Superior Insulation of Fiberglass

Composite and composite

Fiberglass entry doors are made of composite materials in two senses. The fiberglass itself is an advanced blend of silicate glass and plastic resins. This makes for a versatile substance that can be shaped into infinite varieties, but nevertheless remains robust and resistant to elemental damage. But the doors themselves are composite in another way.

In the center of the door is usually a light foam core. The foam core does two things. One, it makes the door much lighter because the interior is essentially hollowed out and devoid of heavy material. Two, the door achieves much greater insulating power with an airy, foam core.

A great insulator

This is because air is a great insulator. Loss of heat, or transmission of heat whether from the outside to the inside or the inside to the outside, can only occur via transmission of molecular vibrations. If the interior is an airy foam core, molecular vibrations become damped out and thus there is less heat loss or heat gain. In theory, insulation has always followed this principle. For example, a down blanket is filled with goose feathers, all of which hold much air and therefore reduce the emission of heat under the blanket to the exterior.

Careful laboratory tests by ThermaTru

In practice, do fiberglass entry doors have better energy efficiency properties? According to ThermaTru, a major fiberglass door company, the answer is yes. They reported testing several of their doors against a normal wood door, and found that the thermal transmission was about 3-5 times less. The engineering metric for measuring thermal transmission is known as the R-factor. For wood doors, the R-factor was 1.75, but for fiberglass doors the R-factor was much higher.

ThermaTru didn't do these tests casually. They took a large chamber that had a cold and hot side. The door was installed to separate the two sides, and measurements were carried out to analyze how much heat was being sent from one side to the other.

Uniformly better than wood

Were there differences in thermal transmissitivity between the different kinds of fiberglass entry doors? Yes. Objectively, some of the thicker ThermaTru doors were expected to be better at insulation. Tests confirmed these expectations. But these differences were much smaller when compared against the wood door.

Energy efficiency for your home

The take-home message is that fiberglass doors can help insulate your home more than a wood door would be able to. However, as a consumer, consider how much heat is actually lost through your door and whether your efforts would be spent on improving something else. For example, the improvement in heat-retention is probably much greater by switching from single to double-paned windows. Consult with a home heating expert who will be able to calculate your best ROI in terms of cost and energy efficiency for the home.