A roof becomes hot when it absorbs solar energy faster than it can release heat. Cool-roof surfaces change that balance by reflecting more incoming sunlight and emitting absorbed heat efficiently.
Solar reflectance measures rejected sunlight
Solar reflectance is the fraction of solar energy a surface reflects. It is measured from 0 to 1. A higher number means less of the sun's energy is absorbed by the roof surface.
Color influences reflectance, but it is not a reliable specification by itself. White products are often highly reflective, while specially formulated “cool colors” can reflect more near-infrared energy than an ordinary product with a similar visible color.
Thermal emittance measures released heat
Thermal emittance describes how effectively a surface releases absorbed heat as infrared radiation. It also uses a 0-to-1 scale, where a higher number generally means the surface can shed absorbed heat more effectively.
A material can be reflective but have lower emittance, or dark but still emit heat effectively. That is why a complete cool-roof comparison considers both properties.
The U.S. Department of Energy's consumer guide defines coolness through the combination of solar reflectance and thermal emittance and notes that savings depend on climate, building characteristics and roof condition.
SRI combines the properties into one index
Solar Reflectance Index, or SRI, is a calculated comparison of how hot a surface is expected to become under defined solar and environmental conditions. It combines solar reflectance and thermal emittance rather than replacing them.
SRI is useful for comparing products, but it is not an energy-bill guarantee. Roof insulation, air leakage, duct location, equipment efficiency, building use, shading and the percentage of roof exposed to sun all affect the outcome.
Initial and aged ratings are different
Dust, biological growth, weathering and surface changes can reduce reflectance over time. A new-product value describes the tested surface at the beginning; an aged value gives a better basis for long-term expectations.
The Cool Roof Rating Council program uses accredited independent laboratories and approved methods to report initial and aged radiative properties. Its Rated Roof Products Directory lets owners compare tested products instead of relying on an unverified “energy efficient” label.
A CRRC rating reports radiative performance. It does not certify installation quality, waterproofing, impact resistance, wind resistance or suitability for a particular building.
Cool surfaces exist for several roof systems
Reflective options can include:
- Elastomeric coatings over approved spray-foam or compatible roof systems
- Single-ply membranes in reflective colors
- Coated metal panels
- Reflective asphalt shingles
- Concrete or clay tile with qualifying surface properties
The exact assembly still must be approved for the roof slope, deck, drainage, wind exposure and use. A reflective coating cannot rescue saturated insulation, failed flashings or unstable foam.
Why cool roofs matter in Arizona
Arizona's strong solar exposure creates an obvious reason to limit roof-surface heat. Potential benefits can include lower roof temperatures, reduced heat flow into the building and lower peak cooling demand. The result will be more noticeable on some buildings than others.
A single-story building with ducts or equipment near the roof can respond differently from a well-insulated building with little conditioned-space exposure. The DOE guide also warns that replacing a mechanically sound roof solely to increase reflectance is rarely economical. Consider cool-roof performance when replacement or recoating is already justified.
Reflectivity does not replace insulation or ventilation
A roof surface, insulation layer and attic or conditioned-roof design solve different parts of the heat-and-moisture problem. Reflective roofing reduces absorbed solar energy. Insulation slows heat transfer. Air sealing limits uncontrolled air movement. Ventilation or a designed unvented assembly manages moisture and temperature according to the building design.
Treating one product label as a complete efficiency strategy invites disappointment.
How to compare products responsibly
Ask for:
- The exact manufacturer and product identifier
- Initial and aged solar reflectance
- Initial and aged thermal emittance
- SRI when relevant to the project requirement
- The product's approval for the existing roof assembly
- Surface-preparation and maintenance requirements
- Warranty terms for both material and workmanship
For an existing foam or coated roof, the inspection should also check adhesion, exposed foam, ponding, cracks, penetrations and coating thickness. America Roofing can evaluate those conditions through its Arizona foam-roofing service and specify a reflective system only when the substrate and drainage support it.
Primary references
Sources used for this guide
- Consumer Guide to Cool Roofs U.S. Department of Energy
- Roof Product Rating Program Cool Roof Rating Council
- CRRC Rated Roof Products Directory Cool Roof Rating Council