By The Roofing Brief Team. Last reviewed: October 2026.
Fiberglass attic insulation comes in two forms: pre-cut batts you lay between joists, and loose-fill you blow in with a machine. Blown fiberglass is among the cheapest ways to insulate an attic floor, at about $1.01 per square foot in material at R-49 in October 2026 Home Depot pricing, but it has one of the lowest R-values per inch of any common attic 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 can win.
Which fiberglass attic insulation to choose: decision table
Match the product to the attic, using the depth and clearance numbers from the maker charts and code sections cited in each row.
| If your attic… | Choose | Threshold or rule that decides it |
|---|---|---|
| Is an open, vented floor with wiring, ducts or odd bays | Blown fiberglass | Fills around obstructions; R-49 needs 17 in of AttiCat or 16.5 in of ProCat |
| Has clear 16 in or 24 in joist bays and good headroom | Fiberglass batts, two layers | PINK Next Gen R-49 batts are 14 in thick in 16 in and 24 in widths |
| Has tight eaves where full-depth batts would block the vent | High-density R-30C (8.25 in) or R-38C (10.25 in) batts at the eaves | IRC R806.3 requires at least 1 in of air space between insulation and roof sheathing at the vent |
| Already has 3 to 4 in of insulation | Blown fiberglass over the top | ENERGY STAR says add R-38 in zones 2 to 3 and R-49 in zones 4 to 8; R-38 is 13.5 in of AttiCat |
| Sees winter design temperatures of -18°F or colder | Batts, or blown fill with extra depth | Johns Manville’s chart adds 1.5 to 2.5 in of Climate Pro at R-38 in that range; it says batts show no R-value loss at U.S. temperatures |
| Will get a plywood storage floor over 2×12 joists | Blown fill or batts sized to the bay | An R-49 batt squeezed into an 11.25 in bay rates about R-42 (NAIMA compression chart) |
| Has recessed cans not rated IC | Replace with IC-rated cans before insulating | NEC 410.116 keeps insulation at least 3 in from a non-IC fixture |
Sources: Owens Corning AttiCat data sheet (Pub. No. 10011273-L, May 2026); Owens Corning ProCat fact sheet (Pub. No. 10019591-C, March 2026); Owens Corning PINK Next Gen manufacturers fact sheet (Pub. No. 10025109-B, May 2026); ENERGY STAR Recommended Home Insulation R-Values; Johns Manville Climate Pro attic coverage card (BIC-260, 6/18); Insulation Institute (NAIMA) compressed R-value chart (BI506, 10/20); 2021 IRC Chapter 8.
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 Owens Corning’s AttiCat chart lists the same figure for installed and settled thickness. 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. For the full side-by-side with cost and grading data, see our comparison of blown-in insulation vs batts.
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 | About R-3.2 to R-3.5 at attic depths (R-30 to R-49); up to R-4.3 for high-density wall batts | About R-2.8 to R-2.9 (AttiCat open-attic loose fill) |
| Material cost at R-49 | About $2.27 per sq ft | About $1.01 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 | Chart lists settled thickness equal to installed |
| Best attic use | Small, open, evenly framed | Large or obstructed attic floors |
Sources: Owens Corning PINK Next Gen submittal sheet (Pub. No. 10024913-C, September 2025); Owens Corning AttiCat product data sheet (Pub. No. 10011273-L, May 2026); material cost from Home Depot and HEP Sales listings, October 2026 (AttiCat at $40.34 per bag; Owens Corning R-49 BF93 batts at $108.98 per 48 sq ft bag). Per-inch and per-square-foot figures are Roofing Brief arithmetic from those documents.
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 small holes cost far more than their area suggests. Under the modeling rule in ANSI/RESNET/ICC 301-2022, an attic labeled R-49 with 5 percent of its area left bare performs at roughly R-21 for the whole ceiling (Roofing Brief parallel-path arithmetic, including about R-1.7 for drywall and air films). Blown fill reduces that failure mode.
Attic batt insulation sizes: R-value, thickness and width
Attic batt insulation is sold by R-value, and each R-value has a fixed label thickness and set widths for 16 in, 19.2 in and 24 in framing. The rows below are the attic-depth unfaced (U) and kraft-faced (K) batts on Owens Corning’s May 2026 PINK Next Gen fact sheet, all 48 in long unless noted. The “C” batts are the high-density cathedral versions that fit more R-value into a shallow space.
| Batt | Label thickness | R per inch | Widths (in) | Sq ft per bag | Source |
|---|---|---|---|---|---|
| R-19 (U) | 6.25 to 6.5 in | 2.9 to 3.0 | 15, 16, 19.25, 23, 24 | 49 to 128 | OC fact sheet 10025109-B |
| R-30 (U or K) | 9.5 in (9 in rolls) | 3.2 to 3.3 | 15, 16, 19.25, 23, 24 | 31.3 to 88 | OC fact sheet 10025109-B |
| R-30C (U or K) | 8.25 in | 3.6 | 15.5, 23.75 | 56.8 to 79.2 | OC fact sheet 10025109-B |
| R-38 (U or K) | 12 in | 3.2 | 16, 19.25, 24 | 42.7 to 64 | OC fact sheet 10025109-B |
| R-38C (U or K) | 10.25 in | 3.7 | 15.5, 23.75 | 41.3 to 63.3 | OC fact sheet 10025109-B |
| R-49 (U or K) | 14 in | 3.5 | 16, 24 | 32 to 48 | OC fact sheet 10025109-B |
Kraft-faced batts carry a vapor retarder, and the same fact sheet says standard kraft facing must be in contact with a thermal barrier such as drywall, while foil-scrim-kraft (FSK) facing can be left exposed. When you add a second layer over existing insulation, use unfaced batts so you do not add a second vapor retarder that can trap moisture between layers. Lay the top layer perpendicular to the joists so it covers the 1.5 in tops of the framing, which otherwise bridge heat straight through the insulation.
Batts lose R-value when squeezed. The Insulation Institute’s chart, compiled from most fiberglass makers, rates an R-49 batt (14 in) at about R-42 when it is compressed into a 2×12 bay of 11.25 in, and an R-38 batt (12 in) at about R-37 in the same bay. If you plan a plywood storage deck, raise it on sleepers or accept the lower number.
What is the R-value of fiberglass attic insulation?
Blown fiberglass in an open attic delivers about R-2.8 to R-2.9 per inch, and fiberglass batts deliver about R-2.9 to R-4.3 per inch depending on product and density. Owens Corning’s AttiCat data sheet (May 2026) calls for 13.5 inches for R-38, 17 inches for R-49 and 20.5 inches for R-60, which works out to R-2.81 to R-2.93 per inch. Owens Corning’s PINK Next Gen batts are R-30 at 9.5 inches, R-38 at 12 inches and R-49 at 14 inches (about R-3.2 to R-3.5 per inch), while its high-density wall batts reach R-3.82 (R-21 at 5.5 inches) to R-4.29 (R-15 at 3.5 inches).
Total R-value is what your energy bill responds to, and for loose fill the bag chart sets it, not a per-inch multiplication. The chart pairs each R-value with a minimum thickness and a minimum number of bags per 1,000 square feet, and you need both: AttiCat’s R-49 row is 25 bags per 1,000 sq ft at no less than 17 inches and 0.688 lb per sq ft. That is why installers mark a depth ruler on the truss and blow to a target inch count while also counting bags. The insulation R-value chart lists per-inch figures for every material side by side.
Blown fiberglass R-value by brand
Blown fiberglass R-value per inch differs by product because each maker blows its fiber at a different density, from about R-2.88 per inch (AttiCat) to R-2.97 (ProCat) at the R-49 row. Three current loose-fill charts, with Greenfiber cellulose for contrast:
| Product | R-30 depth | R-49 depth | R-60 depth | Bags per 1,000 sq ft at R-49 | R per inch at R-49 | Source |
|---|---|---|---|---|---|---|
| Owens Corning AttiCat (27.5 lb bag) | 10.75 in | 17 in | 20.5 in | 25 | 2.88 | OC 10011273-L, May 2026 |
| Owens Corning ProCat (33.5 lb bag, contractor) | 10.5 in | 16.5 in | 19.75 in | 21.5 | 2.97 | OC 10019591-C, March 2026 |
| Johns Manville Climate Pro (contractor) | 10.8 in | 16.9 in | 20.2 in | 23.4 | 2.90 | JM B7600 card, BIC-260 |
| Greenfiber SANCTUARY cellulose | 8.9 in installed, 8.3 settled | 14.3 in installed, 13.3 settled | 17.3 in installed, 16.1 settled | See bag chart | 3.68 (settled) | Greenfiber DIY sheet, Rev F 05-26 |
Every one of these R-values is a lab rating at a 75°F mean temperature, which the FTC R-value rule (16 CFR Part 460) requires for all home insulation labels. Johns Manville’s card notes that at extreme cold, convection through loose fill can pull blown fiberglass below its label value, and Minnesota law requires the stated R-value at winter design temperature. JM’s chart adds 1 in (1.3 bags per 1,000 sq ft) at R-38 for -14°F to -17°F, 1.5 in for -18°F to -22°F and 2.5 in for -23°F and colder, and adds nothing at R-60 in any of those bands.
How much fiberglass insulation does your attic need by climate zone?
ENERGY STAR’s retrofit table, based on the 2021 IECC, recommends R-30 for an uninsulated attic in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. If the attic already has 3 to 4 inches, the table recommends adding R-25 in zone 1, R-38 in zones 2 and 3, and R-49 in zones 4 through 8. Because fiberglass has a low per-inch R-value, meeting these targets means real depth: 17 inches of AttiCat for R-49 and 20.5 inches for R-60. ENERGY STAR estimates that air sealing and adding insulation saves homeowners an average of 15 percent on heating and cooling costs, per its savings methodology. Whether going past those targets pays is the question in our guide to whether you can over insulate a house.
| Climate zone | ENERGY STAR target, uninsulated attic | Blown fiberglass depth (AttiCat chart) | EPA estimated heating and cooling savings from sealing and insulating |
|---|---|---|---|
| Zone 1 (S. Florida, Hawaii) | R-30 | 10.75 inches | 7% |
| Zones 2 to 3 (Gulf, Southern) | R-49 | 17 inches | 9% (zone 2) to 14% (zone 3) |
| Zones 4 to 8 (Mid-U.S. to Alaska) | R-60 | 20.5 inches | 16% (zone 5) to 20% (zone 4C) |
Sources: ENERGY STAR, Recommended Home Insulation R-Values (retrofit table based on 2021 IECC Table R402.1.3); ENERGY STAR Methodology for Estimated Energy Savings, Table 1; Owens Corning AttiCat product data sheet, Pub. No. 10011273-L (May 2026). Depths are minimum settled thickness for AttiCat; other loose-fill fiberglass brands publish different charts.
Find your zone by county on the IECC climate zone map hosted by the DOE’s Building America Solution Center before you buy. The same total R-value protects against summer heat gain and winter heat loss, so a zone 4 attic in a cooling-dominated city and a zone 4 attic in a heating-dominated one share the same R-60 target. For a broader walkthrough of every material by zone, see the attic insulation guide.
How much does fiberglass attic insulation cost?
At R-49, blown fiberglass material runs about $1.01 per square foot and attic-depth fiberglass batts about $2.09 (R-38) to $2.27 (R-49) per square foot, based on October 2026 Home Depot and HEP Sales listings. HomeGuide puts professionally installed blown-in insulation at $1.10 to $1.90 per square foot at R-49, which is $1,100 to $1,900 on a 1,000 square foot attic floor. Its range for installed attic batts, $0.80 to $2.60 per square foot, spans all R-values, so the low end reflects thin batts rather than R-49.
| Option (1,000 sq ft attic floor, R-49) | Cost per sq ft | Job total | Source |
|---|---|---|---|
| DIY blown fiberglass, material only | About $1.01 | About $1,009 (25 AttiCat bags at $40.34) | OC AttiCat chart; Home Depot listing, Oct 2026 |
| DIY fiberglass batts, material only | About $2.27 | About $2,270 | HEP Sales listing, R-49 BF93 at $108.98 per 48 sq ft, Oct 2026 |
| Installed blown-in | $1.10 to $1.90 | $1,100 to $1,900 | HomeGuide blown-in insulation cost guide |
- DIY blown fiberglass, material only at R-49: about $1.01 per sq ft (25 AttiCat bags per 1,000 sq ft at $40.34 per bag).
- DIY batts, material only: about $2.09 per sq ft at R-38 and $2.27 per sq ft at R-49.
- Installed blown-in at R-49: $1.10 to $1.90 per sq ft (HomeGuide).
- Blower rental for DIY blown: Owens Corning’s AttiCat rental guide lists Lowe’s and The Home Depot and gives 24 hours free with the machine when you buy 10 bags; a daily fee may apply after that, so confirm terms at your store.
Higher R-value targets push cost toward the top of each range because they need more material and depth. On the AttiCat chart, R-60 takes 31.5 bags per 1,000 square feet against 14.6 bags for R-30, more than twice the fiberglass, so the zone target drives the number as much as the product does. Air sealing the attic floor before insulating adds cost but protects the R-value you paid for. Shelf prices change by store, region and week, so re-check the bag price before you multiply.
Fiberglass vs blown-in cellulose: what changes in the attic?
The choice usually comes down to fiberglass versus cellulose, and settling is one of the deciding factors. Owens Corning’s AttiCat chart lists the same minimum and settled thickness, so the installed depth is the rated depth. Greenfiber states that its SANCTUARY cellulose R-values are based on settled thickness, which is 90 percent of installed thickness, so the chart has you blow 14.3 inches to end at 13.3 inches settled for R-49. A cellulose job blown only to the settled figure will fall short once it settles.
Cellulose has a higher R-value per inch, about R-3.7 per Greenfiber’s SANCTUARY chart (R-49 at 13.3 inches settled), so it hits a target in about 3.7 fewer inches than AttiCat. Fiberglass trades that for no listed settling allowance, non-combustible glass fibers under ASTM E136, and resistance 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 (AttiCat) | Blown cellulose (SANCTUARY) |
|---|---|---|
| R-value per inch | About R-2.8 to R-2.9 | About R-3.7 |
| Depth for R-49 | 17 inches | 14.3 inches installed, 13.3 inches settled |
| Settling allowance in chart | None (settled equals installed) | Settled is 90 percent of installed |
| Fire behavior | Non-combustible glass (ASTM E136) | Treated with fire retardant |
| Moisture response | Does not absorb moisture (ASTM C1104) | Can slump if wetted |
For a shallow attic where every inch under the roof deck counts, cellulose reaches R-49 at 13.3 inches settled versus 17 inches of AttiCat. For an attic you insulate once and forget, blown fiberglass reaches its rated depth without a settling allowance. The blown-in insulation guide breaks down both loose-fill materials in more detail.
Is fiberglass insulation safe? Downsides to know
The International Agency for Research on Cancer re-evaluated man-made vitreous fibres in 2001 and placed insulation glass wool in Group 3, not classifiable as to carcinogenicity to humans (IARC Monograph 81), the same group as rock and slag wool. That is a different category from asbestos, which IARC classes in Group 1. The practical downside is irritation: Owens Corning’s AttiCat data sheet says fiberglass may cause temporary skin and mucous membrane itching from the mechanical abrasion of the fibers, a condition it calls completely reversible, so cover skin and wear eye protection and an N95 dust mask when you handle or blow it.
The other real downsides are depth and air: fiberglass needs about 17 inches of loose fill for R-49 where cellulose needs 13.3 settled, and neither material stops air leakage, so a leaky ceiling plane undercuts the label R-value until it is sealed.
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. ENERGY STAR’s savings estimate counts air sealing and insulation together, 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; IRC R806.3 requires at least 1 inch of space between the insulation and the roof sheathing at the vent.
- Box out heat sources. Keep fiberglass at least 3 inches from recessed cans not rated IC (NEC 410.116), and away from chimney and flue clearances.
- Set depth markers. For blown fill, 2021 IECC R303.1.1.1 calls for thickness markers on the joists or trusses, at least one for every 300 square feet, facing the attic access.
- Lay batts or blow fill. Butt 16 in or 24 in wide batts tight end to end with no gaps, or blow loose fill evenly to the marked depth ruler. If you are using batts, our guide shows how many attic insulation rolls to buy for your attic.
- Hit the target depth and bag count. For AttiCat, confirm 17 to 20.5 inches for R-49 to R-60, measured across the whole floor, not just near the hatch, and check that you used at least the chart’s bags per 1,000 sq ft.
Ventilation is the step DIY jobs most often get wrong. Fiberglass buried against the soffit chokes the intake air the attic relies on, which traps moisture and can shorten roof deck life; the 1-inch air space in IRC R806.3 is the minimum, and a rigid baffle holds it open under 17 inches of fill. 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 attic insulation, and blown fiberglass performs well on an attic floor because it fills gaps, and Owens Corning’s AttiCat chart lists the same installed and settled thickness. Its weakness is a low R-value per inch, about R-2.8 to R-2.9 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?
Blown fiberglass delivers about R-2.8 to R-2.9 per inch in an open attic, per Owens Corning’s AttiCat data sheet, and attic-depth fiberglass batts about R-3.2 to R-3.5 per inch. To reach R-49, plan on 17 inches of AttiCat or a 14-inch R-49 batt. High-density wall batts reach about R-4.3 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?
Blown fiberglass lists no settling allowance on Owens Corning’s AttiCat chart, while cellulose has a higher R-value per inch but settles to 90 percent of installed depth per Greenfiber, so it must be blown deeper, and it can slump if wetted. For a no-settling install pick fiberglass; for maximum R-value in minimum depth, cellulose can win. Both need an air-sealed attic floor.
How much does it cost to insulate an attic with fiberglass?
At R-49, blown fiberglass material costs about $1.01 per square foot and fiberglass batts about $2.27 per square foot, based on October 2026 retail listings. HomeGuide puts professionally installed blown-in insulation at $1.10 to $1.90 per square foot at R-49, or $1,100 to $1,900 for a 1,000 square foot attic. 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, Owens Corning’s AttiCat chart calls for 10.75 inches for R-30, 13.5 inches for R-38, 17 inches for R-49 and 20.5 inches for R-60. ENERGY STAR recommends R-30 for an uninsulated attic in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. Other brands publish different depths, so follow the chart on the bag you buy.
Is fiberglass insulation as bad as asbestos?
No. IARC Monograph 81 places insulation glass wool in Group 3, not classifiable as to carcinogenicity to humans, while asbestos is in Group 1. Fiberglass can still cause temporary skin and mucous membrane itching, which Owens Corning describes as completely reversible, so wear gloves, long sleeves, eye protection and an N95 mask.