Efficient Spray Applied Radiant Barriers
The use of interior radiation control coating systems reduce radiant heat loads and serves as an environmentally and healthier alternative
By Robert J. Aresty
The green revolution finds the building professional bombarded with sales literature and technical articles concerning improved methods for insulating structures. Radiant barrier products like foils and laminates are well established, but more confusing are paint products that claim to have insulating qualities. ASTM and RIMA (Reflective Insulation Manufacturers Association) have tried to clarify these claims by setting standards and classifying them as Interior Radiation Control Coating Systems (IRCCS) if they meet the criteria for ASTM C1321; this criteria is the “Standard Practice for Installation and Use of Interior Radiation Control Coating Systems (IRCCS) in Building Construction,” having an emittance of e=0.25 or lower.
RIMA’s market survey of coatings that claim to be radiant insulators is a good place for the building professional to start before choosing a product (visit www.rima.net and click ‘Technical Info,’ then ‘Coatings Study’). Interior radiation control coatings were originally developed by the Solar Energy Corporation for use in tubular evacuated solar collectors in 1978. Their Lo/Mit-II MAX product has the lowest emissivity in the industry, e=0.147.
The key number here is the emittance, emissivity, “e,” or the ability of a surface to reflect infrared radiation. The lower the “e,” the less able the surface is to emit its heat energy and the more effective it will be in green construction applications where reducing heat influx is important. Most building materials have a high emissivity (e=0.80 or higher) and tend to radiate their stored heat easily into cooler parts of the structure. The ASTM standard of e=0.25 or less means that a qualifying IRCCS will only emit 25 percent or less of its heat, in effect blocking 75 percent or more of this radiant heat transfer.
Most paints, including aluminum-pigmented coatings, also have emittances in excess of e=0.80. IRCCS are generally silver in color, but are formulated specifically to produce a low emissivity surface when applied to solid substrates such as building walls and roof decks. The most effective use of an IRCCS is on the underside of uninsulated roof decks since they work best when applied to the hottest surface, facing an air space. Their main purpose is to reduce radiant heat loads, lower cooling costs, and increase comfort.
Aluminum foil or metallized film radiant barrier products have slightly lower emittances in the range of e=0.03 to e=0.10. In many cases, however, the cost of installing foil is several times that of spraying an IRCCS. The material cost of IRCCS are generally $0.12-0.20/ft², and are typically installed for $0.45-0.85/ft2 retail, depending on the local market. If the installed cost of a foil radiant barrier product is 30 percent or more higher than the IRCCS, the IRCCS is probably the better investment.
There are many other reasons to use IRCCS in green construction, such as ease of installation. Generally, one operator using standard airless paint equipment can install 1,000 ft2/hr. IRCCS are permeable and will not trap moisture. This eliminates rot and mold potential on the roof sheathing where they are commonly applied. They are electrically non-conductive, and should not be a hazard if contacted by electrical wiring or if the roof is struck by lightning. Also, should a roof leak develop in a building with an IRCCS installed, the source of the leak is easy to find and address. IRCCS have high reflectivity in the visible spectrum as well, so they will also enhance interior lighting levels. Most are low VOC and their dry film is non-flammable, so IRCCS do not require additional thermal barriers like many foam products do. They also have no known degrading effect on roof shingles.
Where an IRCCS may be effective in lowering summer heat loads, but not adequate for retaining heat during the winter months, additional fiberglass or foam insulation may be added to the system as long as a two-inch air space is maintained between the IRCCS and the solid insulation system. IRCCS cannot be assigned an R-value since they only address radiation heat transfer, but when used in conjunction with standard insulation systems, they offer a very cost effective method of raising the performance of the total system in retarding heat flows.
The possible uses of IRCCS products in buildings are numerous, but they are not an instant panacea for reducing heat influx. If they are judiciously and realistically utilized, the building owner will be very pleasantly surprised by the outcome. Remember, it’s a special paint when its emittance is low, and an IRCCS just might be the best solution to enhancing the thermal performance of your green buildings.
Robert J. Aresty is the President and CEO of SOLEC Solar Energy Corporation in New Jersey. He may be reached at firstname.lastname@example.org.