This article was originally published in the July/August 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online July/August 1997



Rock Wool Fills the Void

An installer screeds rock wool off a filled wall using a stud scrubber. Under normal conditions, a face mask is recommended (though not required) when spraying in rock wool.
Mention rock wool to most people in the energy audit and retrofit business and you get a visceral reaction. Immediately they start scratching and coughing, recalling dusty mats from old homes, heavy with 40 years of accumulated mouse droppings. Similarly, ask almost any builder about rock wool and you get a quizzical, bemused look that says, rock what?

Although relatively unknown in building circles, rock wool has actually been around for awhile. It is produced naturally during volcanic eruptions--formed by the action of high winds on lava streams--and was first patented as a commercial product in the United States in 1875.

Rock wool has had a variety of applications, including use as an industrial insulator and as a soil amendment. While it has been used as an attic insulation and retrofit wall insulation for some time, its history as a wall insulation in new residential construction is more recent; it was first applied to open wall cavities of some Texas homes approximately ten years ago. In North Carolina, it is currently being used in attic, cathedral ceiling, and side wall applications.

Rock wool is part of a generic category of materials, called mineral wool, that includes rock wool, slag wool, and fiberglass. The rock wool used for building insulation is often a mixture of basalt, slag (a byproduct from steel furnaces), and limestone. It is manufactured under high heat, and is spun from a molten substance using centrifugal force and compressed air.

Rock wool is applied just like wet-spray cellulose. The material, which contains a starch adhesive, is mixed with a small amount of water and blown into open wall cavities at a density of about 4 lb/ft3. The water activates the glue, which strengthens the bonding of the material to the sheathing and studs. To remove excess and overspray, contractors screed the walls using a motorized roller that runs down the face of the studs.

Rock wool is very effective for insulating behind and around electrical boxes, wires, and pipes. It can fill the most difficult wall cavities, leaving virtually no voids. Rated R-value in wall applications is R-4.1 per inch or about R-14.5 in a 3 1/2-inch wall cavity.

I first came upon rock wool while attempting to insulate my own home. Because of the unusual geometry of my walls, which included numerous full-width triangular braces in the framing, I knew that installing batts in the cavities would be difficult if not impossible. I thus wanted to use a wet spray and decided on rock wool after I found a contractor who was installing it.

At the time, such open-cavity applications of rock wool were still experimental in my area. In attempting to find the right mix, the contractor used too much water in the insulation, and condensation appeared beneath the wall's poly vapor retarder. Fortunately, the contractor carefully monitored the job, recognized the problem, and opened up the poly to let the insulation dry more thoroughly.

What can you do to avoid this situation? Be careful how much water you add to the mix. Clete Kiper, of SprayTech Insulation in North Carolina, has been using rock wool for the past three years. He recommends using just enough water to activate the dry binder. He mixes approximately 1 gallon of water to 55 lbs of rock wool. Kiper's mixture dries quickly, which allows him to tack up his poly several hours after application.

Where rock wool really comes from.
Six years after my installation, I conducted an infrared scan and found no sign of insulation settling in the walls. The 4-lb density may also be effective against infiltration, though independent research has not yet evaluated the system to determine its infiltration control characteristics.

Currently there are eight rock wool manufacturers in the United States and Canada. For more information, contact your local insulation distributor or the North American Insulation Manufacturers Association in Alexandria, Virginia. Tel:(703)684-0084.

--Arnie Katz
Arnie Katz is a senior building science consultant at the Alternative Energy Center's Applied Building Science Center in Raleigh, North Carolina.


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