An Engineering Extension energy feature
An Engineering Extension energy feature
By Mike Dorcey
Radiant barriers can be a cost effective way to reduce energy consumption, but they may not be the best buy for every
house.
Radiant barriers have been the subject of some controversy since their introduction in recent years. But an engineer engaged in research at Kansas State University said the controversy is not over whether radiant barriers work.
"Everybody agrees that they will block radiant heat flow," said Byron Jones, director of the Institute of Environmental Research. "That's not the issue. The issue is how much impact is it going to have on energy consumption, and what are the cost tradeoffs compared to adding more conventional insulation material." Radiant barriers are most effective in climates where the hot season is long and temperatures are high, Jones said. Kansas lies in a zone where the effectiveness of a radiant barrier depends on the individual structure and how homeowners approach the problem of reducing heat loss in winter and heat gain in the summer.
"In Kansas we're on the borderline between insulating for summer or winter. North of here people insulate for winter. That is they use more insulation, which limits the effectiveness of a radiant barrier. South of here radiant barriers are more popular because homeowners insulate less and endure longer, hotter summers. "In Kansas you can do either, but once you have enough insulation (10 inches), the value of a radiant barrier diminishes, because there is little heat flow through this much insulation in the first place," he said. "The air conditioning energy savings that result from installing a radiant barrier depend on the amount of conventional insulation already installed. Savings can range from 5 to 25 percent of the energy required to operate the air conditioner in typical houses."
Jones said a radiant barrier is used with normal insulation materials, such as fiberglass and cellulose, and should not be considered a substitute for insulation but rather something to enhance ceiling insulation. Houses that already have 10 inches or more of ceiling insulation are not likely to gain enough energy conserving difference to make a radiant barrier cost effective, Jones said. People putting insulation into new homes or upgrading insulation levels in existing homes have options.
"In a typical attic with typical insulation (R-19, or 6 inches), a radiant barrier can decrease summer heat flow through an attic by about 20 percent," Jones said. "But this 20 percent is only one factor in the entire energy conservation equation in your house. The actual impact on energy consumption depends on the environment in which it's installed."
"If your house has a lot of south-facing windows with no shades, poor tree shading or a big heat load from within because of appliances or lots of bodies, then a radiant barrier may have only a 5 percent impact or less," he said.
Jones said the controversy over radiant barriers has not been over whether they work but rather at what price they no longer pay for themselves.
"People install radiant barriers because they're normally inexpensive, costing only a few cents per square foot for the material. Where most homeowners incur cost is in the installation," he said.
"If you're going to install a radiant barrier yourself, it's not expensive," Jones said. "But if you're going to pay for installation, assess your building, the cost of adding more insulation and the cost of installing the radiant barrier before going ahead. Consider accessibility, too, in your installation costs because the cost of labor may put a radiant barrier out of the affordability range if your attic is difficult to work in." Jones said there is also some controversy in the building materials industry about how effective radiant barriers are.
Companies making radiant barrier material claim its advantages, of course. "The vast majority of radiant barrier manufacturers are honest and are simply marketing their product," he said. "Unfortunately, both for the homeowner and for the honest companies, there have been some cases of fraudulent claims and unreasonable charges for installing radiant barriers.
"Unethical companies can get away with this because the public -- not just homeowners but contractors and even designers, too -- is unfamiliar with radiant barriers and promoters can play on that lack of understanding."
This leaves the way open for competitors (insulation manufacturers) to deny or minimize the value of radiant barriers.
Jones said his laboratory is currently conducting research that should help settle some of the controversy about radiant barriers by determining under which conditions a radiant barrier is most effective. In a controlled climate chamber, his staff built a simulated attic and is testing a variety of materials under various simulated summer conditions.
"What we're doing is methodically changing the conditions and measuring the results," Jones said. The goal of the research is to produce a database for designers so they can determine when installing a radiant barrier would be effective. Jones' staff will not complete the testing until the end of the year, he said.
Most importantly, Jones cautioned that what his research is demonstrating is that a radiant barrier is a viable, energy conserving technique but is not a simple cure-all.
"Everyone wants a simple answer but this is one situation where there is no simple answer."