Cellulose vs fiberglass in an attic
By Insulation Report Editorial. Last reviewed August 2026. How we research this.
In brief
On a dry, open attic floor both work, and the gap is smaller than the arguing suggests. Cellulose gives R-3.2 to R-3.8 an inch. Blown fiberglass gives R-2.2 to R-2.7. One published cellulose chart reaches R-49 at 13.24 settled inches, where fiberglass needs roughly 18.1 to 22.3.
The trade is weight. That chart puts cellulose at 2.02 lb per square foot at R-49, against about 0.70 for one measured fiberglass bag. Pick cellulose when headroom is short at the eaves, or when it costs less per square foot at your target.
Pick fiberglass when the ceiling below is old plaster or already sagging, when the attic has any leak history, or when you are renting a machine and would rather handle 31 bags than 81. Neither one stops air. If the ceiling under it is not sealed, that decides your result, not the material.
The verdict, by attic
The blown-in guide covers how both go in: the machine, the order of work, and how to check the finished job. It stops short of choosing between them. This page chooses. Read down the table and stop at the first row that matches your attic. The rows that overrule the others come first.
| Your attic | Use | Why |
|---|---|---|
| Wet now, or any live roof leak | Neither, yet | Find the water and fix it first. Then fiberglass, because it dries. See mold in attic insulation |
| Lath-and-plaster ceiling, or drywall that already sags | Fiberglass | About 2.9 times lighter at the same R-value, from the two published charts below. Look at the ceiling from underneath before you add any depth |
| Low clearance: shallow pitch, tight eaves, ducts filling the space | Cellulose | It reaches R-49 in 13.24 settled inches on its own chart. Fiberglass wants 18.1 to 22.3 |
| Bare attic floor, open, dry, normal headroom | Either. Buy on price | Both clear R-60 in a normal attic with room over the joists. Compare material cost per square foot at your target, not per bag |
| Topping up over existing dry material | Either, over either | They stack in any order. Count the inches you have left under the roof, then pick. See adding over existing |
| You are renting the machine and doing it yourself | Fiberglass | 31 bags instead of 81 for 1,200 square feet at R-49, less dust, and less settling to allow for |
| Walls, a knee wall cavity, or a closed sloped ceiling | Dense pack cellulose, by a crew | A closed cavity is a different job with a density target and a wall machine. It is not what either material does on an open floor |
The recommendation. If none of the first three rows describe your attic, the choice is worth very little and you should buy on price. If you want one default: blown fiberglass for a homeowner doing it themselves, cellulose for a crew working to a target in a tight roof. Whichever goes in, the ceiling under it gets sealed first and the eaves get baffles first, or the depth on the invoice is not the depth doing the work.
Want a quote either way?
If you would rather have a crew price it, give us the job and your ZIP and we pass it to a licensed contractor who works in your area. No obligation, and the page above still applies.
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You may not need to choose at all
Before you spend an evening on this, measure. Push a rigid ruler down to the drywall at five spots across the attic, away from the hatch, and take the typical reading rather than the deepest. Then compare it with the recommendation for your climate zone. For zones 4 through 8, ENERGY STAR puts an attic that already has 3 to 4 inches at R-49, and a bare one at R-60. Milder zones get lower numbers. The depth checker does the conversion for you.
If what you have is dry and already within about R-10 of your zone's number, this whole comparison is worth almost nothing to you. Heat flow drops as 1 over R, so the last step up saves a small share of what the first ones did. That R-10 threshold is our estimate, not a measurement. What is worth much more at that point is sealing the ceiling plane: the top plates, the wiring and plumbing holes, the dropped soffit over the kitchen cabinets, the bath fan housing, and the hatch. Loose fiber slows heat through material. It does nothing about air pouring through a hole, and neither material on this page is an air barrier.
We do not sell insulation and we do not install it. That is why this page can tell you the cheapest right answer here is often a caulk gun, a can of foam and a weekend, rather than 81 bags of anything. Still wondering whether the old material should come out? The remove-or-add-over guide runs that test in five steps.
What it costs, at the same R-value
Comparing bag prices is the usual mistake, because the two bags do not cover the same area. A cellulose bag covers less floor than a fiberglass bag at the same R-value, so a cheaper bag is not automatically a cheaper attic. Cost per square foot at your target is the only comparison that means anything. Figures below are for a 1,200 square foot attic floor at R-49.
| At R-49 | Blown fiberglass | Cellulose |
|---|---|---|
| Square feet one bag covers | 39.5 | 14.9 |
| Bags for 1,200 sq ft | 31 | 81 |
| Bag weight | 27.5 lb | 30 lb |
| Bag price we have read | $67.58 | none for this bag |
| Material per square foot | $1.71 | your price divided by 14.9 |
We have read a shelf price on each side, but the cellulose one is a different bag. It is the 25 lb SANCTUARY bag at $15.97, which works out at $1.09 to $1.13 of material per square foot at R-49 on its own coverage card. The 30 lb bag in the table above has no read price here, so here is the line it has to beat. At $67.58 a bag over 39.5 square feet, fiberglass costs $1.71 of material per square foot at R-49. A cellulose bag covering 14.9 square feet matches that at about $25.49. Read the two shelf tickets and the coverage charts beside them, and you have your answer in a minute.
Two warnings on that arithmetic. It is material only, so it says nothing about an installed quote, where labor, air sealing and disposal usually outweigh the material gap. The blown-in cost page builds the installed number from the trade wage and states which inputs are estimates. And the fiberglass price is a retail shelf price read on 2026-08-07. A contractor buys below it, and neither figure is a quote.
Fire, and what an attic floor is held to
Cellulose is shredded paper, so the borate fire retardant blended into it is doing real work. That treatment is what ASTM C739, the cellulose material specification, exists to test. Glass fiber does not burn, because glass is not fuel. Loose-fill glass fiber is made to ASTM C764, and glass fiber batts to ASTM C665. The kraft paper facing on a faced batt is a different matter: paper is fuel, which is why faced batts are not meant to be left exposed.
Most comparisons miss the next part. Federal weatherization rules set fire requirements by where the insulation goes, not just by what it is. For an attic floor, DOE's standards for weatherization materials say that insulation meant for exposed use in attic floors has to meet the same smoldering combustion requirements given for cellulose in ASTM C739. Read that again. The attic floor is held to the cellulose bar whatever you put on it. That is a useful answer to give a nervous relative, and it is checkable.
Neither material changes what you owe the fixtures. Keep the clearance each recessed light's own label asks for, dam around a masonry chimney, and keep both away from a flue. Where a fixture has no label you can read, treat it as one that must be kept clear. Those clearances come from the National Electrical Code, which is a model code that each state or city adopts with its own changes, so the label in front of you is the authority.
When it gets wet
This is the clearest split between them, and it decides more real attics than R per inch does. Glass fiber does not absorb water into the fiber. Water runs through it. If the leak is found and fixed quickly, the layer dries and gets its loft back. Cellulose is paper. It soaks water up, gets much heavier, and the wet patch mats down flat. A matted patch does not spring back, so the R-value in that spot is gone even after it dries out.
The published chart for the cellulose product used on this page is blunt about it. The product is for dry application only, and the sheet says in capitals not to add water to it. There are spray-applied cellulose products for closed cavities, and this is not one of them.
Neither of these is a license to leave water alone. Both materials are porous, and EPA guidance on porous material with mold growing on it is that it comes out rather than gets cleaned. So a soaked attic is a removal job in either material. The mold page works through finding the source, which is nearly always a roof leak, a bath fan venting into the attic, or warm damp air leaking up through the ceiling.
R per inch, and what the gap is worth in inches
Both numbers are ranges on purpose. R per inch moves with density, with the product and with how it went in, so a single figure would be false precision. The R per inch page carries the full set.
| Material | R per inch | Inches to R-49 | Inches to R-60 |
|---|---|---|---|
| Blown-in cellulose | R-3.2 to R-3.8 | 12.9 to 15.3 | 15.8 to 18.8 |
| Blown-in fiberglass | R-2.2 to R-2.7 | 18.1 to 22.3 | 22.2 to 27.3 |
| Loose-fill mineral wool | R-2.8 to R-3.7 | 13.2 to 17.5 | 16.2 to 21.4 |
A manufacturer's own chart lands inside that cellulose range, which is reassuring and rare. The product anchored on this page reaches R-49 at 13.24 settled inches and R-60 at 16.11. That works out at about R-3.7 an inch, near the top of the range. So the honest headline is five to eight fewer inches for the same R-value, not double the performance.
Put that against your framing before it feels decisive. A 2x10 ceiling joist measures 9.25 inches under the American Softwood Lumber Standard PS 20. At R-49 both materials bury it completely, so either way you are building a blanket well above the joist tops. The inches only decide the argument where the roof runs out of room, which is the eaves. Measure the clear height about a foot inside the outside wall, not in the middle of the attic where everything fits.
One caveat on every depth figure here. Loose fill gets denser as it gets deeper, so R-value does not rise in step with depth, and DOE says so. Depth converted from R per inch is an estimate. The coverage chart on the bag actually being blown is the authority, and the calculator works from coverage rather than from a fixed R per inch for that reason.
Settling, and the depth that has to go in
Loose fiber leaves the hose full of air and packs down under its own weight over the following weeks. The published chart handles this by giving two thicknesses for every R-value: what goes in on the day, and what you are left with. The settled figure is the one that has to meet your target.
| R-value | Installed on the day | Settled | Sq ft per bag | Bags per 1,000 sq ft |
|---|---|---|---|---|
| R-30 | 9.13 in | 8.22 in | 26.5 | 37.8 |
| R-38 | 11.49 in | 10.34 in | 20.1 | 49.7 |
| R-49 | 14.71 in | 13.24 in | 14.9 | 67.3 |
| R-60 | 17.90 in | 16.11 in | 11.6 | 86.4 |
The allowance is about 10 percent at every row. At R-49 that is 14.71 inches on the day to end up at 13.24, so roughly 1.5 inches disappear. Fiberglass settles less. That sounds like a point for fiberglass and it is not quite: less settling also means a fresh fiberglass blow can read at target with a ruler on the afternoon it goes in and read short a season later, because it landed fluffed.
So neither material is checked with a ruler alone. Bag count is the proof. Count them against the coverage chart for the product, and set the depth markers to the finished settled total measured up from the drywall. If a crew blows to your settled number on day one in cellulose, you are about 1.5 inches short before the first frost.
Weight on the ceiling
This is the one nobody checks and the one that occasionally matters. Both published charts give a weight per square foot, so the comparison is straightforward.
| At R-49 | Blown fiberglass | Cellulose |
|---|---|---|
| Pounds per square foot | 0.70 | 2.02 |
| Total on a 1,200 sq ft ceiling | 835 lb | 2,424 lb |
Cellulose is about 2.9 times the load at the same R-value. Go to R-60 and it rises to 2.59 lb per square foot, or roughly 3,112 lb over that same ceiling. Spread across every joist in the house that is still a light load, and a sound modern drywall ceiling carries it without complaint. This is not a reason to fear cellulose.
Four cases make it worth a thought. An old lath-and-plaster ceiling, drywall that already sags between the framing, a ceiling with a history of water damage, or any ceiling you have watched cracks open on. In those, look from underneath in daylight before you add depth in anything, and price a ceiling repair into the decision rather than after it. If the ceiling is already marginal, the lighter material is free insurance.
Air, and why dense pack is a wall job
Neither material is an air barrier. Air moves through both, which is the whole reason air sealing is a separate step and not an optional extra. What density changes is how much air moves through the layer, and that is the one real performance difference between them.
On an attic floor at R-49, the cellulose chart works out at about 1.8 lb per cubic foot. Blown fiberglass at the same R-value lands near 0.4, using the midpoint of its depth range, so that second figure is an estimate rather than a published one. The denser material resists air moving through it better. Where you notice it is the eaves, where wind pushes in under the roof and scours the top of a light layer. Baffles fix that for both materials, and they go in before any fiber does.
Dense pack is where this gets misread. In a closed wall cavity a crew blows cellulose to about 3.5 lb per cubic foot, roughly 1.9 times the attic figure. They do it for two reasons: to stop the material settling and leaving a void at the top of the wall, and to slow air moving inside the cavity. Both problems belong to walls.
And here is the part that settles it. Packing to that density buys no extra R per inch. The same sheet's wall rows give R-13 in 3.5 inches and R-21 in 5.5 inches, which is R-3.7 to R-3.8 an inch. The attic rows give about R-3.7. Nearly twice the density, the same R per inch. You also cannot dense pack an open attic floor, because there is no cavity to pack against. Anyone quoting dense pack for your attic floor is describing a wall.
Dust, bags and the day itself
The mainstream retail cellulose product is sold as a low dust version, which tells you what the standard one does. Cellulose dust is heavy paper and borate. It is thick enough to cut visibility in an attic and it settles on everything below the hatch. Fiberglass throws less of it, and what it throws itches. Both need a fitted respirator, sealed goggles, gloves and covered skin, in a space that will be brutally hot by mid-morning.
Then there is the handling. The same 1,200 square foot attic at R-49 is 31 fiberglass bags or 81 cellulose bags, and somebody is cutting every one of them open and feeding it into a hopper. That is the real reason the do-it-yourself row in the verdict table says fiberglass. The DIY comparison works through the rest of that call.
One trap with rental machines. The coverage chart is tied to a specific machine at a specific setting: the attic chart used here names a Volumatic III at third gear with an eight inch gate, and the wall chart a Krendl KS 200. Whatever is behind the rental counter is not that machine. So treat the coverage figure as an estimate on a rented blower, and check the finished job on depth and bag count together rather than on either alone.
Recycled content
Cellulose wins this outright and it is worth knowing what the number means. EPA's procurement guideline tells federal agencies what recovered content to ask for when they buy insulation. It sets cellulose loose-fill at 75 percent postconsumer paper. Fiberglass sits at 20 to 25 percent total recovered materials, in the form of glass cullet. Loose-fill mineral wool sits at 75 percent, from slag.
Those are purchasing recommendations for federal buyers. They are not measurements of the bag in front of you, and a given product can sit above or below them. If recycled content is part of your decision, read the bag and the maker's own published figure rather than a general table, this one included.
Related
Common questions
Cellulose or fiberglass: which is better in an attic?
For a dry, open attic floor, neither is better in general and both reach any target you set. Cellulose gets there in fewer inches, at R-3.2 to R-3.8 an inch against R-2.2 to R-2.7 for blown fiberglass. Fiberglass gets there at roughly a third of the weight. So the attic picks, not the material. Short headroom at the eaves points to cellulose. An old plaster ceiling, or any history of roof leaks, points to fiberglass. If neither applies, buy the one that costs less per square foot at your target R-value.
Can I blow cellulose over existing fiberglass, or fiberglass over cellulose?
Yes, in either order, as long as the old material is dry and clean. They are both loose fiber and neither one attacks the other. Two things to get right. Measure depth from the drywall, not from the top of the old layer, or the finished job comes out short. And write the product name and the bag count on the invoice, because once two materials are stacked you cannot read the total R-value off a ruler. Air seal the ceiling first, whichever way round you do it.
Is cellulose too heavy for my ceiling?
Almost never on sound modern drywall, and it is worth a look from below when the ceiling is not. One published cellulose chart puts the material at 2.02 lb per square foot at R-49. That is about 2,424 lb spread over a 1,200 square foot ceiling, against roughly 835 lb of blown fiberglass for the same R-value. At R-60 the cellulose figure rises to 2.59 lb per square foot. Look from underneath in daylight first if the ceiling is lath and plaster, or if the drywall already sags between the framing.
Does cellulose burn?
It is shredded paper, so the borate fire retardant in it is doing real work, and that treatment is what ASTM C739 tests. Glass fiber does not burn at all, because glass is not fuel. Here is the part most comparisons miss. DOE's weatherization rules set fire requirements by where insulation goes, and for an attic floor they say material for exposed use there has to meet the same smoldering combustion requirement written for cellulose in that standard. So the floor of your attic is held to the cellulose bar whatever you put on it. Keep the clearance the label on each recessed light asks for, in either material.
How much does each one settle?
The published cellulose chart builds in about 10 percent. At R-49 it asks for 14.71 inches on the day to end up at 13.24. That is about 1.5 inches that disappear. Fiberglass settles less, which is why it is the one that can measure fine on install day and measure short a season later. Both are checked the same way: count the bags against the coverage chart for the product actually blown. Bag count is the one number settling cannot erase.
Is cellulose cheaper than fiberglass?
Usually, and you can settle it at the shelf in a minute rather than trusting anyone. Divide each bag price by the square feet that bag covers at your target R-value, which is printed on the bag. We hold one read fiberglass price, $67.58 for a 27.5 lb bag covering 39.5 square feet at R-49, which is $1.71 of material per square foot. A cellulose bag covering 14.9 square feet at R-49 matches that at about $25.49. Cheaper than that and cellulose wins on material. The read cellulose price we hold is a different bag, the 25 lb one, at $15.97 over its own R-49 coverage, or $1.09 to $1.13 a square foot.