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Reflective Insulation Fights Heat

Christopher Brinckerhoff, Associate Editor, Posted 03/01/2017

Airspaces and reflectors create thermal resistance

Covertech Fabricating Inc.'s rFoil reflective insulation.Reflective insulation is like a Thermos bottle for buildings. Thermos bottles have evacuated air cavities and reflective surfaces that provide thermal resistance. Instead of a vacuum, reflective insulation in buildings uses enclosed airspaces at regular pressure. Both reflective insulating technologies reflect a large portion of infrared radiant energy and emit very little, keeping cool things cool and hot things hot.

Monty Millspaugh is vice president of technical services at Reflectix Inc. in Markleville, Ind. Millspaugh has been working in the reflective insulation industry since the 1980s, served in a wide variety of capacities at Reflectix and sat on trade association boards including at Reflective Insulation Manufacturers Association International (RIMA). Millspaugh has used the Thermos bottle analogy during presentations and another way he describes how reflective insulation works is to ask an audience, "What did you learn in school about heat? And invariably the answer is, 'Heat rises,'" he says. "No. Heat does not rise; hot air rises. Heat travels from a warm surface to a cold surface in any direction it has to go. It'll go up, down, horizontal, it'll go all directions."

High Reflectance and Low Emittance

Covertech Fabricating Inc.'s rFoil reflective insulation.Reflective insulation material has two related properties that make it respond to infrared radiant heat: reflectance and low emittance. Reflective insulating surfaces have molecular make-ups that reflect a large portion of infrared radiant energy and emit, or allow to transfer through, a small portion.

David Yarbrough, Ph.D., PE, research engineer and founder, vice president at R&D Services Inc. in Cookeville, Tenn., has been a consultant and author in the building insulation industry for more than

35 years. Yarbrough says in radiation theory, the equation is simplified to reflectance plus emittance equals 1. "So they're complementary in mathematical terms," he says. "If you have a reflectance of

0.95, you automatically have an emittance of 0.05; they add up to 1. Both properties are attributable to the aluminum foil or the metalized film that's being used to produce the shiny surface."

Reflective insulations are installed with a reflective surface facing an enclosed airspace. "The thermal resistance comes from the airspace, not the material," Yarbrough says. "Reflective insulation reduces the radiation across an airspace by a very large percentage."

Reflective Insulations and Radiant Barriers

The simplest way to distinguish between reflective insulations and radiant barriers in building applications is by airspace, Yarbrough says. Reflective insulations have enclosed, non-ventilated airspaces and radiant barriers have ventilated airspaces. "It could be exactly the same material," he says. "It could be installed in a closed cavity, or it could be tacked up in an attic. In one case it would be referred to as a radiant barrier, if it's up in the attic; down in the cavity, it's a reflective insulation."

Another aspect that differentiates reflective insulations and radiant barriers is R-value, in that reflective insulations have one and radiant barriers do not. "Reflective insulations can be evaluated with the same kind of terminology that's used for all other insulations," Yarbrough says. "It is difficult to assign a single value to a radiant barrier because the conditions in an attic change so dramatically from day to day and from winter to summer. Radiant barriers are generally evaluated in terms of energy savings."

R-Value and Heat-Flow

Robert Wadsworth, owner and president at Innovative Energy Inc. in Lowell, Ind., founded the company in 1980. Wadsworth says the most common question he hears about reflective insulation is, what's the R-value? "Depends on how it's installed," he says. "And that's one of the real hard concepts to get across to new customers."

Wadsworth says reflective insulation R-values are determined, in part, by the direction of heatflow, whether it's heat-flow up, heat-flow down or heat-flow horizontal. "And the reason for all that is that in downward heat-flow, convection plays a minimal role in the transfer of the heat," he says.

"Convection just isn't a part of it. There's some conduction, but almost all of it is radiation. So it has a relatively high R-value." Wadsworth says in the case of horizontal heatflow, some convection takes place in the enclosed airspace facing the reflective surface. "These convection loops occur, which reduce the R-value by quite a bit," he says.

In the case of heat-flow up in attics in cold weather, the R-value is reduced by convection, Wadsworth says. Convection occurs in both horizontal and upward heat-flow and plays a significant role in heat-flow up. Examples of R-values for reflective insulation assemblies with heat-flow down are approximately R-13 to R-14, with heat-flow horizontal R-7 to R-8 and with heat-flow up R-5.

Abundant Options

Reflective insulation may be used in roof and wall applications, as well as other empty airspaces including under floors in structures with basements and crawl spaces. Yarbrough says contractors are faced with choosing insulation products from a vast variety of options. "They have a whole menu of possibilities for insulating buildings, and they need to make a choice based on cost, the amount of thermal resistance that's going to be delivered, durability, ease in handling and understanding how to use," he says. "There's a whole series of factors to consider when specifying products. And they generally have quite a barrage of information coming from the very large insulation companies. They probably have more information available about conventional insulations than about reflectives."

Millspaugh says there are projects for which his company recommends installing its reflective insulation by itself and others it recommends combining it with a mass insulation. These combinations of different types of insulations are called hybrid assemblies.

"For example, cathedral ceilings, that would be a hybrid assembly where we would recommend to meet code, say an R-30 fiberglass batt ventilated, and our product either above the vent or below the fiberglass, depending on what part of the country you're in," he says.

Mass insulation manages conductive and convective heat transfer very well, Millspaugh says. "On the other hand, our major performance comes from the third leg of heat loss or heat gain, and that's the radiant transport," he says.

Michael Boulding is president and CEO at Mitex International Inc. in Terrell, Texas. Boulding has been active in the building products industry for more than two decades, including six years serving as president at RIMA. Boulding says reflective insulations can be effective as stand-alone systems in agricultural buildings such as horse barns, warehouses, sheds and other structures that wouldn't otherwise have any insulation. "[When] you're looking for a vapor barrier, you're looking for some insulation value, not a tremendous amount, you're going to be between the 5 to 10 range in an R-value, depending on heat-flow direction and application, but you're going to get a clean finish, you're going to get the reflective part of the insulation, you're going to get some R-value, and it's inexpensive," he says. "If you've got an operation where people are working in the building on a consistent basis, where there's any sort of manufacturing or distribution and you may not want the building to get too hot, then you start getting into the higher hybrid insulations."

Yarbrough says there are opportunities for people to be creative with reflective insulations in attics.

"Under roof rafters in attic spaces, you can make a combination radiant barrier/reflective insulation by trapping air on one side between the roof sheathing and the reflective material, and then on the other side, you have a low-emittance surface facing the ventilated space," he says.

Millspaugh says there is a very large market for retrofitting commercial buildings with reflective insulation. "There are a whole lot of buildings out there that are very difficult to insulate because they've got goods in them, and they're hard to get to. But reflective insulations are a lot easier to install than some of the mass insulations because they can go to the bottom of purlins," he says.

John Starr is vice president and partner at Covertech Fabricating Inc. in Etobicoke, Ontario, Canada. Manufacturers have developed installation methods that make reflective insulations easier to use in retrofitting applications, Starr says. "There's a few of us out there that have come up with a clip system that allows us to put our product and attach it to the underside of the purlin and not touching anything else," he says. "So it's a very easy, quick fix for retrofitting a building with our insulation."

Yarbrough says applications of reflective insulation below floors are attractive in many cases where there aren't requirements for high levels of insulation and there is some space between floor joists. "You can tack reflective insulation on the bottom of the floor joists in a house that has either a basement or a crawl space, and you've trapped several inches of air between the subflooring and the low-emittance material," he says. "What you look for is airspaces that are not being used for something else that you can easily enclose with a boundary with low emittance."

Reflective insulations are not applicable in all projects, sometimes due to the insulation requirements or a lack of uniform space, Yarbrough says.

"A reflective system tends to be more demanding in terms of having a good, uniform space to put it in because it's operating on the basis of an airspace of certain dimensions," he says. "When you change the dimensions of the airspace, you change the Rvalue. Each insulation product has certain characteristics and cost factors that make it fit in one building and maybe not so good for another."

Boulding says the use of reflective insulations as components of hybrid insulations has noticeably increased during the last two to three years. "I think what's interesting about it is that it's starting to bring various manufacturers of various types of insulation together, and that's a good thing," he says. "In the past, you had the mass insulation guys and you had the cellulose guys and you had the reflective guys and you had the foam guys, and they were so far apart, everybody just battling it out for their specific type of insulation. You still get a bit of that, but what we're starting to see, I think, is some of the manufacturers are being more progressive and saying, 'Hey, I've got a reflective insulation that works really well with foam insulation, so why don't we team up with some foam guys and put a system together.' We're starting to see a more common ground, let's build a better system, as opposed to let's just use more of the stuff we happen to make. And it's only taken 50 years, but people are starting to get it."



Reflective Insulation Myths

While use of reflective insulations has been gaining momentum for decades, misconceptions about the products persist in some circles. Here are two myths Reflective Insulation Manufacturers Association International (RIMA) dispels on its website:

Myth: There is no way a product that thin provides much benefit.

Fact: The thickness of reflective insulation has much less to do with the overall thermal performance than does the reflective (low emittance) surface facing the adjoining airspace. The application, including the size of the adjoining airspaces and directions of the heat-flows are the major contributors to the R-values and thermal performances of the insulation systems.

Myth: Radiant barriers and reflective insulation only work in hot regions. They are not suitable for use in cold climates.

Fact: Reflective insulation products excel in hot climates and often are the first choice for insulation in those regions. They provide significant benefits in cold climates where they are used alone or in combination with other insulation materials.

Check out more myths and lots more information about reflective insulations, radiant barriers and interior radiation control coatings at RIMA's website:

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