Cellulose vs Fiberglass Attic Insulation: Which Wins?

For most open attics, blown-in cellulose is the better buy, and blown-in fiberglass wins in a few specific situations, mainly damp attics and hot, humid climates. The material matters less than the prep underneath it, but if you're comparing quotes, here's how the two stack up.

What are you actually choosing between?

You're choosing between two loose-fill materials that do the same job differently: cellulose is shredded, borate-treated recycled paper, and fiberglass is spun glass fiber in a fluffy, loose form.

Both are blown over the attic floor through a hose, both fill around joists and wiring better than batts, and both can reach the R-value your climate calls for. The Department of Energy's recommended attic levels run from roughly R-30 to R-60 depending on climate zone, and most zones call for R-38 or more. Check your zone on the DOE Energy Saver site before you buy anything.

How do cellulose and fiberglass compare side by side?

Cellulose wins on R-value per inch and air resistance, while fiberglass wins on moisture tolerance and minimal settling.

Brand, density, and blowing technique move these numbers. The bag's coverage chart is the authority for your product, and it tells you how much to blow to land on your target R-value after settling.

Where does cellulose win?

Cellulose wins wherever air leakage and winter heat loss are the main problems, because it's denser and stops more airflow than fluffy fiberglass.

That density helps in cold climates, where air moving through a lighter layer cuts into its performance. It also packs around wiring, bracing, and duct runs more completely. If your attic has a finished floor or boxed-in cavities, dense-pack cellulose is the common way to insulate a closed space without tearing it open. The borate treatment helps it resist fire spread, insects, and mold.

The catch is settling. Cellulose compacts over the first years, so if an installer treats the target R-value as the starting depth, a nominal R-49 can end up closer to R-42. Ask for the initial depth, not just the R-value, and have the installer set depth rulers at the joists.

Where does fiberglass win?

Fiberglass wins in damp or humid attics, because it absorbs very little water and dries fast.

Wet cellulose holds water, loses R-value, and can sag and stain the ceiling below. A slow roof leak or a bathroom fan venting into the attic will do far more harm to cellulose than to fiberglass, which gives mold less to feed on. In hot, humid regions where attic moisture is constant, that's a real advantage. It's also noncombustible, which some inspectors like to see near chimneys and recessed fixtures.

It costs you depth. Expect to need somewhere around 30 to 40 percent more thickness for the same R-value, and the lighter fill is more exposed to air movement. In a leaky attic, that shows up on your energy bill.

How much does each cost installed?

A contractor typically charges roughly $1.30 to $3.00 per square foot for fiberglass and $1.50 to $3.50 for cellulose, so the premium for cellulose is usually small.

Price rises with your target R-value, attic access, and whether the crew has to haul bags up a ladder. A 1,200-square-foot attic brought to R-49 often lands between roughly $1,800 and $4,200 with either material. Local labor rates move that more than the material does.

The extras are where budgets blow up:

• Attic air sealing: often $300 to $1,500 depending on how leaky the ceiling plane is. Do it before any new insulation goes down.
• Rafter baffles: typically a few dollars each, to keep soffit vents open so the roof can breathe.
• Old insulation removal: roughly $1 to $2.50 per square foot if it's wet, moldy, or soiled by rodents.
• Fixture clearance: non-IC-rated lights can't simply be buried, so plan on recessed light covers or IC-rated replacements, and follow local code.
• Attic hatch weatherstripping and a rigid-foam cover: cheap, and often forgotten.

Protect the air sealing line in the budget. ENERGY STAR guidance recommends sealing air leaks before adding insulation, and sealing the ceiling plane can do as much for comfort as the insulation itself.

Can you DIY blow-in, and is one material easier?

Yes, you can DIY either one with a blower, and neither is clearly easier: cellulose is dustier and settles, while fiberglass needs more depth and more bags.

Many big-box stores lend an insulation blowing machine free or cheaply with a minimum bag purchase, so check your local store's terms. Budget roughly $0.60 to $1.50 per square foot for materials at typical depths. You'll want a helper at the machine, a long hose, and a plan: work from the farthest corner back toward the hatch, and set depth rulers before you start. Wear a respirator (N95 or better), eye protection, gloves, and coveralls, and lay plywood walkways across the joists so you don't step through the ceiling.

Keep loose-fill clear of chimney flues and combustion appliance vents, and check your local code for the required clearances. Adding loose-fill over an existing floor often needs no permit, but ventilation, framing, or electrical changes may.

Hire an attic insulation contractor if access is tight, the roof framing is complicated, or you need to dense-pack a floored space. Pros also use bulk machines that can blow cellulose at a more consistent density than a rental.

Which should you pick for your situation?

Pick cellulose for cold climates, leaky older houses, and floored attics, and pick fiberglass for hot humid climates and damp attics.

• Cold climate, open attic: cellulose, installed to the settled R-value. The density helps against airflow and winter heat loss.
• Hot-humid climate (Gulf Coast, Southeast): fiberglass, unless you're certain the roof and ducts stay dry.
• Mixed climate, older house with air leaks: cellulose, after air sealing.
• Floored or boxed-in attic: dense-pack cellulose.
• Existing batts underneath: leave them if they're dry, intact, and not compressed, and blow loose-fill over them. Slit or remove any paper or foil facing that could trap moisture, and pull anything wet, moldy, or rodent-soiled.

Our default recommendation for the most common case, a drafty 1970s to 1990s house in a cold or mixed climate, is cellulose with proper baffles and air sealing. Moisture-prone attics are the exception, and there fiberglass is the safer bet.

Your next step: measure your current depth, check your DOE climate-zone target, and get two quotes that list both initial and settled R-value, with air sealing as a separate line item.