Fiberglass vs. Rigid Foam Insulation: Which Is Best?

Installing Rigid Foam Insulation
Installing Rigid Foam Insulation. Getty / Westend61

Most home improvement stores carry two major classes of insulation: fiberglass and rigid foam. Both are designed for residential use. While there is some overlap, each tend to have their own best areas of use. For example, rigid foam is considered the "basement wall insulation" because it resists the moisture given off by basement masonry walls. Fiberglass batts are the "attic ceiling insulation" because they conform to the many obstructions found in attics.

Definitions

  • Rigid Foam:  These are boards of styrofoam, usually in large sizes that can fit an entire wall from top to bottom. This styrofoam is one of three materials: high quality, green-friendly polyisocyanurate; extruded polystyrene; or expanded polystyrene. Rigid foam performs well when subjected to moisture. It does not change dimensions, split, or crack. Walls are often injected with spray-foam insulation. While this foam does become rigid, it does not go under the rigid foam insulation classification.
  • Fiberglass: Fiberglass insulation consists of tightly packed long rolls or individual batts (single unrolled pieces) of spun fiberglass. In contrast to rigid foam, fiberglass is soft and flexible. Fiberglass' millions of air pockets provide a superior thermal barrier but will also trap moisture, leading to mold and mildew growth. Fiberglass should never be used when moisture is even remotely present.

    Where to Use Them

    Rigid Foam

    • Exterior Continuous Insulation: Rigid foam can be used as sheathing--that is, continuous insulation on wall exteriors prior to the installation of house wrap and siding. 
    • Basements: Inside, foam is required when insulating walls where the insulation will be touching masonry. Mostly this means exterior walls in basement build-outs, unless those are daylight-facing walls. The reason is because rigid foam stands up to moisture better than fiberglass.

    Fiberglass

    • Exterior Wall: Rolled fiberglass insulation's main duty is to fill the cavities of above-grade (non basement) exterior walls. These are the internally-accessed cavities of exterior walls; blown-in cellulose insulation would be used when the interior is not accessible.
    • Attics: Thick batts or rolls of fiberglass do an outstanding job of preventing heat loss through the ceiling and roof. Blown cellulose insulation is an alternative to fiberglass. Rigid foam would never be used in attics.

    R-Values

    Fiberglass insulation packs in more R-value per cubic inch than does rigid foam insulation.

    • Rigid Foam: Consider a common rigid foam, Owens Corning FOAMULAR 150. which is two inches thick. This board has a value of R-10.
    • Fiberglass: No fiberglass roll or batt goes as low as R-10. The next highest value is Owens Corning's RF-10 fiberglass roll insulation, with a value of R-13.

    ​These products are used in conjunction with each other; they are not competing products.

    Cost

    Fiberglass insulation is about half of the price of rigid foam insulation.

    Not only that, there will be less waste with fiberglass insulation. Rigid foam produces small pieces and thin sections that are virtually unusable. Fiberglass insulation can be torn off and stuffed into small areas, so very little goes to waste.

    Ease of Installation

    Both products have installation positives and negatives. Foam's main advantage is that it is a clean, irritant-free product and fiberglass' advantage is that it is flexible enough to mold around obstructions.

    • Rigid Foam: Foam can be cut with a fine saw or sliced and snapped (like drywall) with a utility knife. Foam does not require you to "suit up" and protect yourself against fibers as you would with fiberglass. On the downside, its rigidity means that it will not accommodate obstructions in the wall, such as wires, outlet and junction boxes, bracing, etc. The foam must work around such obstructions. Also, because it is difficult to get a tight seal with rigid foam, expandable foam or caulking is often used to fill thin gaps between the foam and studs.
    • Fiberglass:  Fiberglass insulation is an irritant for the skin, eyes, and lungs. Be sure to protect yourself prior to installation, using a respirator rather than a dust mask, eye covering, long gloves and sleeves, and pants. On the upside, fiberglass is flexible and can fit around vents, wires, and wall bracing.