Insulation Types Compared: Fiberglass Batts, Spray Foam, and Rigid Board

Understanding R-Value Before You Choose

Every insulation comparison starts with R-value — a measure of thermal resistance. The higher the R-value, the more effectively the material slows heat transfer. The U.S. Department of Energy provides climate-zone guidelines recommending specific R-values for attics, walls, floors, and foundations depending on your region.

However, R-value alone doesn't tell the whole story. A perfectly rated batt installed with gaps around pipes or wires will underperform significantly. Air movement through and around insulation can reduce effective performance by a larger margin than the material rating suggests. That's why air sealing — covered in detail in our guide to caulk and weatherstripping — should always accompany insulation work.

Fiberglass Batts: Affordable and Widely Available

Fiberglass batts are the pink or yellow rolls found in nearly every home improvement store. They consist of spun glass fibers pressed into pre-sized widths designed to fit standard 16-inch or 24-inch stud spacing.

Where they work best

• Open attic floors and cathedral ceilings with adequate rafter depth
• Interior partition walls for sound control
• Exterior wall cavities in new construction or gut renovations

Limitations to know

Batts do not air-seal on their own. They must be paired with a vapor retarder in climate zones where moisture control is important. Compression reduces R-value, so avoid stuffing batts into undersized spaces. Gaps around electrical boxes and pipes are common installation errors that measurably reduce performance. Professional installation isn't required, but careful DIY work is essential.

Spray Foam: High Performance, Higher Cost

Spray polyurethane foam (SPF) comes in two forms: open-cell and closed-cell. Open-cell foam is softer, vapor-permeable, and less expensive, with an R-value around R-3.5 to R-3.7 per inch. Closed-cell foam is denser, moisture-resistant, and delivers roughly R-6 to R-7 per inch — the highest of all three types covered here.

Where spray foam excels

• Rim joists and band joists where air infiltration is heavy
• Crawl spaces and vented attics being converted to unvented (conditioned) spaces
• Irregular cavities, around pipes, and in hard-to-reach voids

Important considerations

Spray foam must be installed by a licensed professional using proper protective equipment — the chemicals involved are hazardous during application and require cure time before occupancy. Closed-cell foam can also act as a vapor barrier, which may cause moisture problems if misapplied in certain climate zones. Always consult a building performance professional before choosing foam for a sealed-cavity application.

Rigid Board: Versatile and Moisture-Resistant

Rigid insulation boards are manufactured from expanded polystyrene (EPS), extruded polystyrene (XPS), or polyisocyanurate (polyiso). They come in standard sheet sizes, typically 4×8 feet, and range from R-3.8 to R-6.5 per inch depending on the material.

Where rigid board performs well

• Basement walls and below-grade applications where moisture resistance matters
• Exterior continuous insulation over wall sheathing to reduce thermal bridging through studs
• Under slab foundations and in crawl space floors
• Garage doors and attic hatches where space is limited

Installation notes

Rigid board requires mechanical fastening or adhesive and seam taping for air control — it is not inherently airtight at edges. Foil-faced polyiso boards are not appropriate for below-grade use because the facing can trap moisture. Many rigid board products require a fire-rated covering such as drywall when installed in occupied spaces; check local building codes before leaving boards exposed.

Side-by-Side Comparison

The table below summarizes how the three insulation types compare across the criteria homeowners most commonly weigh.

Keep in mind that local labor costs, climate zone requirements, and the specific location in your home will influence which type — or which combination — makes the most sense for your project.