Choosing fiberglass or stone wool for fire and acoustics

March 30, 2016

This blown-in-blanket fiberglass system was specifically designed for closed cavity applications. It fills all voids and gaps, significantly reducing unwanted noise. Photos courtesy CertainTeed
This blown-in-blanket fiberglass system was specifically designed for closed cavity applications. It fills all voids and gaps, significantly reducing unwanted noise.
Photos courtesy CertainTeed

by Ted Winslow
Both fiberglass and stone wool insulation have merit, promote fire protection and sustainability, and offer value to architects, contractors, and property owners alike. This article’s intent is to present a scientific examination of the benefits of using each, in particular with respect to meeting fire and acoustic requirements and codes.

Stone wool (sometimes called rock wool) insulation is not a new product, but its use is growing in the United States, thanks to improved distribution by several major companies. Its name derives from the manufacturing process in which the fiber materials are formed by spinning or drawing molten materials such as slag and ceramics or synthetic materials. Since there are various product subsets, its nomenclature involves putting the parent/raw material to wool (e.g. glass wool, rock wool, etc.).(For a more in-depth technical look at stone wool insulation and its variations, see the article, “The Sustainability of Rock and Slag Wool Insulation” by Angus E. Crane in the November 2008 issue of The Construction Specifier. To read it online, visit

Some consider stone wool easier than fiberglass to cut, handle, and fit into framing cavities. Further, many believe the material’s batts to be more rigid, firm, and dense than their fiberglass counterparts. However, this is out of necessity, not by design. When glass fibers are spun to make fiberglass batts, the fibers are long. As a result, they hang together easily to make a cohesive batt. On the other hand, stone wool fibers are much shorter. Therefore, in order to keep the batts in one piece, they have to be packed to a higher density level and are heavier.

Nevertheless, a good number of installers believe stone wool to be the optimal choice for virtually any project, seeing it as essential to creating a high-performance insulation system in the most progressive builds.

Introduced in 1938, fiberglass is manufactured most often in two different forms—pre-cut batts/rolls and blown-in loose-fill—and is used primarily for thermal and acoustical purposes in wall cavities, attics, and other critical open spaces in building assemblies. Batt products come in pre-made R-values, ranging from R-8 to R-49, whereas loose-fill fiberglass can be blown-in with specialized machines and equipment to meet almost any thermal requirement. Blown-in fiberglass insulation also can be installed over existing batt insulation in renovation projects to increase overall R-value.

Since it is inert and does not settle, properly installed fiberglass insulation maintains its R-value over a long period. Batts and rolls must always be installed at their full designed thickness to achieve the stated value, as compressed fiberglass insulation loses thermal capability. For example, R-20 fiberglass insulation with a thickness of 160 mm (6 1⁄4 in.) is reduced to R-19 once compressed to 140 mm (5 1⁄2 in.). If the material is allowed to regain its original thickness, the full R-value is restored. Installers should always ensure the insulation completely fills construction cavities without gaps or voids.

Fiberglass metal building insulation is a flexible blanket insulation furnished in unfaced rolls. Its primary use is in exterior walls and standing seam roofing systems of pre-engineered metal buildings. Intended to be laminated with an appropriate vapor retarder facing suited to the specific job application, it can be used as an unfaced layer of insulation.
Fiberglass metal building insulation is a flexible blanket insulation furnished in unfaced rolls. Its primary use is in exterior walls and standing seam roofing systems of pre-engineered metal buildings. Intended to be laminated with an appropriate vapor retarder facing suited to the specific job application, it can be used as an unfaced layer of insulation.

In addition, fiberglass is lightweight, flexible, and compression-packaged, speeding jobsite handling and installation, while minimizing warehouse requirements and transportation costs and demands. While batts can be cut to size, they are available in pre-cut sizes fitting standard wall cavities and wall heights, so productivity increases while cleanup is reduced.

Both stone wool and fiberglass share some advantages when it comes to installation. For example, downtime for the two materials is nearly zero in comparison with other insulations like cellulose (applied wet and requiring several days to set up prior to encasing behind drywall) or sprayed polyurethane foam (SPF), which demands 24-hour evacuation. Nothing more than a cutting tool, staple hammer, and minimal personal protective equipment (PPE) are needed to install batts, and no machines or power source are required. Similarly, no specific temperature is needed at time of installation to achieve maximum thermal performance (unlike some other insulation types).

Making a sound case
Before exploring how either material performs within an acoustical context, it is important to keep in mind insulation itself does not have a sound transmission class (STC) number—such ratings are indicated for an entire system, not its components. The use of any insulation can increase the partition assembly STC rating by four to 10 points, depending on the design and size (i.e. thickness) of the wall cavity.

Batts afford easy application of acoustical control to interior walls without altering build practices. Both stone wool and fiberglass soundproofing insulation are available in multiple panel packages, manufactured with good tolerances to fit snugly in between wall studs and ceiling joists. Full-wall assemblies with fiberglass or stone wool will perform comparably in curtailing noise. They fit into the cavities via friction press to ensure there is no air gap allowing sound to leak into adjoining rooms.

Insulation panels are 25 to 76 mm (2 to 3 in.) thick and are rigid enough to be easily cut to shape around electrical outlets and studs. A drywall saw can precisely cut out the notches to ensure each panel fits into place, while walls and ceilings can be finished off with standard or soundproof-grade drywall. No vapor barriers are required for use in interior partition walls.

In simplistic terms, insulation reduces sound’s transfer through the air. The material used in the partition’s structure determines how much sound vibrates through to the other side. Since metal studs have less surface area to transfer sound than their wood counterparts, they yield a higher STC in the wall system. Using resilient channels helps break some of the structural pathway sounds will travel.

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[2]).

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[3]. Cellulose R-values can be overstated due to potential settling over time. This data comes from[4].

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[5]).

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[6]. See also[7]). 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[8].


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