What besides energy savings justifies radiation control for low-slope roofs?
The savings in annual energy costs and other cost savings need to be weighed against the local cost to purchase and install the coating or membrane system. The focus in this fact sheet is on protective coatings with optional reflective pigments (such as aluminum or titanium dioxide) to save on energy needed to cool buildings under the protected roofs. The other cost savings associated with the protective nature of the coating or membrane system may be more significant than the savings in annual energy costs, especially outside a cooling-dominated climate.
Protective coatings have a long history of use before air-conditioned buildings with cooling energy requirements became commonplace. For a brief history of protective coatings and their evolution to the variety of products available today, see the Roof Coatings Manufacturers Association. These products range from low viscosity, non-fibered coatings for penetrating primers and damp proofing coatings to high viscosity, heavy-bodied cements for adhering waterproofing membranes and components and for patching and repairing leaks. In the middle of this range are medium viscosity, fibered and non-fibered products for inter-ply adhesives and top coatings.
Radiation control coatings fall in this medium viscosity category. Protective coatings, in general, use resins ranging from bituminous (such as asphalt) to polymeric (such as acrylic) with reinforcing fillers and a carrier solvent or water emulsification. Reflective pigments are added for radiation control. The coating is left behind as a cured, water resistant film when the carrier solvent or water evaporates.
An important purpose of the coating is to protect the membrane it covers from effects of the ultraviolet (UV) radiation in sunlight. UV degrades asphaltic and some rubber materials, causing them to become brittle and unable to flex during thermal stressing. Ultimately this leads to cracks in the membrane itself or its failure to adhere to flashings. Coatings with optional reflective pigments also protect the membrane from high peak temperatures, lessening the thermal stresses.
As shown by the section on the effect of solar radiation control on energy costs, radiation control savings with little or no thermal insulation are very attractive. Moreover, the cost of a roof, especially with moderate to high levels of thermal insulation, is a significant investment and any projected savings help justify the investment. Often the only cure for water leaks into the building interior is extensive repair or replacement of the membrane. Water held in many types of roof insulation makes them ineffective as thermal insulation. Complete replacement of a damaged roof and disposal of wet insulation is more expensive than construction of a new roof with the same specifications. Even if energy savings due to decreased cooling loads are minimal, protective coatings have the potential to prolong the life of the membrane and add barriers or repair existing barriers against water leakage into the roof system. In this context, energy cost savings can be viewed as a bonus in addition to the other benefits of radiation control.
