Polystyrene vs. fiberglass insulation

Polystyrene and the other rigid insulation products are primarily used in wet applications such as the inside/outside of basement walls and floors. It won’t draw out moisture, whereas fiberglass acts as a wick, and in turn will grow mold, begin to smell, and eventually pack down and loose its R-value, forcing replacement. For that reason, polystyrene panels often are shaped to accept firing strips, making basement applications somewhat straightforward. In fact, thicknesses seem to match those of metal studs, such as 1 ½” and 2”.

3 ½ inches of polystyrene has an R-value of 17.5, whereas 3 ½ inches of fiberglass has only R-13. Sounds good, but a 2’ by 8’ panel of 2” thick polystyrene, plus another 1 ½” panel (about 16 S.F.), will cost as much as a whole roll (40 S.F.) of R-13 fiberglass. Moreover, the rigid panels would be very labor intensive to use throughout the entire house, with electrical outlets and the like. For example, since the panels are 2’ wide and the studs, 16” on center, each panel would have to be cut down, creating substantial waste. Consider also, the fact that studs are not always 16” on center, nor are they always straight. Fiberglass batts are 15” wide, and are flexible to imperfections.

The extra cost would probably far outweigh any energy savings, unless you stay in the house for, say, 50 years. Even so, some applications can still benefit from the use of polystyrene besides basements. For example, vaulted ceilings and the like need to have a “breathing” area above the insulation to prevent ice damming and prolong roof life. This limits the amount of insulation in this restricted space. Polystyrene panels could be beneficial in this situation. For most of the house, though, you can get a very close R-value to polystyrene with a combination of fiberglass, reflective panels, and a vapor barrier, for considerably less money.

There are other types of insulation you may want to consider, including several types of foam insulation which are blown in as a liquid, then, when cured, become rigid. If you were thinking of that as polystyrene, well, it’s not. Polystyrene is sprayed into an extruded mold, and the manufacturing process is volatile. Besides, polystyrene is highly flammable and emits toxic gases when burned. One of the other insulation choices is called Icynene, and although typically used in remodeling, may have a spot in your new construction. Check around. Something should fit your plans.

Hope this helps. It would be interesting to hear other opinions and ideas on the subject.

I'm an owner/builder of a new house in Florida near the beach - very hot and humid. House is ICF construction with barrel tile roof. Was planning to use Icynene open cell insulation in attic but the installation of gas lines in the attic worry me. A sealed attic would be a hazard should a leak in the gas lines occur. Possible plan B: rigid poly with foil back used for radiant barrier, SPF cracks and crevices, install J vents and solar fans in roof, lay rigid poly or batts across bottom trusses for ceiling insulation, and cover the AC ductwork. Not crazy about fiberglass (fibers in air) or cellulose (absorbs moisture). Would like to have your advice and recommendations, as most others are not up on all the products and environment here, or are selling something, i.e. biased.