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Material comparison

Fiberglass vs. Cellulose Insulation: What's the Difference?

September 27, 2026

Fiberglass and cellulose are the two most common materials for blowing insulation into an attic. Both work well when they're installed to the right depth.

The biggest difference is how much insulation you get per inch. Settled loose-fill cellulose is rated at roughly R-3.2 to R-3.8 per inch, while settled loose-fill fiberglass is rated at roughly R-2.2 to R-2.7 per inch. Cellulose therefore reaches a target R-value in fewer inches. Fiberglass reaches the same target by being installed deeper.

That's why the fiberglass vs. cellulose insulation question usually has a practical answer instead of a universal winner. An attic insulated to R-38 performs like R-38 whether it took 11 inches or 13 inches to get there. The choice depends on things like how much depth the space allows, whether the area has had moisture problems, whether the insulation is going into an open attic or a closed wall, and what's already up there.

Quick Comparison

FeatureBlown-in fiberglassBlown-in cellulose
Made fromSpun glass fibers, partly recycled glassRecycled paper, treated with fire retardants
FormsLoose-fill, batts, rollsLoose-fill and dense-pack
R-value per inch (loose-fill, settled)About 2.2 to 2.7 in DOE data; newer products varyAbout 3.2 to 3.8
Settling after installSmallNoticeable, already built into the bag chart
WeightLightHeavier
If it gets soakedNot permanently damaged, but should be checkedBreaks down
Common usesAttic floors, open walls, garages, crawl spacesAttic floors, dense-packing closed walls

What Each Material Is

Fiberglass insulation

Fiberglass insulation is made of fine glass fibers. It comes in several forms: batts and rolls cut to fit between framing, and loose-fill that's blown in with a machine. According to the U.S. Department of Energy, most fiberglass products contain 40% to 60% recycled glass.

Blown fiberglass insulation is common on attic floors because it's light and covers large areas quickly. Batts and rolls are common in open walls, garages, and crawl spaces. More on the forms and where each fits is on our fiberglass insulation service page.

Cellulose insulation

Cellulose is made mostly from recycled newsprint and treated with fire retardants. GreenFiber, one of the larger cellulose manufacturers, uses borate-stabilized formulas.

Blown-in cellulose is installed two main ways:

  • Loose-fill over an attic floor.
  • Dense-pack, where it's blown tightly into closed wall cavities through small holes.

Our blown-in cellulose insulation page covers how we install it.

Cellulose R-Value vs. Fiberglass Insulation Values

R-value measures how well insulation resists heat moving through it. The two numbers people compare are R-value per inch and total R-value. Mixing them up is the most common source of confusion in any cellulose vs. fiberglass insulation comparison.

Cellulose R-value per inch

The DOE's loose-fill data puts the cellulose R-value in the R-3.2 to R-3.8 per inch range after settling. Product charts land inside that range. For R-38, GreenFiber's loose-fill cellulose chart calls for at least 11.7 inches when installed, settling to 10.5 inches. That works out to roughly R-3.6 per settled inch.

Fiberglass insulation values

Fiberglass values vary more by product and density. The older DOE figure for loose-fill fiberglass is R-2.2 to R-2.7 per inch, but current products can do better. Owens Corning's ProPink L77 blown fiberglass calls for a minimum of 13.25 inches in an attic to reach R-38. That's roughly R-2.9 per inch.

Density changes the number a lot. When the same product is packed into a closed wall, Owens Corning rates it at R-15 in a 3.5 inch 2x4 cavity at 1.5 pounds per cubic foot, well above its attic per-inch value.

What the numbers mean in practice

Comparing those two products at R-38: cellulose needs about 11.7 inches at install, fiberglass about 13.25 inches. Both deliver R-38.

The per-inch difference only matters when depth is limited, such as:

  • near the eaves, where the roof comes down low
  • in a wall cavity with a fixed depth
  • in a low-slope roof with little clearance

The real answer for any product is on its bag. Manufacturers of loose-fill insulation have to publish coverage charts showing the minimum settled thickness, weight per square foot, and coverage per bag for each R-value. A generic per-inch number is only a starting point.

Settling: The Depth You Actually Keep

Loose-fill insulation compacts a little after it's installed. How much depends on the material. DOE data from the Building America Solution Center shows loose-fill cellulose settling 10 to 20 percent over time, compared with 2 to 4 percent for loose-fill fiberglass.

That doesn't make cellulose worse. Its coverage chart already accounts for the drop, which is why the initial installed depth is higher than the settled depth.

Settling does create one common problem: an installer who stops at the settled number instead of the initial number leaves you below the R-value you paid for. With cellulose, the initial depth on the bag chart is the one that matters on installation day.

Blown-in Fiberglass vs. Blown-in Cellulose in the Attic

Attic floors are where most homeowners face this choice. ENERGY STAR notes that most contractors use blown-in loose-fill on attic floors, typically fiberglass or cellulose, and describes both as inexpensive and safe. Loose-fill conforms around wiring, pipes, and framing better than batts do, which is why it's the usual choice for existing attics. For a broader blown-in comparison, see Blown-In Insulation: Fiberglass vs. Cellulose.

A few points specific to San Antonio:

  • Recommended levels. San Antonio is in Climate Zone 2. ENERGY STAR's retrofit guidance for Zone 2 lists R-49 when adding to an uninsulated attic and R-38 when the attic already has 3 to 4 inches. These are reference levels. The right level for your home depends on what's already there and how the attic is built.
  • Cold-weather performance doesn't matter here. You may read that low-density blown fiberglass performs worse in very cold attics. DOE ties that concern to attic temperatures below 0°F, which isn't a realistic San Antonio condition. For attics here, the cold-weather argument shouldn't decide between the two materials.
  • Adding on top is normal. DOE notes that different insulation forms can be combined, such as batts over loose-fill or the reverse. Blowing either material over existing insulation is common, provided the existing layer is dry and in good condition.

If you're weighing a top-off against a full redo, our attic insulation page explains how we assess what's already there.

Walls and Other Areas

  • Closed walls. In existing homes with finished drywall, dense-pack is the usual approach. The Building America Solution Center notes that dense-packed cellulose and fiberglass are the preferred methods for closed cavities in existing homes. Dense packing prevents settling, fills the cavity completely, and slows airflow. Cellulose is the most common dense-pack material in DOE-funded weatherization programs.
  • Open walls, garages, and crawl spaces. When the framing is exposed, fiberglass batts or rolls are often the simpler fit. Blown cellulose needs a cavity or surface to hold it in place.
  • Weight. Cellulose is heavier. DOE data puts settled loose-fill cellulose at 1.5 to 2.0 pounds per cubic foot versus 0.5 to 1.0 for loose-fill fiberglass. It recommends against cellulose in mobile home ceilings and floors because of its weight.

Moisture, Fire, and Safety

Water. This is the clearest practical difference. Cellulose breaks down when it gets saturated. Fiberglass is lighter and won't be permanently damaged by saturation. Neither one should stay wet. A soaked patch of either material means there's a roof leak, plumbing leak, or condensation problem to fix first. If your attic has a history of leaks or storm damage, factor that into the choice.

Fire. Fiberglass is made from glass. Cellulose starts as paper, which is why it's treated with fire retardants during manufacturing. Both are sold for residential attics and walls when installed according to the manufacturer's instructions.

Safety note. Loose-fill insulation of either type releases dust and fibers when it's moved or blown. Anyone working in the attic should wear long sleeves, gloves, eye protection, and a properly fitted respirator.

San Antonio attics get extremely hot for much of the year, so schedule attic work for early morning and take breaks.

Step only on joists or walkboards. The ceiling drywall between joists won't hold your weight.

If your existing insulation looks like pebbly, gray-brown or gold granules, it may be vermiculite. The EPA advises treating vermiculite as possibly containing asbestos and leaving it undisturbed. If it's removed, the EPA recommends a trained, accredited asbestos abatement contractor.

Which One Should You Choose?

A practical way to think about the choice:

  • Adding to an open attic with plenty of depth. Either material works. The deciding factors are usually what's already there and your installer's recommendation for your specific attic.
  • Tight spaces at the eaves or low-slope roof sections. Cellulose's higher R-value per inch helps you reach the target in less depth.
  • Attics with a history of roof leaks. Fiberglass handles getting wet better, but the leak has to be fixed either way.
  • Closed, finished walls. Dense-pack is the usual method, and cellulose is the most common material for it.
  • Open walls, garage walls, and crawl spaces. Fiberglass batts or rolls are often the easier fit.
  • Manufactured homes. Weight favors fiberglass.

Common mistakes

  • Comparing R-value per inch instead of total R-value. A deeper layer of fiberglass and a shallower layer of cellulose can both hit the same target.
  • Measuring cellulose on installation day against the settled number instead of the initial installed depth on the bag chart.
  • Covering a problem. Blowing new insulation over wet, moldy, pest-damaged, or vermiculite insulation hides the problem instead of solving it.
  • Blocking attic ventilation. Blowing insulation into the soffit area without baffles can cover the vents.
  • Skipping the air leaks. Gaps around the attic hatch, plumbing, and wiring let air bypass the insulation entirely.

If you're not sure what's in your attic now or how deep it is, that's what an inspection is for. We'll measure what's there, check its condition, and recommend fiberglass or cellulose for your specific attic. Get an Estimate when you're ready.

The Takeaway

Fiberglass and cellulose both do the same job. The difference is how they get there:

  • Cellulose packs more R-value into each inch and fills closed walls well, but it's heavier and doesn't tolerate getting soaked.
  • Fiberglass is lighter, settles very little, handles moisture better, and comes in batts and rolls for open framing.

For a typical San Antonio attic top-off, either can reach the recommended level when it's blown to the depth on the bag chart. The better choice for your home depends on the space, its history, and what's already in it.

References

Frequently Asked Questions

Usually, yes. Mixing materials is common when topping off an attic. The existing layer should be dry, free of mold and pest damage, and not heavily compressed. Heavy cellulose over a very thin layer of fiberglass will compress it slightly, so the new layer is sized to reach the total R-value you want.

Measure the depth in a few spots with a ruler. ENERGY STAR suggests multiplying the depth in inches by 3 for a rough R-value, since common materials fall around R-3 to R-3.5 per inch. Measure away from the attic hatch, where insulation is often packed down or thin.

Under the FTC's R-Value Rule, the receipt for loose-fill insulation should list the coverage area, initial installed thickness, minimum settled thickness, R-value, and number of bags used. Keep it. It's the easiest way to confirm you got the depth you paid for.

For most existing attics, blown-in is easier to install evenly because it fills around wiring, framing, and other obstructions without cutting. Batts can work well in attics with regular joist spacing and few obstructions. Both are rated by total R-value, so a correctly installed layer of either can reach the same target.

Both are generally among the lower-cost ways to insulate an attic. The price of a specific job depends mostly on the attic's size, how deep the insulation needs to go, how easy the attic is to get into, and whether anything has to be removed first. Neither material is automatically cheaper. An estimate based on your attic's actual depth and condition is the only reliable way to compare.