How to Know What Insulation is Behind Your Walls

What it is: Popular before World War II, mineral wool comes in two main forms: rock wool, which is made by spinning molten rock into masses of tiny intertwined fibers similar to cotton candy; and slag wool, which is made from the by-product, or slag, that forms on the surface of molten metal.

How it works: Formed into blankets, batts, or boards, mineral-wool fibers block both sound and temperature exchange through walls. It can also be found as loose-fill insulation.