Insulation can be one of the most important factors in achieving energy efficiency in a building. It works primarily to slow the flow of heat through a building envelope, but it also can seal the envelope, preventing outside drafts and air leakage from unconditioned spaces, thereby maintaining indoor air quality. Insulation not only saves money by reducing heating and cooling loads but also is a key factor in achieving comfortable living and working spaces.

All common insulations use air or some other gas contained within bubbles or pockets in the insulation to decrease the insulation's conductivity, because a gas conducts heat more slowly than a solid. Also, to prevent convective heat loss, which occurs from the flow of gas, the gas must be kept as still as possible. The thin fibers in fiberglass insulation, for example, create small pockets of air and thus restrict air circulation. In theory, an inch of air can achieve an insulation value of R-5.5. Realistically, however, the best air-filled insulations only achieve R-4.5 to R-4.7 per inch (meaning that two inches of insulation would give you R-9 to R-9.4), and most are rated considerably lower. To achieve higher values, hydrochlorofluorocarbons (HCFCs) are used in place of air, because they have a lower conductivity.

There are three basic types of insulation: fiber, foam, and reflective.

Fiber insulation is available in either loose-fill form or in batts. Loose-fill insulation consists of fiberglass, cellulose, rock wool, or some other type of fibers that are blown into wall cavities or attic joist spaces. If properly installed, loose-fill insulation can provide more complete cavity coverage than batts, because the fibers can fill around wires, piping, and other obstacles. Loose-fill is usually installed by specialized contractors, whereas batts can be installed by nearly anyone with a desire to insulate a space. R-values per inch for loose-fill insulation range from R-2.2 for fiberglass to about R-3.2 for rock wool or cellulose.

Batts are available made of fiberglass, cotton, or rock wool, and all achieve an R-value of about R-3.2 per inch.

Be aware that loose-fill insulation can settle over time. When loose-fill insulation is located in a wall, its settling creates a void above the insulation that can serve as a conduit for heat. In an attic, settling results in a nonuniform distribution of the insulation, which can reduce its effectiveness. Some types of insulation use acrylic-based or other binders to help prevent settling, though there is some question as to how well they work.

Loose-fill cellulose can be mixed with water and blown in wet, usually without any added binders. This sticky mixture molds itself into gaps and seals them to a degree, which helps to eliminate air leakage and infiltration, and has an R-value of about R-3.5 per inch. Loose-fill cellulose can also be installed dry with a blowing machine and a reduced-size application nozzle that packs the insulation tightly, creating dense-pack, or high-density cellulose. Though this method does not seal as well as wet-spray cellulose, dense-pack cellulose also reduces air infiltration. Because it is packed tightly, little settling occurs and, unlike wet-spray cellulose, it can be used on wall insulation retrofits. It too has an R-value of around R-3.5

Foam insulation comes in either rigid sheets or spray. Rigid foam insulation generally has a higher R-value per inch than fiber insulation, because it uses HCFCs instead of air to create pockets or bubbles in the foam sheet, thereby achieving values from R-3.6 (expanded polystyrene) to R-7.7 (isocyanurate). Rigid foam insulation is also easy to install with nails or glue. Its cost per unit R, however, is much higher than that of fiber.

Sprayed-in foam can be used in open or closed cavities as well as around ducts or pipes that pass through the building envelope. Low-density urethane spray foams can achieve up to R-11 per inch, though most foams are rated much lower, with values around R-4 to R-6. Like wet cellulose, spray foams are effective at sealing out drafts.

Care should be taken in selecting rigid foam insulation, as it can contribute to insect problems. Carpenter ants and termites will tunnel through polystyrene and polyisocyanurate foams to either create nesting cavities or to create a protected passage to wood inside a building. To combat this problem, one company has added a boric acid insect repellant to its foam insulation. Testing to date has shown the treatment to be fairly effective in keeping insects away.

Reflective insulation is different from the other kinds in that, instead of reducing conductive heat flow, it reduces radiant heat flow. It also does not use a gas to insulate. In its most basic form, it consists of a single sheet of a reflective material, which reflects heat emitted from a warm surface back to that surface. It must be positioned adjacent to an air gap to be effective, otherwise heat will simply conduct through to the next solid layer that it touches.

Manufacturers' claims for the R-values of reflective insulation need to be examined closely, because the R-value can change depending on where the insulation is used. For example, in floor joist spaces above an unheated basement, convective heat losses are minimal, as the warm air will stay near the floor. As a result, radiant transfer is the primary method of heat loss, and thus a single reflective barrier can have an equivalent R-value of R-8. In a ceiling joist application, the warm air will rise away from the conditioned space, instead of into it, and so convection will be the primary method of heat loss. In this case, reflective insulation will have little effect in stopping heat loss and should not be used in place of fiber or foam insulation. A single layer of reflective insulation next to an air space in a wall can have an equivalent R-value of around R-3.

Evaluate the area to be insulated. Accessibility to the area needing insulation will often be the first determining factor for choosing what type of insulation to use. New construction obviously allows for more options than an insulation retrofit, as the spaces needing insulation can be more easily reached. Table 1 serves as a general guide to where each type is commonly used.

Available space for the insulation also affects the choice. For example, to create an R-14 barrier using isocyanurate rigid foam, which has an R-value of 7 per inch, you'd need only two inches of space in which to fit the insulation. To achieve this level of insulation using fiberglass would be much cheaper; however, it would require about four and a half inches of space to accommodate it, which may not be available in a given circumstance.

Perform a cost-benefit analysis. A cost-benefit analysis will help to determine the most cost-effective type of insulation to use and what the ultimate R-value of the building envelope should be. The selection will vary according to the local climate and is affected by the law of diminishing R-eturns. This law says that each additional unit of R-value contributes less energy savings than the previous one (Figure 2). The effect can be seen by looking at the U-value curve, which quickly flattens as R-values continue to climb. In practical terms, this means that adding R-10 insulation to a wall that already has R-40 insulation will save very little additional energy. In deciding on the ultimate R-value of the building envelope, it is also important to remember that many communities mandate minimum R-value levels.

Consider environmental and health factors. In 1994, the National Toxicology Program listed fiberglass as a suspected carcinogen. Fiberglass is potentially more dangerous in loose-fill form, as airborne fibers are more easily produced by handling it than when handling batts. It also uses phenol formaldehyde as a binder. Phenol formaldehyde is a pollutant both in the manufacturing stage and in buildings, because it continues to off-gas from the insulation into the surrounding spaces for several years. One company has developed a potentially safer type of fiberglass whose fibers are laminated together without the use of a binder. These fibers are actually stronger and less likely to break and form airborne respirable fibers than is fiberglass containing formaldehyde.

Cellulose insulation is made from recycled newspaper. Some concerns have been raised about the health risks of ink residue, but the boric acid additive that is used as a fire retardant and vermicide is generally considered safe.

Chlorofluorocarbon (CFC)-blown rigid foams have been replaced with alternative foams that use HCFCs because of the damage that CFCs do to Earth's protective stratospheric ozone layer. Though HCFCs are not as damaging to the ozone as CFCs, they still have a negative effect and are slated for elimination over the next two decades. Possible replacements include hydrofluorocarbons and hydrocarbons.

The best available insulation products have a maximum R-value of about R-11 per inch. Several new insulation technologies exist that have an R-value of 20 or more, but they have not been developed as building insulation products. They include gas-filled panels, vacuum insulation panels, and aerogels.

Gas-filled panels contain multiple pockets of sealed polymer film filled with low-conductivity argon, krypton, or xenon gas, which have R-values per inch of 7.2, 12.4, and 20 respectively. Vacuum insulation panels use a vacuum held between two gas-impermeable layers of metal to create R-values of 25 to 40 per inch. Aerogels are low-density solids that resemble wisps of frozen smoke. They are made most commonly from silica and offer R-values of 15 to 35 per inch. All three of these are currently used in appliances such as ovens and refrigerators, but they are too expensive to compete with traditional building insulations. One company, however, is developing a granulated transparent aerogel that will be used to create skylights and windows with R-values of 8 to 20. The first of these should be available in late 2002 or early 2003.

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