What makes a material a good insulator, and what makes it good thermal mass?
Given that the purpose of insulation is simple, to maintain a temperature difference, the principle criteria for selecting an insulation material is its resistance to the passage of heat. The technical term is “thermal conductivity.” You probably are familiar with R-values, which are a function of the thermal conductivity of a material and its thickness. If the only thing insulation needed to do was to retard the passage of heat, then you could shop solely on that basis and just look for the most heat resistance (or highest R-value) for the money. Unfortunately, it’s not that simple. For purposes of this page, however, it’s sufficient to understand that what makes good insulating material is low thermal conductivity and enough thickness to make a difference, usually expressed in terms of an R-value. Examples of material that’s good for insulation are fiberglass, rock wool, cellulose, and polystyrene or polyurethane foam. What makes material good thermal mass is a little more complicate
