pollen. Over a few years, a white roof can turn gray, dropping its SRI and reducing its efficiency. Experienced Roofers in Philadelphia know that the choice of material affects how dirt adheres. We prefer materials with a smoother surface texture that allows rain to wash away dirt naturally, preserving the energy-saving properties. We also recommend periodic cleaning plans to keep the reflectivity numbers high.
Another technical consideration is “dew point.” When you cool down a roof surface significantly, you change the moisture dynamics of the assembly. If not properly insulated, a cool roof in a humid climate can actually increase the risk of condensation forming on the underside of the deck in winter. This happens because the roof deck stays colder than it used to. This is why we don’t just roll out a white layer and leave; we calculate the insulation requirements to ensure the dew point stays outside the building envelope, preventing internal rot.
Finally, we look at the reinforcement of the membrane. Whether it is polyester scrim in TPO or a fiberglass mat in modified bitumen, the internal structure must be able to handle the movement of the building. Cool roofing is more than a coat of white paint. It is a system that balances energy reflection, heat release, and moisture control. Getting it right requires a contractor who understands building science, not just waterproofing.