Pop(ular) Rocks

Can rocks be among the most popular sustainable building materials?

sustainable materials, energy efficiency, building construction, walls, energy


Richard King


Marguerite Anglin


Anna Ishii

Buildings are definitely changing for the better. They are being created with more recycled material, increased energy efficiency, and safer components. You might ask, "Is there a single material that can do all that?" Of course, the answer is yes... rocks?

Okay, not really rocks, more like minerals—and not ones found wholly in nature either. Slag is a byproduct of smelting, a process used to remove impurities in ferrous and non-ferrous metals like steel. Slag often contains silicon, aluminum, and magnesium and has been re-used for other purposes for decades.

Slag is recycled for various uses in concrete and road construction, and ballast on railroads. Surprisingly (and most important to sustainability), it is also used in building insulation, where it is known as mineral wool. Given the worldwide trend toward greater environmental responsibility, it's no wonder that mineral wool insulation's popularity has grown. With 70-90% recycled content, it has 30% more thermal capacity per inch than common house (batt) insulation, creating greater energy savings in less space!

Mineral wool is also safer. In contrast to most rigid board insulations, mineral wool is not plastic (it contains no petroleum), so it's not flammable. As such, it meets new code requirements for improved safety of exterior wall construction. It is also highly water resistant, making it an excellent candidate for use in exterior walls.