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, hydrofluorocarbons (HFCs) 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 HFCs 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. When an air gap exists in a building’s shell, heat moves across the gap via infrared radiation—the same kind of radiation one feels from a campfire. The amount of radiant heat transfer depends on the temperature difference between the two surfaces, the emissivity of the warmer surface, and the absorptivity of the cooler surface. If used properly, reflective materials—single-sheet radiant barriers or multiple-layer products like reflective bubble-pack insulation—can significantly slow this radiative heat transfer by reducing the warm surface’s emissivity or increasing the cool surface’s reflectivity (the latter effectively reduces absorptivity). Reflective insulation must be adjacent to an air gap to be effective; otherwise, heat will simply conduct through to the next solid layer. A single layer of reflective foil next to an air space can provide an equivalent R-value of over R-3 in a wall system, and as much as R-8 in a floor-joist application where radiant heat flows downward. Reflective foils without an air space have an equivalent R-value of 0.
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.