Batt Insulation: Fiberglass, Mineral Wool, and Cotton

Last week's column addressed cellulose insulation, which is the greenest and one of the most effective insulation materials available. But it isn't always practical or affordable to install cellulose. To insulate walls with cellulose, it's usually necessary to hire an insulation contractor, and if the job is very small--bumping out and reinsulating one wall of your home, for example--the cost may be prohibitive for that small improvement. This is where batt insulation makes sense.

Batt insulation is comprised of a fiber material, most commonly fiberglass, that is held together with a binder and cut to standard dimensions to fit between wall studs, rafters, or joists. It is sold in compressed rolls or bundles, and it expands to the rated thickness when opened up. Some batt insulation has facing on one side made of asphalt-impregnated kraft paper, foil-coated paper, or vinyl some is fully encased in plastic to prevent fiber-shedding; and some is sold unfaced.

By far the most common type of batt insulation--and the most common type of insulation period--is fiberglass. It is made by melting glass and spinning the molten liquid to produce thin fibers, much the way cotton candy is made, but at a much higher temperature. All fiberglass insulation produced in the U.S. today contains at least 30 percent recycled glass. The most common source of that recycled content is glass cullet from window glass manufacturing; the rest is recycled beverage containers.

A binder is added to the fiberglass to hold the fibers together. The most common binder is phenol-formaldehyde, which may result in low levels of formaldehyde emissions over time--though most of the formaldehyde is removed during a baking process. One fiberglass insulation manufacturer, Johns Manville, has switched entirely to an acrylic binder to eliminate formaldehyde offgassing.

There has been concern that glass fibers might be carcinogenic, like asbestos, but worry about this has decreased in recent years following new studies. If fiberglass insulation is properly installed (so that the insulated cavity is separated from the living area by an air barrier), I am not too worried about fiber shedding once installation is complete. But it is very important to wear gloves, goggles, and a dust mask when installing fiberglass. Also, handle the batts carefully to minimize dust generation.

Mineral wool is similar to fiberglass, but the raw material is rock (usually basalt) or slag (a waste material produced mostly from iron ore smelting). As with fiberglass, phenol-formaldehyde binder is typically used in the manufacture. Mineral wool batts are higher-density than fiberglass batts, block sound more effectively, and are more fire resistant--properties that are particularly important in commercial buildings, where most mineral wool insulation is installed.

A newer type of batt insulation is made from recycled cotton. Two manufacturers produce cotton batt insulation that is totally free of formaldehyde and can be installed without gloves or other precautions. Bonded Logic, makes the UltraTouch line of cotton insulation, and InsulCot makes a similar product carrying the InsulCot brand. A nontoxic, boron-based (UltraTouch) or phosphorus-based (InsulCot) flame retardant is added to keep the material from igniting if exposed to fire.

A wonderful thing about installing cotton insulation is that at the end of the day your eyes and arms aren't itchy. The material also has a very high recycled content--being made mostly from factory denim scrap. I have found, though, that cotton insulation is harder to cut than fiberglass, and it doesn't expand to its rated thickness as quickly. It is also a lot more expensive than fiberglass and harder to find. Locally, Renew Building Materials & Salvage on Putney Road in Brattleboro, carries Bonded Logic insulation.

Most batt insulation insulates to between R-3.3 and 3.7 per inch, depending on the material and density. Usually, the insulating performance and air tightness isn't as good as blown-in or damp-spray cellulose.

Properly installing batt insulation can be tricky. To get the best energy performance, the batts must entirely fill the wall or ceiling cavities. This means pushing the batts in at the edges and pulling the face out to avoid leaving air pockets. If wires or pipes extend through the cavity, split the batt so that insulation surrounds the wire or pipe and fully fills the cavity. If you simply compress the batt behind or in front of the penetration, your overall insulating value (R-value), will be significantly compromised.