Blown-In Cellulose or Fibreglass for Your Attic?

Both blown-in cellulose and loose-fill fibreglass will bring an attic up to code when they are installed to the right depth over a sealed ceiling. Cellulose packs more R-value into each inch and resists air movement a little better, while fibreglass is lighter, settles less and shrugs off moisture. For most open attic floors the choice matters less than the depth, the air sealing underneath and the eave baffles, but there are a few houses where one material is clearly the better fit.

Here is how the two compare on the points that show up in real quotes.

What each material is

Cellulose is shredded paper, mostly recycled newsprint and cardboard, treated with borate compounds for fire and pest resistance. It is blown through a hose and lands as a dense, grey, fluffy layer that fills around wires, truss webs and junction boxes.

Loose-fill fibreglass is glass spun into fine fibres and chopped into small tufts. It is usually white or pink, much lighter by volume, and goes in with the same kind of blowing machine. It is not the same product as fibreglass batts, which are the pre-cut rolls often found stuffed between joists in older attics.

R-value per inch

Cellulose typically lands at about R-3.2 to R-3.8 per inch once settled. Loose-fill fibreglass varies more by product, usually around R-2.2 to R-2.9 per inch. Each bag label states how many inches and how many bags per 1,000 square feet are needed to reach a given R-value, and a good installer follows that chart rather than eyeballing it.

What this means for you:

• To reach R-60, cellulose needs roughly 16 to 19 inches of settled depth.
• Fibreglass needs roughly 20 to 26 inches for the same R-60, depending on the product.

That extra depth is rarely a problem in a tall attic. It becomes one near the eaves on low-slope roofs, in shallow attics over bungalows and older urban houses, and anywhere the insulation has to stay below the top of a baffle.

Settling and depth markers

Cellulose settles after it is blown in, often by 10 to 20 percent over the first months. Manufacturers account for this by listing an "initial installed thickness" that is higher than the "settled thickness." If the crew blows only to the settled number, the attic ends up short.

Fibreglass settles much less. That makes it a bit more forgiving if the installer is in a hurry, but it does not remove the need for depth rulers. Either way, ask for attic rulers (cardboard depth markers) stapled to framing every few hundred square feet so you can check the depth after the job, and ask for the bag count on the invoice.

Air movement and very cold attics

Neither material is an air barrier. Warm, moist air leaking up from the house will move through both of them, which is why the attic air sealing checklist comes first on any job.

Cellulose is denser, so it slows air drifting through the layer itself more than light fibreglass does. Building science research has shown that very light loose-fill fibreglass can lose some of its effective R-value in extremely cold attics, when air starts circulating within the insulation layer. Newer fibreglass products are blown denser than older ones, and adding depth helps, but in the coldest parts of Canada and the northern US this is one reason many contractors lean toward cellulose for attic floors.

Moisture and roof leaks

This is where fibreglass has the edge. Glass fibres do not absorb water. If a small roof leak or a bath fan dumping into the attic gets the insulation damp, fibreglass dries out and usually recovers once the source is fixed.

Cellulose absorbs moisture. A small amount is not a disaster, and cellulose can buffer brief humidity, but a persistent leak can leave it matted, heavy and prone to mould on the wood around it. Wet cellulose usually needs to be removed and replaced.

Neither material forgives a moisture problem for long. Bath and kitchen fans should vent outside, not into the attic, and roof leaks should be fixed before any new insulation goes in.

Weight on the ceiling

Cellulose is several times heavier than fibreglass for the same volume. On most houses with 16-inch joist spacing and standard drywall, an R-60 layer of cellulose is fine. Where ceilings are 1/2-inch drywall on framing spaced 24 inches apart, or old plaster that is already sagging or cracked, the added weight deserves a look. Some contractors use fibreglass in those attics, or recommend checking the ceiling before going to full depth.

Fire and pests

Cellulose's borate treatment gives it a fire rating that meets code for attic use, and borates also discourage insects. Fibreglass does not burn. Both are acceptable over typical attic framing.

Neither one should be blown against chimneys, recessed lights not rated for insulation contact, or live knob-and-tube wiring. Those need dams, covers or an electrician's sign-off first, whichever material you choose.

Rodents will nest in either one, so sealing their entry points matters more.

Cellulose is also dustier to install, so the crew should protect the hatch area and keep the blowing machine outside. Both usually take a day or less once the air sealing is done.

Typical cost ranges

Prices change with region, attic size, access and how much prep is needed, so treat these as typical ranges rather than quotes:

• Adding blown-in insulation to bring an existing attic up to R-49 or R-60 often lands around $1.50 to $3.50 per square foot of attic floor in many US and Canadian markets, including labour.
• Cellulose and fibreglass are usually close in price for the same R-value. Cellulose often uses fewer bags because of its higher R per inch, fibreglass needs more depth but the material is lighter.
• Air sealing, baffles, hatch insulation and removing old wet or rodent-damaged insulation are often priced separately and can add more to the total than the choice of material.

When you compare quotes, compare the target R-value, the settled depth, the bag count and what prep is included, not just the material name.

Which one fits your house

Cellulose is often the better pick when:

• the attic is open, tall and easy to reach
• you live in a very cold climate and want the most R-value per inch
• the attic is shallow or has tight eaves, where fewer inches for the same R-value helps
• the ceiling framing is sound and spaced at 16 inches

Fibreglass is often the better pick when:

• there is a history of small roof or condensation problems still being monitored
• the ceiling is older plaster or 1/2-inch drywall on 24-inch spacing
• someone in the house is sensitive to dust during installation

Plenty of attics end up with one material topped over the other, such as fresh cellulose blown over old fibreglass batts. That is generally fine as long as the old layer is dry, not soiled and not hiding vermiculite.

Frequently asked questions

Is cellulose or fibreglass better for attic insulation?

Neither is better everywhere. Cellulose gives more R-value per inch and slows air movement within the layer more, which helps in very cold attics. Fibreglass is lighter, settles less and handles occasional moisture better. A sealed ceiling and the correct depth matter more than which one is used.

Can you blow cellulose over existing fibreglass insulation?

Yes, in most cases. New cellulose can go over old fibreglass batts or loose-fill as long as the existing layer is dry, free of rodent contamination and not compressed by stored items. The air sealing should be done first, which may mean moving the old insulation aside at leak points.

How long does blown-in attic insulation last?

When it stays dry and undisturbed, both cellulose and fibreglass can last for decades. Roof leaks, rodents, foot traffic and venting bath fans into the attic are what usually shorten their life. Checking the depth markers and looking for damp spots every few years is a good habit.

Does cellulose insulation attract mould?

Dry cellulose does not support mould growth on its own, and its borate treatment helps. When it stays wet from a roof leak or condensation, the cellulose and nearby wood can develop mould. Fixing the moisture source before insulating is what prevents it.

Get a quote for your attic

If you are not sure what is in your attic now or how deep it is, a short inspection settles it. Fill out the quote form with your city, the type of house and what you are noticing, such as cold rooms, ice dams or high heating bills. A specialist who works in your area checks the attic and gives you a written quote with the material, target R-value, settled depth and air sealing spelled out. For basement walls, see the spray foam comparison.