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Fiberglass Insulation

Dateline: 06/29/98

As I write this at 6:00 PM, it is still 100 degrees outside, and it looks like the temperature will reach the triple digits all week. The huge mushroom cloud I can see outside my window is a forest fire, nearly 100 miles away, spreading rapidly in the high temperatures and dry conditions.

At a time like this, you may be wondering why I'm writing an article about fiberglass insulation. After all, it's a product normally associated with keeping warm in the winter.

The purpose of insulation, though, is to slow down the movement of heat through your walls. The insulation doesn't care which way the heat is moving, so it does just as good a job keeping the summer heat out as it does the winter heat in.

In this article, I'll explain how fiberglass is able to act as an insulator. First, though, we'll need to take a quick look at heat transfer in general.

Heat and Temperature

Heat is a measurement of the motion of the molecules which make up a substance, and represents the energy contained in the substance. In hot substances, the molecules are moving relatively quickly. In cold substances, the molecules are moving slower, but the motion is still called heat.

Temperature, on the other hand, is a measure of how something feels to us. An object with a low heat content probably feels cold, but the object still contains heat.

Heat always moves from a warm object to a colder one. The important thing to remember is that cold isn't transfered, because cold is just a qualitative description of low heat. A warmer object may cool off, but that is because it is losing heat, not gaining cold.

Conduction, Convection, and Radiation

Heat can be transfered in one of three ways: conduction; convection; or radiation.

The most efficient method is conduction, which is the transfer of heat between two objects in contact. The transfer occurs as the fast-moving molecules of the hot object bump into the slower-moving molecules of the cold object. The fast molecules give up some of their energy, slowing down, and this energy goes into speeding up (and thus heating up) the slow molecules.

A slower method of heat transfer is convection, which occurs in fluids or gases. A cool fluid in contact with a warm solid will heat up through conduction. The warmer fluid drifts into the cooler fluid, setting up a convective current. Because material must actually be moved, convection is less efficient than conduction.

The least efficient method of heat transfer is radiation. In this case, heat moves through space without the assistance of a physical substance. This is how the Sun's heat reaches the Earth. The radiative heat is transfered directly into a solid object, but it passes readily through transparent materials such as air and glass.


When you run your air conditioner during the summer, the air inside your house is much cooler than the air outside. Because nature always tries to equalize temperatures, heat will begin flowing into your house.

Sealing all the drafts prevents direct convective currents from forming (the hot outside air can't flow into the house), and drawing the curtains prevents the Sun's radiant energy from heating your inside floors, walls, and furniture.

If you've done all of this, then the only way for the heat to get into your house is to move through the walls (and the slab or basement floors, and the roof). There is no way to prevent this transfer of heat, but you can slow it down. To keep your house cool, the rate at which heat enters must be lower than the rate at which your air conditioner can remove it.

In a manner similar to electrical wiring, the walls have a resistance to heat transfer. The higher the resistance, the longer it takes heat to move through the walls.

If the walls are solid, then the heat is transfered through conductance, which is the most efficient (and thus fastest) method. Resistance can be increased by using a material which is a poor heat conductor, or by increasing the wall thickness.

Most building materials, however, are fairly good heat conductors, and the good insulators make poor houses. Thick walls such as adobe or straw bale make good insulators, but these materials aren't as common as wood studs or bricks.

The ideal insulating approach would be to force the heat to be transfered as radiation. Unfortunately, this would require a vacuum between the siding and the interior wall. This method is used in the laboratory for storing very cold liquids, but is impractical for something like a house.


After radiation, convection is the next best option. It's a less efficient transfer mechanism than conduction, and it turns out that air is a pretty good insulator.

If you just leave a gap between the inner and out walls of your house, the only way to transfer heat is through convective air currents. (Well, heat will also be conducted through the wall studs, but for our purposes we can ignore them.) This is better than a thin, solid wall, but it's still not good enough under most conditions.

The problem is that the moving air has a direct path from the warm outside wall to the cool inside wall. It would help things a lot to find a way to slow down the air currents.

This is where fiberglass insulation comes in handy. Fiberglass insulation is made from very fine strands of glass held together in a thick, random mat. Between the fibers are many small pockets of air.

The glass strands themselves are very poor conductors of heat. In order for heat to be transfered through the mat, then, it must be carried convectively through the tiny air pockets. But the randomness of the strands means there is no direct path through the mat, so the heated air must take a very circuitous route through the wall.

The insulating properties of fiberglass mat depend on this random path. If the mat is compressed, the path becomes shorter, and the heat gets through quicker. That is why you should not tightly pack insulation.

Any material that provides this type of air path can be used as an insulator. My old house had cellulose insulation (basically shredded newsprint) in the attic: it worked fine as an insulator, but presented somewhat of a fire hazard.

A promising new class of materials are the aerogels. They have the lowest density of any solid material--they are almost entirely made of air--but until their costs come down they will be limited to specialized applications such as satellites.

Related Links

I searched the Internet for a good introductory page on heat transfer, but most sites are too specialized or don't cover the basics very well. If you have any suggestions, please send them to me at composite@aboutguide.com.

There are a few pages which merit a look:

  • Glossary of Heat Transfer - List of terms, including heat, conduction, convection, etc., with brief definitions.
  • Heat and Temperature - Fairly detailed description of the relation between heat and temperature, but not much on heat transfer.
  • Heat Transfer Experiments - Relatively easy experiments to demonstrate the principles of heat transfer; the figures show experimental setups, but also help illustrate the principles.

Owens Corning is probably the most well-known producer of fiberglass insulation (they also make fiberglass for use in composites). Their insulation page has some information about insulation, but focuses on such topics as selection and proper installation.

All figures on this page were drawn specifically for this article by Barry Berenberg.

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