Choosing the right insulation can be confounding. No other building material comes in so many forms—from ground-up newspapers to foam boards to translucent gel. Understanding how insulation works can help us find the right material for a particular project.
The word insulation comes from the Latin
insula—an island. Insulation attempts to create a climate-controlled enclosure by slowing heat flow—quite a trick, since thermal energy will always keep moving until equilibrium is reached, and your “island” of conditioned air is tiny compared with the outdoors. It gets even trickier when you consider that heat moves in three different ways: through conduction, convection, and radiation.
Thermal conduction is the movement of heat through direct contact: one molecule, literally vibrating with thermal energy, bounces into an adjacent molecule, transferring some of that energy. If you touch a hot wood stove, your hand will rapidly gain heat through conduction.
Convection, by contrast, is the movement of molecules through a fluid or gas. It transfers heat because differences in temperature tend to cause air to move, carrying its heat energy. Air is warmed by a wood stove, becomes buoyant, and moves upward through a room, spreading its energy.
Radiant heat flows by way of infrared waves. You can stand several feet from a fire and feel its glow. We are heated by the sun’s radiation, 93 million miles away.
R-value measures resistance to thermal conduction; a material with a higher R-value per inch is a better insulator. Most types of insulation work because they have millions of pockets of gas trapped within their structure. These pockets slow heat transfer from high-energy, warm air molecules to low-energy air molecules on the other side of the wall. Fiber insulation, such as fiberglass and cellulose, relies on trapped air. The R-value of some foam insulation is enhanced by trapped gases: low-conductivity blowing agents. The insulation materials themselves—plastics in foams or wood fiber in cellulose—are also inherently resistant to conduction.
Insulation materials stop convection in two ways. The material itself interrupts the air movement, and the individual pockets of trapped air are small enough that air currents don’t form within them. Some insulation materials also form an effective
air barrier, stopping air from flowing through the building enclosure and carrying heat (and moisture) with it.
Materials differ in their emissivity—their ability to radiate heat. Low-emissivity (low-e) coatings such as foil facings, when adjacent to an air space, help slow heat radiation from a warm object to cooler objects. Radiant barriers have a place in specialized applications, as the low-e coatings in windows and the underside of roof sheathing, but paint and insulation manufacturers are notorious for exaggerating their benefits.
June 1, 2011
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