How to Know What Insulation is Behind Your Walls

What it is: A lightweight, shiny mineral that resembles mica flakes.

How it works: Vermiculite absorbs water and is fire resistant, giving it myriad uses, from fluffing up garden soil to soundproofing floors. The mineral was popular during much of the 20th century as insulation, thanks in part to its easy application: It could simply be poured by the bagful between ceiling joists.