Fiberglass, Cellulose & Wool Insulation: What's the Difference?

Building Insulation is any object in a building used as insulation for any purpose. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation (e.g. for vibrations caused by industrial applications). Often an insulation material will be chosen for its ability to perform several of these functions at the same time.

Insulation is an important economic and environmental investment for buildings. By installing insulation, buildings use less energy for heating and cooling, and occupants experience less thermal variability. Retrofitting buildings with further insulation is an important climate-change mitigation tactic, especially in locations where energy production is carbon intensive. Local and national governments and utilities often have a mix of incentives and regulations to encourage insulation efforts on new and renovated buildings as part of efficiency programs in order to reduce grid energy

Fiberglass Insulation

Fiberglass is a common type of fiber-reinforced plastic using glass fibers. The fibers may be randomly arranged, flattened into a sheet (chopped strand mat), or woven into glass cloth. The plastic matrix may be a thermoset polymer matrix—most often based on thermosetting polymers such as epoxy, polyester resin, or vinyl ester resin—or a thermoplastic.
Unlike glass fibers used for insulation, for the final structure to be strong, the fiber’s surfaces must be almost entirely free of defects, as this permits the fibers to reach giga-pascal tensile strengths.

The process of manufacturing fiberglass is called pultrusion. The manufacturing process for glass fibers suitable for reinforcement uses large furnaces to gradually melt the silica sand, limestone, kaolin clay, fluorspar, colemanite, dolomite and other minerals until a liquid forms. It is then extruded through bushings. Chopped strand mat or CSM is a form of reinforcement used in fiberglass. It consists of glass fibers laid randomly across each other and held together by a binder.

Health Hazards

NTP considers fibrous glass dust to be “reasonably anticipated [as] a human carcinogen [Certain Glass Wool Fibers (Inhalable)]”. Similarly, California’s Office of Environmental Health Hazard Assessment (“OEHHA”) published a November, 2011 modification to its Proposition 65 listing to include only “Glass wool fibers (inhalable and bio-persistent).”
Fiberglass will irritate the eyes, skin, and the respiratory system. Potential symptoms include irritation of eyes, skin, nose, throat, dyspnea (breathing difficulty); sore throat, hoarseness and cough. Scientific evidence demonstrates that fiberglass is safe to manufacture, install and use when recommended work practices are followed to reduce temporary mechanical irritation. Unfortunately these work practices are not always followed, and fiberglass is often left exposed in basements that later become occupied.

Dry or Blown-In Cellulose Insulation (loose fill)

Dry cellulose is used in retrofitting old homes by blowing the cellulose into holes drilled into the tops of the walls. It can also be blown into a new wall construction by using temporary retainers or netting that is clamped in place then removed once the cellulose has reached the appropriate density. This form of application does settle as much as 30%-40%. Look to the coverage chart on any product application instruction. R-value based on settled thickness. Use of given instruction charts are for estimating purposes only. Job conditions, application techniques and settings on equipment will influence actual coverage. Do not add water to this product. Due to variance in application moisture levels and temperatures cellulose will settle. Settling could leave gaps in a wall as the insulation compacts vertically, or it could leave space between a floor and the insulation beneath it. In addition, a dense-pack option can be used to reduce settling and further minimize air gaps. Dense-pack can only be done in a closed cavity by placing pressure on the cavity, and should be done only by an experienced installer.

Loose fill in walls is an antiquated technique of using cellulose in wall cavities. The home performance industry and its accrediting bodies support the dense-pack standard of insulating wall cavities, which does not settle. This method stops the stack effect and convective loops in wall cavities.
Spray-Applied Cellulose (wet-spray cellulose)

Spray-applied cellulose is used for applying cellulose to new wall construction. The differences are the addition of water to the cellulose while spraying as well as adding some kind of moisture retardant such as chlorine to prevent mold cultures. In some cases the insulation might also mix in a very small percentage of adhesive or activate a dry adhesive present in the cellulose. Wet-spray allows application without the need for a temporary retainer. In addition, wet-spray allows for an even better seal of the insulated cavity against air infiltration and eliminates settling problems. Wet-spray installation requires that the wall be allowed to dry for a minimum of 24 hours (or until maximum of 25% moisture is reached) before being covered.

Stabilized Cellulose

Stabilized cellulose is used most often in attic/roof insulation. It is applied with a very small amount of water to activate an adhesive of some kind. This reduces settling and decreases the amount of cellulose needed. This can prove advantageous at reducing the overall weight of the product on the ceiling drywall helping prevent possible sag. This application is ideal for sloped roofs and has been approved for 5:12 (41.66%) slopes.

Low-Dust Cellulose

The last major type of cellulose insulation on the market is low-dust variety. Nuisance levels of dust are created during application of most types of dry insulation causing the need for simple dust masks to be worn during installation. This kind of cellulose has a small percentage of oil or similar dust dampener added. This may also be appropriate to homes where people are sensitive to newsprint or paper dust (though new dust will not be created after installation).

Wool Insulation

Wool insulation commonly comes in rolls of batts or ropes with varied widths and thicknesses depending on the manufacturer. Generally, wool batts have thicknesses of 50 mm (2 in) to 100mm (4 in), with widths of 400 mm (16 in) and 600 mm (24 in), and lengths of 4000 mm (13 ft 4 in), 5000 mm (16 ft 8 in), 6000 mm (20 ft) and 7200 mm (24 ft).
Wool insulation costs significantly more than conventional fiberglass insulation, but does not require the use of protective gloves, and may have significantly lower health risks to both the building occupants and the installation crew. It can be used in the roof, walls and floors of any building type as long as there are spaces to put the insulation in.
Some wool used to manufacture insulation is the wool discarded as waste by other industries due to its color or grade. Sheep wool insulation is often treated with borax to enhance its fire retardant and pest repellent qualities. The level of Borax is relatively low only 4% dry weight although the scouring baths have a higher load of 8–9%.
There are some companies that use DE (Diatomaceous Earth) as pesticide. DE is believed to be harmless and is used in feed and animals to stave off parasites. DE has the drawback of having to be applied in two steps to be effective and the wool having to be installed loose.

Rock Wool

Mineral wool is any fibrous material formed by spinning or drawing molten mineral or rock materials such as slag and ceramics.
Applications of mineral wool include thermal insulation (as both structural insulation and pipe insulation, though it is not as fire-resistant as high-temperature insulation wool), filtration, soundproofing, and hydroponic growth medium.

High-temperature mineral wool is a type of mineral wool created for use as high-temperature insulation and generally defined as being resistant to temperatures above 1,000 °C. This type of insulation is usually used in industrial furnaces and foundries. There are several types of high-temperature mineral wool made from different types of minerals. The mineral chosen results in different material properties and classification temperatures.
Alkaline Earth Silicate Wool (AES wool)

AES wool consists of amorphous glass fibers that are produced by melting a combination of calcium oxide (CaO−), magnesium oxide (MgO−), and silicon dioxide (SiO2). Products made from AES wool are generally used in equipment that continuously operates and in domestic appliances. AES wool has the advantage of being bio-soluble—it dissolves in bodily fluids within a few weeks and is quickly cleared from the lungs.

Alumino-Silicate Wool (ASW)

Alumino-Silicate wool, also known as refractory ceramic fiber (RCF), consists of amorphous fibers produced by melting a combination of aluminum oxide (Al2O3) and silicon dioxide (SiO2), usually in a weight ratio 50:50. Products made of alumino-silicate wool are generally used at application temperatures of greater than 900° C for equipment that operates intermittently and in critical application conditions.

Polycrystalline Wool (PCW)

Polycrystalline wool consists of fibers that contain aluminum oxide (Al2O3) at greater than 70 percent of the total materials and is produced by sol–gel method from aqueous spinning solutions. The water-soluble green fibers obtained as a precursor are crystallized by means of heat treatment. Polycrystalline wool is generally used at application temperatures greater than 1300° C and in critical chemical and physical application conditions.


Kaowool is a type of high-temperature mineral wool made from the mineral kaolin. It was one of the first types of high-temperature mineral wool invented and has been used into the 21st century. It can withstand temperatures close to 1,650° C (3,000° F).

Scroll to Top