UPDATED on April 5, 2016
Most green builders who need a layer of horizontal insulation under a concrete slab specify expanded polystyrene (EPS), an affordable product that performs well in this application. If a builder specifies high-density EPS rated for below-grade use, the product is very durable.
That said, many green builders don’t like EPS. Some object to the fact that polystyrene is made from petroleum, while others worry about possible health problems associated with the brominated flame retardants that polystyrene manufacturers add to EPS.
These builders have always wished there was a good, affordable way to insulate concrete slabs without the use of foam.
What about mineral wool?
Mineral wool insulation is available in several different forms. Some builders have looked at the densest available mineral wool panels and wondered, “Are these insulation panels dense enough to install under a slab?” When asked that question, mineral wool manufacturers have usually answered “no” — at least until the last few months.
A couple of years ago, I asked Iain Stuart, a technical solutions coordinator at Roxul, about using Roxul under concrete slabs. Stuart told me flatly that Roxul insulation products were “not for horizontal use under slabs.”
After several years of dithering, however, the engineers at Roxul, a manufacturer of mineral wool insulation, recently gave builders the OK to use horizontal mineral wool insulation under concrete slabs.
The announcement was oral
The first sign that Roxul was ready to change its public position came last fall. In the November 2014 issue of Environmental Building News, former editor Alex Wilson shared the news that a Roxul representative at a Passive House conference in Portland, Maine, announced that the company would “stand behind” the use of Roxul mineral wool insulation installed horizontally under slabs.
Ever since Wilson shared that news, I’ve been waiting to see…
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