Is Air An Insulator

This article was written by Jacob Verneris of Thermaxx Jackets

If you’ve dealt with insulation in the past, you may have heard someone say that air is a good insulator. This statement may pose many questions if you are unfamiliar with insulation or the properties of air. To get to the bottom of this, you must ask yourself, “Why is air a good insulator?”

In general, gases are better insulators than liquids, which are better insulators than solids. Density is a large factor which affects a substances insulation capability. Density is dependent on intermolecular spacing, or the distance between a material’s molecules. For a better visual, let’s look at the different physical phases of H2O. In its solid state, as ice, its molecules are packed much closer together in relation to its gaseous state as water vapor. The greater the spacing between a material’s molecules, the harder it is for heat to transfer through that material. Air is a good insulator because it is a gaseous substance, therefore its spread-out molecular configure resists heat transfer to some degree.

Think of the last time you had a hot cup of coffee or tea. When you pick the cup off the table to take a sip, you can feel the warmth of the drink get hotter as your face gets close. The reason why you do not feel the same warmth of that coffee or tea from the table is because the air is acting as an insulator.

An example of air as an insulator is a double-pane window. Double-pane windows are two sheets of glass with a stagnant space of air or gas between the two panes. This small space of air between the two layers of glass reduces the ability of heat transfer via convection. Convection is the transfer of heat by the movement of fluid matter, fluids being only liquids and gases.

Because this region of air has very minimal movement, it makes it very hard for heat to transfer through this medium.

Other examples of air as an insulator include insulated coffee cups. If you are drinking out of a paper cup, your hand will feel the warmth of the coffee, maybe even so much that you need a cup sleeve to comfortably hold it. But if your drink is in an insulated cup, you may barely feel any heat while holding it. Not only is it cooler to the touch, but it also keeps the coffee hot for a longer period of time. Two types of insulated cups are thermos style cups and polystyrene cups, often referred to as styrofoam. Thermos style cups work in the same way as double-pane windows do. They have an inner and outer layer that are separated by air to reduce convection. Polystyrene is a type of insulation that can be composed of more than 95% air. If you have used these types of cups before, you know that there is only a minor change in the outside temperature of the cup from the coffees heat.

There are various ways to measure how good of an insulator something is. A common scale uses a term called the k-factor. The k-factor is measured in BTU-in / hr – ft2 – °F. If you are unfamiliar with the different measuring scales, please visit our “Insulation Ratings: R Factor, K Factor & C Factor” page for more information. Below is a table showing the thermal conductivity, or k-factor, of air in comparison to other common types of insulation.

The thermal conductivity, or k-factor, of air in comparison to other common types of insulation

TemperatureAirFiberglassPyrogelCryogel
-200˚F0.095--0.096
-100˚F0.13--0.10
0˚F0.16--0.11
32˚F0.17-0.14-
75˚F0.180.21-0.12
100˚F0.19--0.12
200˚F0.22--0.13
212˚F0.22-0.16-
300˚F0.240.30--
392˚F0.27-0.19-
500˚F0.290.42--
572˚F0.31-0.24-
752˚F0.35-0.32-
932˚F0.39-0.44-
1112˚F0.42-0.62-

The insulation materials listed in the table above take advantage of air in different ways than perhaps a double-pane window does. Fiberglass, for example, has a low density and is composed of woven silicon embedded with tiny glass crystals. The fiberglass traps air within itself, limiting heat transfer.

Although air has great insulative properties, sometimes it isn’t practical to rely on it alone. In order for air to be properly utilized, there must be a boundary present. In other words, if air is confined, then it can work at its full potential. To put this concept in a better perspective, let’s look at a well-insulated house. The outer walls, doors, and windows act as a boundary when properly sealed. If it’s a cold day and the house is being heated, the walls, doors, and windows will keep the cold temperature out and the heat inside. But now let’s say there is a draft in a door or window. There will be a significant decrease in the houses ability to keep the heat inside and the cold out. You can even tell the difference in the houses temperature and it will cost significantly more to heat.

So let’s ask ourselves again, is air a good insulator of heat? The answer to this question is yes if it is utilized properly. Air must be within a boundary in order to reduce heat transfer to its best ability, such as its application in double-pane windows, insulated coffee mugs, and commonly used types of insulation.

Read more about the 5 most common thermal insulation materials

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