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Fiberglass Insulation: A Complete Guide

Most of us probably understand that insulation serves a pretty essential purpose in our homes. Though we may never touch that pink, fluffy, cotton candy-looking stuff in our attic, we know that somehow or another, it helps us keep our homes at a comfortable temperature all year round. A well-insulated home will not only protect you and your family against cold in winter and excess heat in summer but can also play an essential role in reducing noise pollution. In the best cases (such as passive houses), a well-insulated home with a sealed building envelope might need minimal additional heating and cooling.

Unfortunately, at least nine out of every ten homes are under-insulated, leading to higher energy bills and enormous corresponding greenhouse gas emissions. Improving the insulation in your home, then, is one of the best ways to increase energy efficiency, save money on your energy bills, and reduce your family's carbon footprint. Though there are dozens of different options for high-efficiency insulation (be sure to check out this three-part Rise guide on all-things insulation), fiberglass continues to be the most popular option.

According to a 2019 report, fiberglass batt insulation accounts for 52 percent of all insulation applications in new single-family homes. Blown fiberglass comprises another 19 percent of new home builds. Between the two, almost three out of every four homes continue to use fiberglass as their primary insulation material. Despite its apparent popularity, what are the pros and cons of fiberglass insulation? Below, we offer an in-depth guide to the different types of fiberglass insulation, where it comes from, and the advantages and drawbacks of this popular insulating material.

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By Tobias Roberts, Rise Writer

Apr 7, 2025 · 7 min read

What Is Fiberglass Insulation?

Fiberglass insulation, as its name implies, is made of extremely fine glass fibers. This insulation material comes in both batts, rolls, and as a loose-fill product. As we will see below, blown-in fiberglass is another alternative that might make sense in certain areas. Different materials offer insulation to our homes by creating small, porous spaces where air pockets can provide adequate heat flow resistance. Fiberglass insulation achieves this because the tiny, fine glass fibers create thousands of little air pockets that help conductive and convective heat flow.

Where Can Fiberglass Insulation Be Installed?

Fiberglass insulation is often placed in unfinished walls, floors, and ceilings. It is generally placed between studs, joists, and beams during the construction process. It can also be used as duct insulation and for insulating tank-style water heaters. Because of its nature, fiberglass insulation is not waterproof (though it does offer a certain degree of moisture resistance). So, you cannot use it in foundations or other areas where excessive exposure to moisture is a threat.

What Types Of Fiberglass Insulation Can You Buy?

Fiberglass insulation comes in batts or rolls, can be blown-in, and can also be manufactured into boards.

The most commonly seen type of fiberglass insulation is batts. These batts range in different thicknesses, generally from three to six inches. Fiberglass batts are a popular option with homeowners because they are a relatively easy DIY project. Similarly, you can also find fiberglass insulation in rolls. The main difference between fiberglass insulation batts and rolls is in how you install them. Batts are installed in varying pieces, while larger fiberglass rolls get turned out between walls and ceiling framing.

Blown-in fiberglass insulation is another standard product used for insulating homes. It is made of small, plastic filaments that are strengthened with glass that is spun into fibers. Blown-in fiberglass insulation is generally used in areas where you need to fill in uneven or hard-to-reach areas. You can use it around pipes, wires, and other objects inside the wall cavity. Because even a small insulation void can negatively affect your home's thermal performance, blown-in insulation helps tackle those potentially costly air gaps.

Less common, though still available, are fiberglass boards. Fiberglass board insulation is manufactured from inorganic fiberglass. It uses a thermosetting resin, which allows the fiberglass to be formed into flexible, semi-rigid, or rigid boards of varying densities. Fiberglass boards can be used in walls and ceilings. Because they can be installed directly over drywall, wood, concrete block, and many other solid surfaces, they are another easy, DIY insulation project for homeowners.

How Is Fiberglass Insulation Made?

The National Institute of Standards and Technology (NIST) explains that fiberglass insulation is made by a process wherein spun-glass fibers are formed into batts. Using heat, streams of vitreous melt are "either spun into fibers after falling onto rapidly rotating flywheels or drawn through tiny holes in rapidly rotating spinners. This process shapes the melt into fibers."

Blown-in fiberglass insulation incorporates plastic filaments and the small filaments of glass. In contrast, fiberglass boards or panels rely on plastic resins to give specific stability to the boards.

Which Type Of Fiberglass Insulation Best?

The best type of fiberglass insulation will depend on the thickness of the batt or roll. The US Department of Energy reports that a 3.5-inch thick fiberglass batt will offer an R-value of R-15 at $0.34 to $0.40 per square foot. In comparison, an 8-inch thick batt will have an R-value of R-25 at $0.37 to $0.45 per square foot. As we mentioned above, blown-in fiberglass insulation can help properly insulate hard-to-reach areas and seal air gaps. However, blown-in insulation can release tiny particulate matter into the air inside your home. You will most likely need to contract a professional for insulation.

What Are The Cons of Fiberglass Insulation?

Fiberglass insulation has three main drawbacks:

It can cause health concerns related to skin, eyes, and lungs during installation, and there are mold risks if it gets wet.
It loses efficiency when wet.
The manufacturing process is energy-intensive.

One of the reasons so many homeowners prefer fiberglass insulation is that it is a relatively easy DIY project. However, getting fiberglass in your skin can be painful and lead to long-term rashes. When installing, make sure to wear a mask and protective goggles, especially if you are cutting or sawing the batts, as the small fibers can cause damage to your eyes and lungs.

Another concern related to fiberglass insulation is that if it gets wet, it essentially loses all of its insulation capacity until it dries out. If the moisture problem is not addressed, mold growth can also occur in fiberglass insulation, thus compromising your home's indoor air quality.

The other drawback to fiberglass insulation is that it is extremely energy-intensive to manufacture. However, the Insulation Institute says that installing insulation in four million houses (4% of all North American housing stock) would reduce carbon in the atmosphere, comparable to planting 667 million acres of trees.

How Do You Remove Fiberglass Insulation From Skin?

If you get slivers of fiberglass in your skin during insulation, you can gently wash the affected area under running water and pat dry. Using a piece of tape on any visible glass fibers protruding from your skin is another way to remove fibers.

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What Is The R-Value Of Fiberglass Insulation?

Loose-fill fiberglass insulation insulates to R-values between 2.2 to 2.9 per inch.
Fiberglass batts and rolls have an R-value of 2.9 to 3.8 per inch.
Blown-in fiberglass insulation ranges between R-values of 2.2 and 2.7 per inch.

How Thick Does Fiberglass Insulation Need To Be?

The thickness of your fiberglass insulation will depend entirely on your local climate. The US Energy Star website offers a helpful map to determine your recommended insulation levels. For example, if the recommended insulation level is R-15 for your walls, you would need roughly five inches of fiberglass batts.

How Long Does Fiberglass Insulation Last?

When properly installed and protected from extended exposure to moisture, fiberglass insulation can last between 80 and 100 years. Fiberglass insulation can be ruined due to getting wet and becoming matted. Similarly, when fiberglass batts get broken by human interference, this can also shorten fiberglass insulation's lifespan.

How Much Does Fiberglass Insulation Cost?

Fiberglass insulation batts and rolls generally cost between $0.64 and $1.19, including installation. Blown-in insulation runs from $1 to $1.50 per square foot, with installation included. Compared to other types of insulation, fiberglass is generally one of the cheaper alternatives on the market. Because fiberglass batts can improve energy efficiency by 25 to 30 percent, the payback period for enhancing your home's insulation can be relatively short. This payback is especially quick for drastically under-insulated houses with lots of air gaps and leaks.

How To Install Fiberglass Insulation

Fiberglass batt and roll insulation are relatively easy to install on your own. You will need to remove the drywall on your wall or ceiling to fit the fiberglass batt into the stud bay, making sure it is tight up to the top plate. Press the batt into place and use a small utility knife to trim the lower end of the batt adjacent to the bottom plate. You want to ensure a tight friction fit between the studs. You will need to cut the batt when you come to electrical boxes, wiring, or other obstacles inside the wall or ceiling. It is a good idea to cover the fiberglass batt with a 3-mil polyethylene vapor barrier when finished.

How To Cut Fiberglass Insulation

When cutting fiberglass insulation, make a simple, straight cut with a sharp utility knife. Avoid sawing the batt or roll as this could cause the small glass fibers to become volatile and affect your indoor air quality.

How To Dispose Of Fiberglass Insulation

Fiberglass insulation can easily last a lifetime. If you do decide to replace it, however, it is possible to recycle fiberglass. Like cellulose insulation, fiberglass insulation often contains a percentage of recycled material. So, instead of sending it to a landfill, try to find your nearest recycling center.

Fiberglass insulation is one of the most popular insulation alternatives for homeowners in North America for many reasons. It is relatively inexpensive, durable, and easy to install on your own. As with all types of insulation, it can also radically improve your home's energy efficiency and thermal performance. However, it has health and environmental drawbacks too, so, as always, you will have to choose based on what is right for you and your family.

Rise Writer

Expert. Practical. Sustainable.

Article by

Tobias Roberts

Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.

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