How fiberglass insulation works

Fiberglass insulation is designed to be fluffy and take up as much of a framing cavity as possible. Fiberglass insulation is also designed to be encapsulated, which means covered on all sides, like a box. For maximum efficiency of fiberglass insulation, the encapsulation should be sealed not allowing air movement. But here is the confusing part. The woven fibers that make up fiberglass is not an insulation material. The layers of woven fiberglass create air pockets that are designed to trap the air inside the framing cavities and prevent thermal changes from occurring. While most people accept this explanation, there are some major problems with this scientific explanation:

fiberglass insulation

Problem 1: There is no such thing as an air tight framing cavity. Air movement is imminent unless closed cell spray foam insulation is used to completely air seal the cavity. If this material is used, then the fiberglass should be affective, but not until ALL of the joints of the cavity are sealed from air movement.

Problem 2: Fiberglass is made of woven glass fibers. Glass is a fantastic conductor of energy, meaning that it absorbs the heat or cold from the exterior and interior very well. If the air pockets of the insulation are the insulator, then the temperature of the fiberglass material directly affects the temperature of the air pockets. Therefore, when it is cold, the insulation is less effective and when it is hot, the insulation is less effective. The efficiency of fiberglass is then not efficient.

Problem 3: The more significantly different the temperatures are when they meet, the more moisture will be produced. If fiberglass conducts the energy from both the inside and outside, there can be significant moisture build up within the wall cavities. This is the main reasoning for vapor barriers. Protecting the paper on the drywall from becoming infested with mold is a problem that is almost exclusively associated with fiberglass insulated homes because of the vast thermal changes that occur within the framed cavities of the house.

Fiberglass insulation is not a very good insulation material. It is, however, minimum code and will work with excessive vapor barrier and air sealing methods that will lock out the air movement and use the fiberglass as simply a minimal barrier from the elements. But is this really what we have in mind when we build a home?

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