Rigid foam board and fiberglass insulation are two of the most common choices in home renovation, and each earns its place in a different part of the building envelope. Foam board resists moisture well, which makes it a strong pick for basements and exterior walls. Fiberglass is flexible and effective at holding heat, so it fits naturally between studs, joists, and rafters. Knowing how the two materials behave tells you which one belongs in each assembly.
The decision usually starts with the building assembly itself, and the differences between fiberglass, cellulose, spray foam, and rigid foam insulation for energy efficiency shape how the whole envelope performs. Start there, then narrow the choice to fiberglass and rigid foam for the specific cavity you are insulating.
Major Differences Between Fiberglass and Rigid Foam
The two products look and behave differently from the moment you buy them. Fiberglass arrives as flexible rolls or batts, with rolls running 24 to 40 feet long and batts sized at 3-1/2 inches and up to fit between framing members. Rigid foam comes as 4-by-8-foot sheets in thicknesses from 1/4 inch to 2 inches, cut from polystyrene or polyisocyanurate.
A survey of construction insulation materials from spray foam to mineral wool puts fiberglass and rigid foam in context before you commit to a single product line.
| Feature | Fiberglass Insulation | Rigid Foam Board |
|---|---|---|
| Form | Rolls or batts up to 40 ft long | 4-by-8 ft sheets, 1/4 to 2 in thick |
| Material | Melted glass spun into fibers | Polystyrene or polyisocyanurate |
| R-value per inch | About 3.2 to 4.3 | About 3.6 to 6.5 by type |
| Moisture resistance | Absorbs water, loses R-value when wet | Resists moisture well |
| Best locations | Stud, joist, and rafter cavities | Basements, sheathing, continuous layers |
| Fire behavior | Noncombustible | Combustible; needs a thermal barrier |
Both products ship in versions with and without facings. Kraft-faced fiberglass batts include a built-in vapor retarder for exterior walls, while unfaced batts suit attic floors where the material sits between and over the joists. Foam board faces range from plain plastic skins to foil layers that add a radiant barrier when installed against an air space.
How the Materials Are Made
Fiberglass starts as sand and recycled glass melted at high temperature, spun into fine fibers, and bound into blankets. Rigid foam is made by expanding plastic resin with a blowing agent into closed cells, then forming it into boards with a smooth facing or foil skin.
Where Each Material Performs Best
For exterior continuous insulation, contractors install foam boards over the sheathing and under the siding, a technique many builders use to save energy with rigid foam while cutting thermal bridging through the studs.
Basement Walls and Foundations
Foam board is the standard choice for basement walls because it can sit against concrete without wicking moisture into the living space. Fiberglass batts against a damp foundation wall soak up water, lose insulation value, and can trap mold behind the drywall.
Interior vs Exterior Placement
On the interior, foam boards are fastened to the concrete and covered with a thermal barrier such as 1/2-inch drywall. On the exterior of a foundation, boards run below grade with a protective coating or drainage layer over the top.
Stud, Joist, and Rafter Cavities
Fiberglass shines inside wall cavities, attic floors, and floor joist bays because it compresses around wiring, pipes, and outlets. Batts friction-fit between studs at standard spacings, and rolls cut to length fill odd bays without special tools.
R-Values and Thermal Performance
R-value measures resistance to heat flow, and the two materials reach different numbers per inch of thickness. Fiberglass batts deliver about R-3.2 to R-4.3 per inch, so a 3-1/2-inch batt in a 2×4 wall rates around R-13 to R-15, and a 5-1/2-inch batt in a 2×6 wall lands at R-19 to R-21. Rigid foam compresses more insulation into less space: EPS runs about R-3.6 to R-4.0 per inch, XPS about R-5.0, and polyisocyanurate about R-5.6 to R-6.5 at 75 degrees F, with some loss of performance in cold weather.
Climate matters because each foam type responds differently to temperature, and the EPS, XPS, and polyiso boards used for exterior sheathing and foundations each carry their own long-term R-value curve.
| Material | R-value per inch | Notes |
|---|---|---|
| Fiberglass batt | 3.2 to 4.3 | Standard fill for wall and attic cavities |
| EPS | 3.6 to 4.0 | Bead board, allows moisture vapor through |
| XPS | 5.0 | Smooth closed-cell surface, resists water |
| Polyisocyanurate | 5.6 to 6.5 | Foil-faced, highest per inch in warm conditions |
Thickness drives the final R-value more than the material choice in most retrofits. A wall cavity only accepts so much insulation, so the per-inch number decides how much thermal protection the assembly gets. Foam board’s advantage shows in thin assemblies: a 1-inch XPS sheet delivers roughly the same R-value as a 3-1/2-inch fiberglass batt in one third of the depth.
Why Air Sealing Changes the Math
Insulation slows conducted heat, but air leakage bypasses it entirely. A cavity filled with fiberglass still lets air move through the wall if gaps remain, while a continuous layer of foam board doubles as an air barrier when the joints are taped. That is why many high-performance walls pair batt insulation with a thin foam layer on the outside.
Cost, Installation, and Labor
Material prices vary by region, but fiberglass is usually the cheaper buy. Batts run roughly $0.50 to $1.50 per square foot, while foam board runs about $0.60 to $2.50 per square foot depending on type and thickness. Installed costs widen the gap because foam demands more cutting, fastening, and sealing time.
Floor insulation over a crawlspace is a common test case, and the fiberglass vs rigid foam decision there comes down to labor as much as material cost.
- Measure the cavity width and cut the batt about 1/2 inch wider than the opening with a utility knife or scissors.
- Press the batt into the cavity and let friction hold it in place.
- Split the batt around wiring and outlets, tucking the material behind the cables.
- Fill gaps around pipes and junction boxes with offcuts.
- Check for voids where the batt meets the framing and fill them before closing the wall.
What Foam Board Installation Adds
Foam board installation starts with measuring and cutting each sheet, either with a fine-tooth saw or a hot knife. Panels are fastened with cap nails, adhesive, or both, and the seams get taped or foamed to stop air movement. Cutting around pipes and outlets takes more time than slipping a batt into a cavity.
For most homeowners, the labor difference favors fiberglass in open cavities and favors a contractor for large foam jobs such as full exterior sheathing or foundation insulation.
Contractor quotes for the same wall can differ by 30 percent because cutting and fitting dominate the labor line. Get at least three bids, ask how the crew handles seams and penetrations, and confirm whether the price includes taping foam joints or air-sealing the top and bottom plates.
Durability, Moisture, and Indoor Air Quality
Durability depends on where the insulation sits. Fiberglass does not rot, but it soaks up water and loses most of its R-value while wet, so it must stay dry. Rigid foam shrugs off moisture in most situations, though XPS absorbs some water over years of ground contact and EPS can wick water if it sits in standing water.
Pests treat the two materials differently. Fiberglass provides no food value, but mice and other rodents will tunnel through it to reach warm spaces, and their nests compress the batt. Foam board is also chewable and can hide insect activity, so keep both materials sealed behind rodent-proof barriers in rural and wooded lots.
Moisture and Vapor Behavior
Foam board works in direct contact with concrete and below grade, which is why foundations and basements are its home turf. Fiberglass needs a vapor barrier or a dry cavity to perform, and it should never touch a wet surface.
Air Quality During and After Installation
Fiberglass releases dust and loose fibers while it is handled, so installers wear long sleeves, gloves, and a respirator. Once enclosed in a wall, it is inert. Rigid foam produces little dust, but some boards carry a solvent smell at first, and the material must be covered with a thermal barrier in occupied spaces because it is combustible.
Selecting the right foam board starts with the exposure, because each rigid foam type handles moisture, temperature, and load differently.
How to Choose Fiberglass or Rigid Foam
A decision checklist keeps the choice honest. Answer these questions before buying:
- Is the space below grade or in contact with concrete? Choose foam board.
- Is the cavity open and framed at standard spacing? Fiberglass batts go in fast and cheap.
- Do you need maximum R-value in a thin profile? Rigid foam wins per inch.
- Is the exterior wall getting a continuous insulation layer? Use foam board over sheathing.
- Does the project need a DIY-friendly product? Fiberglass is easier for first-timers.
- Is sound control a priority? Fiberglass in interior partitions absorbs noise better than foam.
Mixed assemblies cover most homes: batt insulation inside the framing and a foam layer on the exterior or in the basement. That combination controls moisture at the foundation, cuts thermal bridging at the studs, and keeps material costs in check.
Room-by-Room Recommendations
Basement: foam on the walls with a thermal barrier. Attic: fiberglass batts or blown fiberglass on the floor, with foam only at the eaves for air sealing. Exterior walls: batts in the cavity, foam outside. Crawlspace: foam on the walls or rigid board under the floor joists.
Environmental factors also enter the decision. Fiberglass contains recycled glass and can be recycled again, while foam boards depend on blowing agents, so the environmentally friendly rigid insulation choices today use low-global-warming alternatives and recyclable facings.