A Better Kind of Batt Insulation

roxul insulation cutSometimes good things come out of bad situations. For example, I had been in the middle of a nasty home-improvement job when I ended up being introduced to a better way to insulate using a product that I still believe is the best in its class.

A small attic space needed a dozen new recessed light fixtures, and that meant I had to cut access holes into the ceiling, wiggle my body into impossibly tight spaces covered by existing fiberglass insulation, then run new electrical cables before buttoning everything up again. It was the itch and airborne dust of the fiberglass I dreaded most, and my fears were completely legitimate.

As the new fixtures went in, I needed more insulation to do a neat job, and that’s what prompted me to buy something other than fiberglass to fill disturbed areas. I’d heard from contractor friends that Roxul mineral wool batts was better, and that made me curious enough to try it. I now know exactly what they mean.

There are two reasons why I immediately liked using Roxul:

  1. personal comfort
  2. superior workability

Roxul batts are noticeably denser than other batts and less prone to releasing fibers into the air. In fact, you can take a piece of Roxul, rip it apart with your hands in strong sunlight and almost nothing can be seen floating around in the air. With very few loose fibers falling off, there’s also nothing to irritate your skin, either. The firm consistency of Roxul means it slices precisely and is easy to fit accurately into existing spaces without sagging — a perennial problem with other fiber-based insulation materials.

Roxul is made by heating basalt rock and slag until it’s liquid, then spinning the material into thin fibers as it cools and solidifies. The process reminds me of how they make cotton candy, though the basalt version has characteristics that the sweet pink stuff on a stick does not.

roxul insulationIn tests I completed my shop, Roxul proved to be surprisingly water repellent. You can pour water right on top and it simply beads up and rolls away. Sure, you can get Roxul to absorb water, but you have to work pretty hard at it. The whole issue of water absorbency matters when it comes to insulation because sometimes roofs leak. And when drips land on traditional, fiber-based insulation, it simply goes soggy and collapses under its own weight, losing most insulating properties in the process while spawning mold growth.

Greater batt density offers other advantages, too. It reduces the movement of air through the insulation, boosting energy performance in situations where a building envelope might not be completely sealed. Reduced air movement through fibers also reduces the tendency for condensation to build up within batts during cold winter weather.

As it turns out, the best tool for slicing Roxul is not a utility knife. A serrated bread knife works better. The extra density and firmness of the product is why. You can easily measure and cut accurately to within less than a quarter of an inch, making for precise installations that permit no air leakage around batt edges.

You’d think that all these advantages would have to come at a higher cost, but not necessarily so. Comparing prices of Roxul and fiberglass with equal R values, relative costs per square foot are slightly more or slightly less for each type, depending on market conditions. For most jobs, price differences are so small they don’t matter nearly as much as a more pleasant working experience, a better installation and superior energy performance.