Fiberglass attic insulation comes in two forms: pre-cut batts you lay between joists, and loose-fill you blow in with a machine. Fiberglass is the cheapest attic insulation on the market, running $0.30 to $2.00 per square foot installed, but it carries the lowest R-value per inch of any common material, so hitting the R-49 to R-60 most U.S. attics need means more depth than cellulose or foam. For a flat, open attic floor, blown fiberglass usually wins on cost and coverage. For a small or accessible attic you insulate yourself, batts win.
This guide compares fiberglass batts against blown fiberglass head to head, gives the real R-value per inch for each, maps depth to the DOE zone target, and shows where fiberglass beats cellulose and where it loses.
Fiberglass batts vs blown fiberglass: which belongs in your attic?
Blown fiberglass suits most attic floors because it flows around wiring, framing, and odd bays to leave no gaps, and it settles only 1 to 3 percent over its life. Fiberglass batts suit small, accessible, evenly framed attics where a homeowner can lay them without a blower rental. The two products use the same glass fibers but perform differently once installed, so the attic layout decides the winner more than the material name.
Batts are rectangles of fiberglass sized to standard 16-inch or 24-inch joist spacing. You cut and fit them by hand. Blown fiberglass is chopped loose fill fed through a hose from a hopper, building an even blanket across the whole attic floor. The table below is the practical split.
| Factor | Fiberglass batts | Blown fiberglass |
|---|---|---|
| R-value per inch | R-3.0 to R-4.3 (density dependent) | R-2.2 to R-2.7 (attic loose fill) |
| Installed cost | $0.30 to $1.50 per sq ft | $0.50 to $2.00 per sq ft |
| DIY friendly | Yes, no machine needed | Harder, needs blower rental |
| Gap risk | High around wiring and odd bays | Low, fills around obstructions |
| Settling over time | Can sag or compress | 1 to 3 percent, holds depth |
| Best attic use | Small, open, evenly framed | Large or obstructed attic floors |
The single biggest reason blown fiberglass dominates finished attic-floor jobs is gap control. A batt cut short around a truss or notched around a junction box leaves a thermal hole, and thermal holes drive most of the heat loss. Blown fill eliminates that failure mode.
What is the R-value of fiberglass attic insulation?
Fiberglass batts deliver R-3.0 to R-4.3 per inch depending on density, while blown fiberglass in an attic delivers R-2.2 to R-2.7 per inch as low-density loose fill. That per-inch number is the lowest of any mainstream attic material, which is why fiberglass needs more depth than cellulose or spray foam to reach the same total R-value. Standard-density batts sit near R-3.0 to R-3.7 per inch; high-density batts reach R-3.82 to R-4.29.
Total R-value is what your energy bill responds to, and it is per-inch value multiplied by installed depth. A blown fiberglass attic at R-2.5 per inch needs roughly 18 to 22 inches of depth to reach R-44 to R-49. That is why installers mark a depth ruler on the truss and blow to a target inch count, not to a guessed bag total. The insulation R-value chart lists per-inch figures for every material side by side.
How much fiberglass insulation does your attic need by climate zone?
The Department of Energy recommends R-30 in the warmest zones and up to R-60 in the coldest. Most of the continental United States falls in the R-49 to R-60 band. Because fiberglass has a low per-inch R-value, meeting these targets means real depth: roughly 20 to 24 inches of blown fiberglass for R-49 to R-60. Meeting the recommended R-value cuts heating and cooling costs by 10 to 20 percent versus an under-insulated attic, per the DOE.
| DOE climate zone | Recommended attic R-value | Approx. blown fiberglass depth |
|---|---|---|
| Zone 1 (S. Florida, Hawaii) | R-30 | 12 to 14 inches |
| Zones 2 to 3 (Gulf, Southern) | R-38 | 15 to 17 inches |
| Zones 4 to 5 (Mid-U.S.) | R-49 | 19 to 22 inches |
| Zones 6 to 7 (Northern) | R-49 to R-60 | 20 to 24 inches |
| Zone 8 (Alaska) | R-60 | 24 inches or more |
Find your zone by ZIP code on the DOE climate zone map before you buy. The same total R-value protects against summer heat gain and winter heat loss, so the target does not change between a cooling-dominated and a heating-dominated climate within the same zone. For a broader walkthrough of every material by zone, see the attic insulation guide.
How much does fiberglass attic insulation cost?
Fiberglass attic insulation runs $0.30 to $1.50 per square foot for batts and $0.50 to $2.00 per square foot for blown fiberglass, installed. On a typical 1,000 square foot attic floor, that is roughly $500 to $2,000 depending on form, target R-value, and whether a crew or you do the work. Fiberglass is the cheapest attic insulation by material, which is its main selling point.
- DIY batts, material only: $0.15 to $0.50 per sq ft.
- Installed batts: $0.30 to $1.50 per sq ft.
- Installed blown fiberglass: $0.50 to $2.00 per sq ft.
- Blower rental for DIY blown: often around $200 per day, sometimes free with a bulk bag purchase.
Higher R-value targets push cost toward the top of each range because they need more material and depth. A cold-zone R-60 attic uses close to twice the fiberglass of a warm-zone R-30 attic, so location drives the number as much as the product does. Air sealing the attic floor before insulating adds a few hundred dollars but protects the R-value you paid for.
Fiberglass vs blown-in cellulose: what changes in the attic?
The choice usually comes down to fiberglass versus cellulose, and the deciding factor is settling. Blown fiberglass settles only 1 to 3 percent, holding its installed depth for the life of the attic. Blown cellulose settles roughly 20 percent, so an attic blown to R-49 in cellulose can drift toward R-38 as it compresses, unless the installer over-blows to account for it.
Cellulose has a higher R-value per inch, around R-3.2 to R-3.8, so it hits a target in less depth up front. Fiberglass trades that for lower long-term settling, non-combustible glass fibers, and immunity to the moisture-driven slumping cellulose can suffer. Neither stops air on its own, so both need an air-sealed attic floor to perform.
| Factor | Blown fiberglass | Blown cellulose |
|---|---|---|
| R-value per inch | R-2.2 to R-2.7 | R-3.2 to R-3.8 |
| Settling over life | 1 to 3 percent | Around 20 percent |
| Depth to hold R-49 | Holds installed depth | Needs over-blow to hold |
| Fire behavior | Non-combustible glass | Treated with fire retardant |
| Moisture response | Does not slump | Can slump if wetted |
For a shallow, hot attic where up-front depth matters more than 20-year retention, cellulose can be the pick. For an attic you insulate once and forget, blown fiberglass usually holds its rated performance longer. The blown-in insulation guide breaks down both loose-fill materials in more detail.
How to install fiberglass attic insulation the right way
Whether you batt or blow, the sequence is the same: seal air leaks first, protect ventilation and heat sources, then build the fiberglass to the depth your zone needs. Skipping the air-seal step is the most common mistake, because loose fill and batts both let air pass and an unsealed attic floor bleeds heat straight through the insulation.
- Air seal the attic floor. Caulk and foam every top plate, wire penetration, plumbing stack, and gap before adding insulation.
- Install baffles at the eaves. Keep soffit vents clear so intake air reaches the ridge, or insulation blocks the airflow the roof needs.
- Box out heat sources. Keep fiberglass off recessed cans not rated IC, and away from chimney and flue clearances.
- Lay batts or blow fill. Butt batts tight with no gaps, or blow loose fill evenly to the marked depth ruler.
- Hit the target depth. Confirm 19 to 24 inches for R-49 to R-60, measured across the whole floor, not just near the hatch.
Ventilation is the step most DIY jobs get wrong. Fiberglass buried against the soffit chokes the intake air the attic relies on, which traps moisture and can shorten roof deck life. Pair insulation depth with clear intake at the eaves, covered in the attic ventilation guide. If you are weighing every material rather than just fiberglass, the best insulation for attic breakdown compares them by situation.
Frequently asked questions
Is fiberglass good for attic insulation?
Yes. Fiberglass is the most common and cheapest attic insulation, and blown fiberglass performs well on an attic floor because it fills gaps and settles only 1 to 3 percent. Its weakness is a low R-value per inch, R-2.2 to R-2.7 blown, so it needs more depth than cellulose or foam to reach the R-49 to R-60 most attics require.
What is the R-value of fiberglass attic insulation?
Fiberglass batts deliver R-3.0 to R-4.3 per inch depending on density, and blown fiberglass delivers R-2.2 to R-2.7 per inch as attic loose fill. To reach an R-49 target, plan on roughly 19 to 22 inches of blown fiberglass. High-density batts reach the top of the range near R-4.29 per inch but cost more per square foot.
Are fiberglass batts or blown fiberglass better for an attic?
Blown fiberglass is usually better for an attic floor because it fills around wiring and framing with no gaps and holds its depth. Batts are better for small, accessible, evenly framed attics where a homeowner can install them without renting a blower. Gap control is the deciding factor: a poorly cut batt leaves a thermal hole that blown fill avoids.
Is fiberglass or cellulose better for attic insulation?
It depends on priorities. Blown fiberglass settles only 1 to 3 percent and holds its R-value long term, while cellulose has a higher R-value per inch but settles about 20 percent and can slump if wetted. For long-term retention pick fiberglass; for maximum R-value in minimum up-front depth, cellulose can win. Both need an air-sealed attic floor.
How much does it cost to insulate an attic with fiberglass?
Fiberglass attic insulation costs $0.30 to $1.50 per square foot for batts and $0.50 to $2.00 per square foot for blown fiberglass installed. A 1,000 square foot attic runs roughly $500 to $2,000, depending on target R-value and whether you DIY. Higher zone R-values push toward the top of the range because they need more material and depth.
How many inches of fiberglass insulation do I need in my attic?
For blown fiberglass, plan on about 12 to 14 inches for R-30 in the warmest zones, 19 to 22 inches for R-49 in the middle of the country, and 24 inches or more for R-60 in the coldest zones. Because fiberglass has a low R-value per inch, it needs more depth than cellulose or foam to reach the same total R-value.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.