What Is Fiberglass Insulation?

Fiberglass, also known as glass wool, is a glass fiber-reinforced plastic with a texture similar to wool. It’s one of the world’s most popular insulation products, with insulative uses for walls, attics, ducting, fireproofing, and soundproofing. 

Fiberglass Insulation

How Fiberglass Insulation Works

During the manufacturing process, fiberglass is filled with tiny air bubbles, giving it insulative properties. When heat flows through the insulation, the small air bubbles disrupt conduction, slowing or stopping heat loss. Tiny air cells also disrupt the natural convection of heat flow–further slowing heat loss.

Manufacturers use similar air-trapping technology in other insulation products like mineral wool and rigid foam board. 

Fiberglass Insulation and R-Value

R-value is the measure of thermal resistance to heat conduction. Thicker fiberglass wool forces heat to travel through more trapped air, providing a better R-value. For instance, 3 ½” thick fiberglass batts have an R-12 value and 5 ½” batts have an R-20 value.

For fiberglass to retain its R-value, you can’t compress it. For example, packing 5 ½” batts into 3 ½” wall studs decreases insulation performance since the compression squeezes air out of the batts. Also, when fiberglass gets wet, it provides almost zero R-value.

Blown-in fiberglass insulation in attics retains its insulation value because the weight of the product alone doesn’t squeeze out the air.

Types of Fiberglass Insulation

The versatility of fiberglass makes it a popular option for many insulation products. It is also fire-resistant (it does melt) and mold resistant.

Fiberglass Batt and Blanket Insulation

Since the process was patented in 1933, fiberglass batts and blankets have become one of the most used and recognizable types of insulation. They are available as unbacked or backed products, with backing options including kraft paper, foil, or other materials. When installed properly, the backing acts as a vapor barrier.

Installers can use fiberglass batt and blanket insulation on the undersides of ceilings and floors to keep heat in or out–as required. It also acts as a soundproofing product. Most overhead applications require wood, plastic, or metal strapping to keep the product in place until installers add drywall overtop.

Fiberglass Batts

The most common length for fiberglass batts is 48 inches, and common widths fit 16” on center framing and 24” on center framing. Average thicknesses are 3 ½” and 5 ½”. 

Fiberglass batts are easy to cut for use in smaller cavities, such as those around windows and doors and non-standard stud cavities. In order for fiberglass batts to work well, installers must fit them properly – even small misfits allow air to escape, and packing it too tight lessens its value.

Fiberglass Blankets

Fiberglass blankets are available in rolls–usually 50’ long. They come in multiple sizes and thicknesses and, once unrolled, re-aerate quickly. 

Installers can cut blankets to fit framing wall cavities. Cutting eliminates the join at the 4’ mark but makes for a more time-consuming installation process.

Fiberglass Blown-In Insulation

Shredded fiberglass is a popular loose-fill attic insulation. Like cellulose insulation, it fills cavities better than batts and blankets. When combined with an active binder, installers can spray shredded fiberglass on the undersides of structures like ceilings, where it will stick and dry out.

Installers can also spray shredded fiberglass into wall cavities and hold it in place with netting until the drywall is installed. At approximately R-2.5 per inch, blown fiberglass is less efficient than batts or blankets. To match a standard R-19 batt, blown-in fiberglass must be 7.5″ thick.

Fiberglass Duct Insulation

A popular product for HVAC ducting is insulated foil-backed fiberglass. The foil holds the fiberglass together and provides radiant insulation value. Fiberglass duct insulation is denser than batt, blanket, or shredded products. It contains heat and cold inside the metal and acts as soundproofing for noise traveling through the ducts.

Duct insulation is available as a wrap for round pipes and as a rigid fiberboard for rectangular ducting. Some manufacturers make rigid fiberglass ducts, eliminating the need to install metal ducting.

Fiberglass Insulation Longevity

Fiberglass insulation lasts up to 80 years if kept dry and free of insect or pest infestations. Infestations and moisture reduce its longevity.

Cost of Fiberglass Insulation

Fiberglass blanket insulation costs between $0.55 and $1.02 per square foot for material only–depending on R-value. You can DIY fiberglass blanket insulation or hire an installer. Loose-fill blown-in attic insulation costs approximately $1.00 – $2.80 per square foot supplied and installed–depending on R-value.

Adding insulation and sealing gaps in your home will save an average of $200.00 per year. The National Insulation Association (NIA) says that return on investment takes from 6 months to 2 years in badly insulated houses. 

Fiberglass Wool and Health

In 2011 the United States removed fiberglass insulation from its carcinogen warning list. The International Agency for Research on Cancer reclassified all thermal and acoustical fiberglass wools as not classified human carcinogen in 2001.

As positive as these developments are, breathing tiny glass particles into your lungs, where they may stick, is not healthy. Fiberglass can irritate the nose, throat, eyes, and skin, and insulative fiberglass in appliances may cause a disease similar to asbestosis.

Some years ago, manufacturers began using oil to bind the glass particles to the fiber, which reduces the number of fibers available for inhaling and cuts down on the itchiness. You should still take precautions. Wear a good N-95 mask or respirator, gloves, and a disposable hazmat suit when handling fiberglass.