Choosing fiberglass or stone wool for fire and acoustics

Fiberglass batt insulation provides building occupants long-lasting comfort through increased thermal performance, reduced noise levels, and moisture absorption resistance. It should not settle or lose R-value over time. Today’s sustainable formulations also mean installers can expect less itch and dust, plus improved odor and cutability.

While fiberglass and stone wool insulations were designed originally to thermally insulate, both deliver the added benefit of keeping sound from bouncing back into a room or vibrating through. Their sound-reducing capabilities are inherent—specific chemicals may be added to select materials as a fire retardant, but nothing added to the insulation itself enhances its properties for sound absorption.

An R-11 batt offers little difference in sound properties when compared to an R-13 batt; what matters is consistently filling the cavity with the insulation material. To further enhance a wall system’s acoustic performance, additional elements like resilient channel or acoustic gypsum boards can be added into the design.

While all insulation buffers sound, there are specialty products on the market. For commercial use, specialty fiberglass board products are manufactured for sound in a variety of densities and thicknesses. However, according to testing performed at several independent laboratories, variation in density has little or no effect on overall STC ratings. Insulation thickness remains the determining factor. (Visit

Sound absorbent by nature, fiberglass and stone wool insulation significantly reduce sound transmission in wall, ceiling, floor, and HVAC assemblies. The first 25 mm (1 in.) of these materials in a building cavity can increase an assembly’s sound transmission class (STC) value by three or four points (in some constructions). Each additional 25 mm can further increase the STC rating by two points.

Fiberglass and stone wool insulation materials are also very good sound-absorbers because they have many interconnecting air pockets. Other effective sound-absorbers, called resonators, typically employ small perforations or slots allowing sound to enter, but not to escape easily. Wood slat panels and slotted concrete masonry units operate on this principle.

Fire safety as a factor
Common insulations perform in very different manners when it comes to fire safety. For example, inorganic batts or blankets of stone wool or fiberglass (properly installed and secured in place) are noncombustible, and accepted as fire blocks in wood-frame walls per the International Building Code (IBC).

Naturally noncombustible, fiberglass and stone wool maintain this inherent quality for the entire product life. As such, they require no additional fire-retardant chemical treatments. Many building codes also recognize fiberglass insulation as an acceptable firestop in wood- and steel-framed wall assemblies.

Both insulation types withstand very high temperatures (i.e. up to 1177 C [2150 F]), and are suitable for meeting/exceeding stringent fire/smoke rating requirements such as National Fire Protection Association (NFPA) 220, Standard on Types of Building Construction, and ASTM E136, Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 C.

Thermal performance ranges for insulation for 50 x 100-mm (2 x 4-in.) and 50 x 150-mm (2 x 6-in.) walls found in manufacturers’ data and submittal sheets. Calculations for open- and closed-cell sprayed polyurethane foam (SPF) made by taking the R-value per inch value that is contained in American Society of Heating, Refrigeration, and Air-conditioning Engineers’ (ASHRAE’s) 2005 Handbook of Fundamentals. Further, sprayfoam R-values assume ‘full-cavity’ installations, but this can be difficult to actually achieve. Data comes from Cellulose R-values can be overstated due to potential settling over time. This data comes from

However, it is important to note most vapor retarder facings used on fiberglass insulation are flammable. Faced insulation should be covered soon after installation with gypsum board or other code-approved finished material. Further, member manufacturers of the North American Insulation Manufacturers Association (NAIMA) place the following caution on kraft paper and foil retarders (batt and roll insulations):

This vapor retarder is flammable and should NOT be exposed.(This information comes from NAIMA Facts 30 (NO. BI472 8/97). Visit

However, these products, when properly installed in rigid compliance with a code-approved thermal barrier, pose no fire hazard. In applications such as warehouse ceilings or storage areas where the vapor retarder is left exposed, flame-spread-resistant (FS-25) faced fiberglass insulation 
is available.

A sustainable difference
Batts can be removed from an existing building and re-installed, making them among the few reusable forms of insulation (demonstrating lasting sustainable performance). Both stone wool and fiberglass can be sustainable in other ways, as well.

For example, fiberglass insulation offers among the highest renewable and recycled content in the industry, and delivers ongoing energy savings that lowers the carbon footprint. About 0.5 kg (1 lb) of fiberglass insulation saves 12 times as much energy in its first year in place as the energy used to produce it, continuing savings for the life of the building. (See NAIMA Facts 45 (NO. N012 10/15). Visit
2016/03/N012-Fiber-Glass-and-Rock-and-Slag-Wool-Insulation-Materials-for-a-Sustainable-Planet-2014-Numbers.pdf. See also Further, it is made from abundant silica sand and an average of 50 percent recycled post-consumer glass product. The material comes compression-packaged, allowing for more product on each truck, reducing transportation demands and impact on the environment.

Since 1992, the plants of NAIMA members—which include manufacturers of both fiberglass and stone wool—have been able to divert more than 21 billion kg (46.3 billion lb) of recycled materials from the wastestream. From that same year to 2008, the fiberglass insulation industry recycled more than 8 billion kg (18 billion lb) of pre- and postconsumer glass containers.

Many fiberglass manufacturers have plants that use up to 40 percent or more recycled materials in their wares. Insulation providers, fiberglass and stone wool alike, are currently exploring ways in which their use of such recycled materials can be increased without compromising the performance of their products.

Both fiberglass and stone wool can help block noise, and each has its own fire safety dynamics. Either insulation material can provide sensible solutions for savings and sustainability. In the end, building professionals will make their choice based on preference, price, profitability, and performance.

Ted Winslow is product manager, building science, systems, and technical marketing for CertainTeed Insulation. He serves the company as a technical resource on topics ranging from code reviews to sustainability programs and oversees development of CertainTeed insulation systems. Winslow holds a bachelor’s of science degree in mechanical engineering from Temple University. He can be reached at

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2 comments on “Choosing fiberglass or stone wool for fire and acoustics”

  1. While both fiberglass and stone wool are great choices for insulation, either one may become a better choice depending on the details of the project you’re working on. It’s nice to see that both are very sustainable materials, especially in an age where most people are doing their part to be more environmentally friendly. Thanks for sharing.

  2. We are building our first home and I don’t know what to use for installation. I know fiberglass the usual thing to use. Thanks for the information about the specs of it.

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