Insulation: Mineral Wool Or Polyiso?

Choosing the ideal roofing insulation material for a facility project depends on several factors.

By John B. Letts
From The February 2017 Issue

When facility executives are faced with the important decision of selecting roofing insulation for a project, there are several critical factors that must be considered. Specifiers and contractors can certainly provide valuable insight, but it’s just as beneficial for facility executives to be knowledgeable about both types of insulation before making a decision. After all, insulation plays a crucial role in minimizing energy costs and making an impact on a building’s carbon footprint. Roofing insulation should not only meet building codes, but also offer the most value related to performance and durability. Each insulation option brings its own distinct benefits to a commercial roofing project.

roofing insulation
(Photo: Firestone Building Products)

Mineral Wool Insulation

Due to its components, mineral wool goes by many names, including mineral fiber, rock wool, slag wool, and stone wool insulation. Slag wool is more commonly used, and accounts for about 80% of the mineral wool industry. Mineral wool originated in Germany in 1871 and was one of the first insulation materials to be commercially produced. To manufacture mineral wool, molten glass, stone, or slag and other raw materials are heated and spun into very thin fibers, much like how cotton candy is spun. When the material is produced, binders are used to hold the fibers together and form it to meet specific product needs. Mineral wool is commonly available as blanket and loose-fill insulation.

Polyiso Insulation

Polyisocyanurate, more commonly referred to as polyiso insulation or ISO, is a closed-cell rigid foam board used in more than 70% of commercial roof construction. Polyiso was originally developed in the 1930s and was once used as insulation for beer barrels. Since then, it has evolved to become an environmentally friendly roofing and wall insulation solution. The boards, which are typically 4′ x 8′, are sandwiched between a top and bottom facer.

Comparing Properties Of Insulation

When selecting insulation, comparing and contrasting factors like fire protection, R-value, and compressive strength must be considered. Here’s how polyiso insulation matches up to mineral wool in key categories:

Fire Protection. Thanks to its flame retardant chemicals, mineral wool is highly fireproof, which makes it a popular choice for commercial buildings where fire performance is the most critical factor. Mineral wool is composed of noncombustible batts, or precut sections of insulation, made up of inorganic fibers that have an impressive melting point of more than 1,000°F.

Polyiso insulation also has outstanding fire performance—polyiso is a thermoset material and isn’t susceptible to melting. Building owners and facility managers need insulation that can stand up to the heat without deteriorating over time.

Thermal Performance and Weight. Mineral wool is more often found in walls than in roofs because of its weight, and has a relatively low R-value of 3.8. Mineral wool insulation is 4.5 times heavier than polyiso and requires twice as many boards to be installed, which can increase project costs. However, that density means that mineral wool boasts better sound control properties than polyiso.

Polyiso offers the highest R-value per inch of any rigid foam board insulation. This means it resists the flow of heat and keeps the interior of a building warm or cool, depending on the season, helping to keep energy costs low. Polyiso insulation typically has an R-value ranging from R-5.6 to R-8 per inch, which is 45% more than mineral wool insulation. As gas escapes, however, the R-value of polyiso can drop over time, but foil and plastic facings can help stabilize that number.

Compressive Strength. Here, compressive strength is directly related to durability and is defined as the ability of a rigid foam board to maintain its shape when force is applied. Mineral wool may be thick insulation, but its compressive strength is only 11 pounds per square inch (psi). This means that factors like mechanical fasteners, foot traffic, vibrations, and other external pressures can cause significant damage to the insulation. When mineral wool is stepped on, it does not fully recover back to its original state, which can result in reduced thermal performance.

Polyiso insulation has significantly higher compressive strength than mineral wool and can range from 16 psi to 25 psi. This ensures it will maintain its shape despite foot traffic and routine maintenance to the roof, making polyiso a durable choice.

Materials and Environmental Impact. Some sustainability experts have expressed concerns regarding the binder that ties the fibers of mineral wool together. Typically, the binder is a phenol formaldehyde or urea-extended phenol formaldehyde, an element that poses a potential health concern and may be harmful to air quality.

Although mineral wool is typically made from 70% recycled content, unlike polyiso, it cannot be recycled and re-used on reroofing applications. It requires approximately 85% more energy to produce than polyiso and has a global warming potential (GWP) that is 3.5 times higher.

roofing insulation
Mineral wool is commonly available as blanket and loose-fill insulation while polyiso is frequently used for roofing and wall insulation due to its small carbon footprint and high R-value. (Photo:

The facers used in polyiso insulation are made up of cellulosic material with 15% chopped fiberglass and recycled material. According to the Polyisocyanurate Insulation Manufacturers Association (PIMA), the energy savings potential of polyiso insulation over a typical 60-year building life span is equal to up to 47 times the initial energy required to produce, transport, and install it. These factors, along with polyiso’s zero ozone depletion potential; its opportunity for reuse; and its lower GWP compared to mineral wool, make polyiso an eco-friendly insulation option.

Polyiso is equipped with shorter fasteners as well as thin, light boards. It therefore requires fewer insulation pallets than mineral wool. This has less environmental impact and can significantly reduce costs associated with labor, handling, and crane fees.

While both mineral wool and polyiso insulation offer their own distinct properties, selecting which one to use can depend largely on the specific roofing project.

Letts is the technical director, insulations in the technology department of Firestone Building Products Co. He has more than 30 years of experience in urethane technology, from research and technology to technical service and plant support. Letts’ primary experience is in polyisocyanurate insulation board and its performance in roof and wall systems. He was the past chairman of the technical committee of PIMA (Polyisocyanurate Manufacturers Association) and received his doctorate degree in chemistry from Ohio State University.

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  1. Great article – really clarifies a lot of the different things you hear out there in the marketplace. The stat about polyiso saving 47 times more energy than is required to produce it is a good way to show how environmentally friendly it is. It would have been great to get the same number for mineral wool, but this article makes it clear that because of the high weight, lower R-value and very high energy needed to melt the rock/slag and spin it into fibers, the mineral wool number would be a lot lower

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