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There's nothing like talk of an energy crisis to get people pondering their insulation. Whether it's stuffed into an attic crawlspace or sandwiched into the cavities of a wall, the yardstick most people use to measure insulation is R-value. The more Rs you've got, the better equipped you'll be to ride out the next energy drought. But even for those well-prepared homeowners who can recite the R-value of their walls faster than their own Social Security numbers, this most fundamental measurement of insulation is often misunderstood.
Few homeowners know what "R" is, never mind its value. Even fewer realize that the R-value stamped on the kraft-paper face of fiberglass batts has little to do with a home's ability to hold onto precious conditioned air, which can account for 50 to 70 percent of a home's energy consumption, according to the U.S. Department of Energy. Most people know higher R-values are better, but beyond that the term remains pretty much a puzzle. Why bother unraveling this mystery, aside from the satisfaction you might get impressing a Home Depot salesman with your knowledge? Because in older homes and even in new construction - where building codes establish minimum R-values for the thermal envelope or foundation, exterior walls and roof - real-world efficiency can be all over the map. Understanding R-values (and their limitations) can help homeowners choose the most cost-effective insulation and decide where and how to install these products - including whether to do it themselves.
What is R-value? The "R" in R-value stands for resistance. The more resistant a material is to the conduction of heat, the higher the R-value. Resistance is important because when there's a difference in temperature between two sides of a surface, heat will always move toward the cooler side. Inside a house on a cold day, warm interior air pounds on the walls toward the cooler exterior. The R-value in the walls is there to pound back, keeping the heat from escaping. In summer, the same process occurs except the assault is from the outside in. High-R insulation, such as fiberglass, cellulose and foam, defends this line by severing the routes that heat travels. "Fiberglass is sort of like your hair on a bad hair day," said Andre Desjarlais, an engineer at the Oak Ridge National Laboratory, a research arm of the DOE.
"The energy has to go from one fiber to another, and that's a rather tortuous path." Cellulose and foam (whether rigid panels or sprayed in place) contain tiny pockets of trapped air that block conduction. The result is the same - lots of dead ends that slow or stop the migration of heat. Fiberglass, cellulose and foam are more than 95 percent air (or gas) and have R-values from 2 to 7 per inch or more. "The air itself really becomes the insulation," said Gene Meyer, an engineer at Kansas State University and an expert on home insulation. A 2x4 of framing lumber, by comparison, has an R-value of about 1 per inch, still far more resistant to heat transfer than high-density materials like aluminum or steel, which have R-values near 0. Plywood and other wood-composites used in home construction have R-values slightly higher than solid wood, but compared to insulation, pose relatively little resistance to conduction. R-values and home insulation R-value is just one measurement of thermal efficiency, and it's a finicky one at best. Add up all the R-values in a typical 2x4 wall - from the R-0.67 of the siding to the R-0.45 of the drywall - and you end up with what may seem like a downright cozy R-20. Tempting as it may seem to add up all the Rs and start counting your savings, the path to energy efficiency is not quite so direct, and it's easy to get lost. For starters, the R-value of insulation is determined and tested by manufacturers and rating agencies under ideal conditions, not in the real world.
The U.S. government requires manufacturers to measure R-value at a mean temperature of 75° and a deltaT (the difference in temperature from one side to the other) of 40° to 50° F. What about when the outdoor temperature is below zero? Depending on the type of insulating material, the actual R-value could be lower. "R-values can change as the temperature difference increases, but in most cases not significantly," Meyer said. One exception may be fiberglass. Some tests have shown that fiberglass loses R-value under extreme temperature differences due to air movement within the insulation itself, especially in attics. (Charles Cottrell, director of technical services for the North American Insulation Manufacturers Association, says fiberglass insulation may lose some of its R-value in extremely cold conditions, but the loss is minor "even in some of the most severe climates of the U.S.") Foam insulation - whether sprayed or in rigid sheets - also may lose some R-value as the tiny cells of gas inside the insulation dissipate and are replaced by air, although the R-values for foam products are supposed to take this aging into account. Gas-filled, high-efficiency windows potentially have the same problem, although the amount of loss that might occur over time is the subject of debate. How well insulation is installed also plays a big role. Just a few weak links - gaps around windows and doors, ill-fitting or compressed fiberglass batts (often found around wiring and plumbing) or slight settling in cellulose-filled stud bays - and the actual R-value of a wall (or whole-wall R-value) plunges. "Leaving 5 percent of a wall uncovered could increase energy loss by as much as 50 percent," Desjarlais said. Compressed and settled insulation produces similar losses.
"You can have the best insulation, but it won't perform nearly as well as the number on the label if it's not properly installed." That's not all. Every stick of wood, plywood, hardboard or steel framing acts like a thermal freeway, allowing heat to bypass the high-R insulation barriers inside the wall cavities. A 20 to 25 percent framing factor - the amount of wood framing in a wall - is common, and it could reduce the overall R-value of the wall by 40 percent or more. Add to this the quality of the framing, the air-tightness of wall and roof assemblies and the air-resistant qualities of the insulation itself, and you've got lots of factors driving overall energy efficiency. Even comparing R-values among different types of insulation can be tricky. "The R-value for one insulation product means something completely different in another product," said Fred Lugano, an insulation contractor and consultant in Charlotte, Vt. "It's a good way to compare differences among the same class of insulation. A 1-inch foam panel with an R-value of 7 is twice as resistant as a 1-inch foam panel with R-3.5, but the R-value stamped on a package of insulation doesn't have much to do with how it will perform in a building."
How much R-value do you need? They may be ambiguous, but R-values do mean something. Better insulation translates into lower energy bills and greater comfort - less cycling of air-conditioners and forced hot-air blowers, and a more evenly conditioned home. The more extreme the climate - a greater disparity between indoor and outdoor temperatures during heating and cooling seasons - the more valuable each incremental boost in R-value. Fortunately, figuring out how much R-value you need is work that's already been done for you. Most local or state building codes establish minimum R-values for foundations, walls and roofs, roughly parallel to recommendations published by the DOE for every region of the country. These recommendations take into account climate and energy source (usually electricity, oil or gas) and fold them into an economic equation that also factors in the current and projected cost of energy. A homeowner in Minnesota with electric heat is going to get more value out of an R-22 wall than their next door neighbor with a gas-fired boiler, who may be able to get away with R-18. The amount of energy lost is the same, but the value of that energy is greater for the electricity consumer who pays more for heat energy. The DOE recommendations assume a "payback" period of about 10 years, the amount of time it would take for the cost of the insulation to pay for itself in energy savings. Even with energy prices likely to rise, ratcheting up R-values beyond this payback period may have only marginal effects on long-term energy savings.
There is another reason that more R isn't always better. A home's thermal envelope is like a bucket with lots of holes, some small and some gaping. Especially in older homes, the easiest and most cost-effective way to increase energy efficiency is to seal the house against air leakage. The value of piling on R-100 worth of insulation in the attic of a house with leaky walls and windows would be negligible; spending the time and money on air sealing would have a bigger impact on energy efficiency. "Although it plays a big role, R-value is not the only metric when talking about the thermal envelope," Desjarlais said. "You really need to take a balanced approach."