Thermal Properties of Insulation Materials

Fiberglass and Mineral Wool

• Fiberglass and mineral wool are typically available in the form of pre-cut batts or rolls and installed in unfinished walls, ceilings and floors between studs, rafters and joists. Fiberglass insulation is available in high- and medium-density varieties; high-density fiberglass has an R-15 value in a standard 2-by-4-stud wall, while medium-density fiberglass has an R-13 value in the same space. Mineral wool insulation has approximately the same R-value as high-density fiberglass.

Foam Boards

• Rigid foam insulation boards may be made of polyurethane, polystyrene or polyisocyanurate. Both polyurethane and polyisocyanurate boards are made from a closed-cell foam that contains a low-conductivity gas within its cells. This gas gradually escapes from the foam over time, reducing the thermal resistance of the material in a process called thermal drift. Foil facing on the foam boards help stabilize thermal drift, and foam-faced boards have a maximum stable R-8.7 value per inch. Polystyrene boards do not suffer from thermal drift and have a maximum R-5 value per inch.

Polyurethane Foam

• Polyurethane foam insulation is also available in form that can be sprayed in place in existing construction. These foams are sprayed into wall cavities where they expand and fill the cavity. Because they conform to the size and shape of the cavity, they produce a tight seal and are very effective insulators. Sprayed foam is subject to thermal drift, but high-density varieties achieve an R-6.5 value per inch even after thermal drift occurs. Low-density varieties have an R-3.6 value per inch.

Cellulose

• Cellulose insulation material is an environmentally friendly alternative to other manufactured materials. It is produced from recycled wood fibers, most commonly old newsprint paper. The paper is shredded into a fibrous pulp that can be piled into cavities between studs and rafters. Cellulose insulation has a maximum R-3.8 value per inch, similar to high-density fiberglass and mineral wool.