Fiberglass Insulation

Fiberglass is a type of fiber primarily composed of glass that is used in a wide variety of applications, and is predominantly employed as a residential and commercial thermal insulator. Fiberglass is also used to create products as varied as automobile bodies, boat hulls, arrows, roofing, shower curtains, and tent poles.

As an insulator, it slows the spread of heat, cold, and sound in structures, cars and aircraft. By trapping pockets of air, it keeps rooms warm in the winter and cool in the summer and thereby serves as a convenient method to increase energy efficiency. Fiberglass is an attractive choice for home insulation because it poses no fire hazard.

According to some estimates, thermal insulation (made from fiberglass and its alternatives) conserves 12 times as much energy as is lost in its production, and it may reduce residential energy costs by up to 40%.

Glass has been woven into small amounts of coarse fibers for many centuries, even by the ancient Egyptians and Phoenicians, but fiberglass did not exist in its modern form until 1932 as a result of an accident.

A researcher named Dale Kleist was attempting to create a vacuum-tight seal between two glass blocks when a jet of high-pressure air turned a stream of molten glass into fine fibers. He had unintentionally discovered an effective method to produce large amounts of fiberglass particles, a method that he would refine in later years.

Fiberglass was trademarked in 1938 as Fiberglas® and was subsequently used in clothing, boat hulls, fishing rods, and eventually automobile bodies in 1953 when Fiberglas® partnered with Chevrolet.

In homes, fiberglass insulation can be installed in various parts of the building envelope. It can be pink, yellow, white or green, depending on its manufacturer, and has a spongy feel. Commonly found in blanket form, called batts, it is available in bags containing standard pre-cut lengths and widths.

Batts are typically stapled into place. It also comes in bags as loose fill that can be blown into attic, wall and floor cavities. Most fiberglass batts are manufactured with a paper or foil backing that faces the direction of warmth. When installed correctly, it creates a continuous membrane that retards the passage of moisture and reduces the likelihood that fibrous particles will enter the living space.

It is important that the backing always faces the warm side of the structure in which the insulation is installed. Batts are available in different thicknesses, with the thicker batts offering a higher resistance to heat flow. This resistance is known as R-value, with common R-values for walls being R11 to R19, and R30 to R38 for ceilings.


It is important for InterNACHI inspectors to understand the health risks associated with exposure to fiberglass insulation. These risks are not, at present, fully understood or agreed upon, but it is generally accepted that, in certain situations, it has the potential to cause physical harm.

Small particles that come into contact with skin can lodge in pores and cause itchiness, rashes and irritation. When inhaled, particles can cause coughing, nosebleeds, and other respiratory ailments. Very fine airborne particles are capable of becoming deeply lodged in the lungs and are believed by many to cause cancer and other serious afflictions.

OSHA considers this threat to be serious enough that it requires fiberglass insulation to carry a cancer warning label.

When it is disturbed, fiberglass insulation releases particulates into the air which may be inhaled by those installing or removing it, or by property inspectors crawling through attics or crawlspaces.

If you must disturb fiberglass insulation, wear gloves, long-sleeved shirts, pants and goggles. A respirator with a particulate filter should be used to prevent inhalation of the potentially dangerous fibers.

Before removing fiberglass insulation, it is a good idea to dampen the area to prevent particles from entering the airspace. Afterwards, wash your hands with water, preferably cold water, as warm water can expand pores which have trapped particles and allow them to travel deeper into your skin.

Insulation R Value Chart

Insulation R-Values for Location, Heat Type & Area*
LocationHeat TypeAtticWall FloorCrawl Space Wall**Basement Wall
Zone 1Natural Gas38-4913131311
Oil Furnace38-4913131311
Electric Furnace38-4913131311
Electric Baseboard38-4913131311
Heat Pump38-4913131311
LPG Furnace38-4913131311
Zone 2Natural Gas381313-191311
Oil Furnace381313-1913-2511
Electric Furnace38-491319-252511
Electric Baseboard38-491313-2513-2511
Heat Pump381313-191311
LPG Furnace38-491319-302511
Zone 3Natural Gas30-381313-1913-2511
Oil Furnace381313-191311
Electric Furnace381313-1913-2511
Electric Baseboard381313-191311
Heat Pump30-3813131311
LPG Furnace38-491313-3013-2511
Zone 4Natural Gas38-491325-302511
Oil Furnace4913302511
Electric Furnace38-491325-302525
Electric Baseboard4913302511
Heat Pump38-491313-2513-2511
LPG Furnace4913302511-25
Zone 5Natural Gas3813252511
Oil Furnace4913302511-15
Electric Furnace4913302525
Electric Baseboard4913302511
Heat Pump3813302511
LPG Furnace4913302525
Zone 6-8Natural Gas4913302525
Oil Furnace4913302525
Electric Furnace4913302525
Electric Baseboard4913302525
Heat Pump4913302525
LPG Furnace4913302525

*Ranges resulted from the slection of two different zip codes within the same zone (i.e. Dover, DE and Chattanouga,TN for zone 4)

R-value of Materials and Depths
MaterialR-value/in3 1/2″5 1/4″10″12″15″
Fiberglass (batt)3.1 – 3.410.8 – 11.916.3 – 17.831.0 – 34.037.2 – 40.846.5 – 51.0
Fiberglass blown (attic)2.2 – 4.37.7 – 15.011.5 – 22.622.0 – 43.026.4 – 51.633.0 – 64.5
Fiberglass blown (wall)3.7 – 4.312.9 – 15.019.4 – 22.637.0 – 43.044.4 – 51.655.5 – 64.5
Mineral Wool (batt)3.1 – 3.410.8 – 11.916.3 – 17.831.0 – 34.037.2 – 40.846.5 – 51.0
Mineral Wool blown (attic)3.1 – 4.010.8 – 14.016.3 – 21.031.0 – 40.037.2 – 48.046.5 – 60.0
Mineral Wool blown (wall)3.1 – 4.010.8 – 14.016.3 – 21.031.0 – 40.037.2 – 48.046.5 – 60.0
Cellulose blown (attic)3.2 – 3.711.2 – 12.916.8 – 15.032.0 – 37.038.4 – 44.448.0 – 55.5
Cellulose blown (wall)3.8 – 3.913.3 – 13.619.9 – 20.838.0 – 39.045.6 – 46.857.0 – 58.5
Polystrene Board3.8 – 5.013.3 – 17.519.9 – 26.238.0 – 50.045.6 – 60.057.0 – 75.0
Polyurethane Board5.5 – 6.519.2 – 22.728.9 – 34.155.0 – 65.066.0 – 78.082.5 – 97.5
Polyisocyanurate (foil-faced)5.6 – 8.018.2 – 28.029.4 – 42.056.0 – 80.067.2 – 96.084.0 – 120.0
Open Cell Spray Foam3.5 – 3.612.2 – 12.618.4 – 18.935.0 – 36.042.0 – 43.252.5 – 54.0
Closed Cell Spray Foam6.0 – 6.521.0 – 22.731.5 – 34.160.0 – 65.072.0 – 78.090.0 – 97.5

**Crawl space walls that are vented or have moist problems should not be insulated.