Back in May of this year I wrote about a new rigid mineral wool insulation product from Roxul and how it can be used in place of foam-plastic insulation materials like polystyrene in certain applications. I’ve been revising the BuildingGreen Guide to Insulation Products and Practices and dug back into lots of insulation products. There are some new mineral wool developments to report.

Before getting into the details, here’s a little background: Mineral wool, variously referred to as rock wool, slag wool, and stone wool, was one of the first insulation materials to be widely produced commercially — starting back in 1871 in Germany.

Rockwool International, the world’s largest producer of mineral wool and the parent company of Canadian manufacturer Roxul, began production of the material in 1937. The U.S. company Thermafiber, one of the largest U.S. producers of the material and a company poised for rapid growth today, was founded in 1934.

Mineral wool is made by melting the raw material, which can be stone (such as basalt) and/or iron ore slag, at very high temperature, spinning it like cotton candy to produce very thin fibers, coating those fibers with a binder to hold them together, and forming it into the insulation batt or boardstock material to meet specific product needs.

Mineral wool lost most of its market share when less-expensive fiberglass insulation came along, but unique properties of the material have been fueling a comeback in recent years — and this year the world’s largest fiberglass insulation company, Owens Corning, purchased Thermafiber. With this development, I’m expecting to see a lot of attention paid to mineral wool in the coming years — led by a new product introduction last week.

Why I like mineral wool

Mineral wool is highly fire resistant, which has long made it an insulation material of choice in many commercial buildings. It achieves its fire resistance without the use of any flame retardant chemicals, which are widely used in most foam-plastic insulation materials — and which I believe to be a huge downside of those products.

Mineral wool is a heavier and denser insulation material that fiberglass, giving it better sound-control properties and more effectively restricting air movement through it. When produced in boardstock form, mineral wool can be rigid enough to work as insulative sheathing, like extruded polystyrene and polyisocyanurate.

Mineral wool can also contain very high recycled content by using iron ore slag (a waste product from steel manufacturing). Some mineral wool products on the market have over 90% recycled content — higher even than cellulose insulation, though it is made from post-industrial rather than post-consumer recycled material.

The downside to mineral wool

There are three downsides to mineral wool. One is that mineral fibers can break off and become airborne; when we breathe those fibers in they can cause health problems. In the past there was some concern that mineral wool and fiberglass fibers might be carcinogenic, like asbestos. While those concerns have largely been dismissed, the fibers are still respiratory irritants. Installers of mineral wool should always wear quality dust masks, and the material should be adequately covered with drywall or coatings that prevent fibers from entering the indoor air in a building.

The second downside is the binder used to glue the fibers together. Manufacturers use a phenol formaldehyde or a urea-extended phenol formaldehyde binder. Formaldehyde is a known human carcinogen, and if a lot of it escapes into the indoor air, that would clearly be a health concern. Fortunately, the processing drives off nearly all of the free formaldehyde in the material, so formaldehyde emissions from mineral wool have extremely low formaldehyde levels — in some cases as low as background formaldehyde levels.

Nonetheless, there is a perception problem with formaldehyde binders — if not a real problem — and manufacturers are working on alternatives — as has occurred with fiberglass insulation. I fully expect that within a few years one of the mineral wool manufacturers will announce a biobased binder that works with mineral wool and the industry will fairly quickly convert to such a binder.

The third downside to mineral wool is that it can be hard to work with. Mineral wool boardstock is more compressible than rigid foam-plastic insulation, so installing strapping over it may take special care. In the batt form, the insulation doesn’t compress as easily as fiberglass to squeeze into odd corners and around wires. That can make mineral wool harder to work with — but it should also prevent some of the worst installation problems that occur with fiberglass. (The effectiveness of all types of batt insulation depends to a very significant extent on the care taken during installation.)

Thermafiber’s new mineral wool batt insulation

The latest news with mineral wool is the introduction by Thermafiber (now an Owens Corning company) of UltraBatt, a flexible batt insulation product for 2×4 or 2×6 walls. This follows Roxul’s introduction of a widely distributed mineral wool batt insulation product, ComfortBatt, several years ago.

UltraBatt is a fairly dense batt (not compressible like fiberglass batts) that offers very good sound control as well as relatively high insulating values. The 3 1/2-inch batts for 2×4 walls provide R-15, and the 5 1/2-inch batts for 2×6 walls provide R-23 — though, as with all cavity-fill insulation, the actual “whole-wall” R-value will be lower, due to thermal bridging through the studs.

UltraBatt is comprised of 70% post-industrial recycled content. As for pricing, the national distributor Menards showed the online price to be about $31 per 40 square feet in the 3 1/2-inch batts, or about $0.77 per square foot. This compares with unfaced CertainTeed fiberglass batts at about $23 for 88 square feet, or $0.26 per square foot. The installed cost of dense-packed cellulose, meanwhile, is typically $1 to $2 per square foot for a 2×4 wall, though the pricing of any contractor-installed insulation is very dependent on the project.

I have not seen test data on formaldehyde (or other) emissions from UltraBatt, but I was told by Owens Corning that testing is underway and findings will be reported in 2014. I suspect that, like Roxul’s ComfortBatt, the formaldehyde emissions will be very low.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.