Attic insulation is the single highest-ROI energy retrofit available for most homes. The temperature differential between an attic and the conditioned space below is enormous (30 to 50 degrees Celsius difference in summer, 20 to 30 in winter), so heat flow across that boundary is the dominant load on heating and cooling systems. Adding insulation reduces that heat flow proportionally. Choosing the wrong insulation type means paying more than necessary or buying performance you cannot install correctly. This guide walks through the four main insulation categories, their costs, and when each one fits.
Understanding R-value
R-value measures thermal resistance. Higher R-value means more resistance to heat flow. R-value is additive: stacking R-19 batts to a total thickness of R-38 doubles the thermal resistance.
R-value per inch varies by material. Fiberglass batts: R-3.0 to R-3.5 per inch. Cellulose loose-fill: R-3.5 to R-3.7 per inch. Mineral wool: R-3.7 to R-4.0 per inch. Open-cell spray foam: R-3.5 to R-3.8 per inch. Closed-cell spray foam: R-6.0 to R-7.0 per inch.
The DOE recommends R-49 to R-60 for attics. R-49 in fiberglass is about 16 inches deep. In cellulose it is 14 inches. In closed-cell spray foam it is 7 to 8 inches. Material choice affects both cost and how much vertical space the insulation occupies, which matters if you want to store anything in the attic.
The other consideration is air sealing. Insulation slows conducted and radiated heat flow but does not stop air flow. A poorly sealed attic with high-R insulation performs much worse than a well-sealed attic with moderate insulation. Spend the first dollars on air sealing (caulking electrical penetrations, gasketing the attic hatch, sealing duct boots, plugging chases). Then add insulation on top.
Fiberglass batts
Fiberglass batts are pre-cut rectangles of fiberglass insulation, faced with kraft paper or foil or unfaced. They come in widths sized for standard joist spacing (14.5 inches for 16 inch on-center framing, 22.5 inches for 24 inch on-center framing).
Cost: cheapest material per R-value at 0.40 to 0.70 dollars per square foot for R-38. DIY-friendly because batts cut with a utility knife and lay between joists without specialized equipment.
The performance challenge is fit. Batts must fully fill the bay without compression and without gaps. Compressed insulation has lower R-value. Gaps around wiring, plumbing, and obstacles let air bypass the insulation. Professional fiberglass batt installation pays attention to these details. DIY installation often does not, and the as-installed R-value is 70 to 85 percent of the rated value.
Fiberglass is also irritating to skin and lungs. Wear long sleeves, gloves, and an N95 mask during installation. Modern fiberglass uses safer binders than older pink batts but still produces airborne fibers during cutting and placement.
Cellulose loose-fill
Cellulose is shredded recycled newspaper treated with boric acid for fire and pest resistance. It blows into the attic as loose-fill, filling around obstacles, conforming to joist spacing, and self-leveling.
Cost: 0.50 to 0.80 dollars per square foot for R-49, installed. DIY-feasible by renting a blower from a building supply store and buying bags of insulation.
Cellulose has the best installed performance of the loose-fill materials because it fills around obstacles, conforms to non-standard joist spacing, and does not require attention to gaps. Air movement through cellulose is also lower than through fiberglass, providing better real-world performance.
Settling is a real concern. Loose-fill cellulose loses 15 to 25 percent of its installed thickness within 2 to 3 years. Buy enough material to allow for settling. Installer crews should blow to about 20 percent more thickness than the target.
Moisture is the main weakness. Wet cellulose compacts heavily and loses most of its R-value. Cellulose does not support mold (the boric acid treatment is fungistatic) but it loses thermal performance when wet. Fix roof leaks promptly.
Mineral wool
Mineral wool (rock wool, slag wool) is spun from molten basalt or recycled industrial slag. It comes as batts or as loose-fill blown insulation. The fibers are denser than fiberglass, providing slightly higher R-value per inch and significantly better sound dampening.
Cost: 0.70 to 1.20 dollars per square foot for R-38 batts, higher than fiberglass.
Mineral wool advantages: fire resistance (it does not melt or burn below about 1000 degrees Celsius), water resistance (it sheds water and dries quickly, retaining most R-value after wetting), and sound dampening (rooms below mineral-wool-insulated ceilings are noticeably quieter).
Mineral wool disadvantages: cost, and installation difficulty. The dense fibers do not cut cleanly with a utility knife; you need a serrated wool knife. Batts are heavier than fiberglass, more tiring to install in tight attic conditions.
Mineral wool is the best choice for attics in fire-prone wildfire areas because the embers that ignite home fires often enter through attic vents and the insulation directly underneath is the first combustible surface. Mineral wool resists ignition.
Spray foam
Spray foam is a two-part polyurethane mixed at the spray gun and applied as liquid that expands to 30 to 100 times its initial volume. It adheres to the substrate and hardens into a rigid foam layer.
Two types: open-cell and closed-cell. Open-cell is lower density (0.5 pound per cubic foot), R-3.5 per inch, vapor-permeable, less expensive. Closed-cell is higher density (2 pound per cubic foot), R-6.5 per inch, vapor-impermeable, more expensive.
Cost: closed-cell spray foam at R-49 installed runs 4.00 to 6.00 dollars per square foot. Open-cell runs 1.50 to 3.00 dollars per square foot. Compared to 0.50 to 0.80 for cellulose, spray foam is 4 to 10 times more expensive per R-value.
Spray foam advantages: best air sealing of any insulation type (the foam fills every crack and adheres to surfaces). Highest R-value per inch (closed-cell) for cases where vertical space is limited. Adds structural rigidity to roof sheathing.
Spray foam disadvantages: cost. Off-gassing during cure (you cannot occupy the home for 24 to 48 hours after installation). Requires professional installation. Can hide rot or pest damage if applied over compromised wood. Removal for any subsequent roof repair is extremely difficult and expensive.
Spray foam is justified for unvented attic conversions (where the entire roof deck becomes the insulated boundary), for very limited vertical space (cathedral ceilings, knee walls), and for severe ice dam control. It is overkill for typical attic floor insulation where cellulose at one-quarter the cost performs comparably.
Radiant barriers
Radiant barriers are foil sheets installed on the underside of the roof rafters to reflect radiant heat from the roof back upward. They are not insulation in the R-value sense but reduce radiant heat gain in hot climates.
In hot sunny climates, a radiant barrier reduces summer attic temperatures by 10 to 20 degrees Celsius and reduces cooling costs by 5 to 10 percent. In cold climates, the benefit is minimal because the heat flow is conductive (not radiant) during winter.
Cost: 0.30 to 0.70 dollars per square foot installed. Adds to existing insulation rather than replacing it.
Radiant barriers in cold climates can actually cause condensation problems by trapping moist air against cold surfaces. Skip them outside hot climates.
How to choose
For most retrofits, blown cellulose is the best value. It performs well in real-world installations, costs less than half what spray foam costs, and DIY installation with rented equipment cuts the cost further. Target R-49 to R-60 depending on climate zone.
For new construction or major renovations where the roof deck is exposed, closed-cell spray foam on the rafters creates an unvented sealed attic, which is the highest-performance configuration and worth the cost premium for new builds.
For homes in wildfire zones, mineral wool batts or loose-fill mineral wool for the air-sealing-plus-fire-resistance combination.
For homes with limited budget and an unsealed attic, prioritize air sealing first (200 to 600 dollars in caulk, foam, and weatherstripping pays for itself within a season), then add cellulose to bring the R-value to spec.
For more on home efficiency improvements see our smart thermostat ROI guide and our methodology at /methodology.
Frequently asked questions
How much insulation do I need in my attic?+
The DOE recommends R-49 to R-60 for attics in most US climate zones, with cold climates needing R-60. R-49 is roughly 14 inches of loose-fill cellulose or 16 inches of fiberglass batts. Most older homes have R-19 to R-30, well below current recommendations. Adding insulation to bring an attic from R-19 to R-49 cuts heating and cooling costs by 10 to 25 percent in most climates.
Is spray foam worth the extra cost?+
For most attics, no. Spray foam costs 3 to 4 times more per R-value than loose-fill cellulose. The air sealing benefit is real but you can achieve similar air sealing with caulk and weather stripping at much lower cost, then add cellulose for thermal resistance. Spray foam is justified for unvented attic conversions, very cold climates with severe ice damming, or homes where the attic floor cannot be made airtight.
Will more insulation cause moisture problems?+
Only if you skip the air sealing step. Insulation slows heat flow but does not stop air flow. Warm humid air leaking from the home into a cold attic condenses on cold surfaces (rafters, sheathing) and causes mold and rot. Before adding insulation, seal the air leaks. After sealing, the higher insulation level is safe.
How long does insulation last?+
Fiberglass batts last 30 to 50 years if undisturbed. Cellulose loose-fill settles to about 80 percent of its original thickness within 2 to 3 years and stabilizes. Spray foam lasts 80 plus years. The exception is anything that gets wet. Wet fiberglass loses 40 percent of R-value and supports mold. Wet cellulose can compact and lose almost all insulation value. Fix roof leaks before they soak the insulation.
Can I install attic insulation myself?+
Loose-fill cellulose and fiberglass: yes, with a rented blower. Cost runs 0.50 to 0.80 per square foot in materials. Fiberglass batts: yes but with care to avoid gaps and compression that reduce performance. Mineral wool batts: yes, similar to fiberglass. Spray foam: no. Professional installation only, due to chemical handling and proper mixing requirements. DIY spray foam kits exist but produce inconsistent results.
