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Fiberglass vs Cellulose

What they are

Fiberglass insulation is made from spun glass fibres. As loose-fill it looks like fluffy white, pink or yellow tufts; as batts it comes in rolls or pieces. Cellulose is made mostly from recycled newsprint or cardboard ground into fibres and treated with fire retardants such as borate compounds. It is blown as loose-fill or dense-packed into walls. Both are common, widely available and accepted by building codes when installed to the manufacturer's instructions.

Comparison

Depth and weight

Because cellulose has a higher typical R per inch, you need fewer inches for the same R, but it weighs more. Very deep cellulose over thin drywall ceilings, especially with wide joist spacing, can be a concern; manufacturers list maximum coverage weights and ceiling requirements. Fiberglass needs more depth for the same R; make sure baffles and hatch dams are tall enough.

Settling

Cellulose settles more after installation. Bag charts list both installed (initial) thickness and settled thickness. Blow to the installed thickness so you end up at the settled R. Fiberglass loose-fill settles less but still has a minimum thickness on the chart. Use depth markers either way.

Moisture and leaks

Cellulose can absorb water from roof leaks or condensation and may compact, lose R and promote mould. Fiberglass does not absorb much water, but wet fibreglass still performs poorly until dry. Either way, fix leaks and ventilation problems first.

Air sealing

Dense cellulose slows air movement more than loose fiberglass, which some studies have found helps performance in cold weather. But neither replaces proper air sealing at the ceiling plane; see air sealing first.

Environmental and health notes

Cellulose has high recycled content. Fiberglass also often uses recycled glass. Both produce dust during installation; wear PPE. Neither should be confused with vermiculite, which looks like small pebbles and may contain asbestos.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: target R49 total over 1,000 sq ft, with R10 already present. To add R39: fiberglass loose-fill at an assumed R2.5/in needs 15.6 in; cellulose at an assumed R3.5/in needs about 11.1 in settled. If the cellulose bag chart says 12.5 in installed for that R, blow to 12.5 in so it settles to about 11 in.

How to choose

• Choose cellulose if you want fewer inches, extra air resistance and high recycled content, and your ceiling can carry the weight.
• Choose fiberglass if leaks are a worry, weight matters, or the attic has limited depth at the eaves but your baffles allow extra height.
• Get the bag chart before buying either and run the numbers with the attic insulation calculator.

Installation differences

Fiberglass loose-fill is light and blows quickly; it is easy to blow too fluffy, which lowers R per inch and density. Cellulose is heavier and needs more machine power, and more dust control at the hopper. Both need the correct machine settings from the manufacturer.

Over the years

Cellulose's settling happens mostly in the first months. Fiberglass stays near installed depth. Both can be disturbed by people working in the attic.

Fire safety

Fiberglass is non-combustible glass fibre, though binders and facings can burn. Cellulose is treated to meet fire standards. Neither should be in contact with chimneys, flues or non-rated recessed lights; follow code clearances and labels.

Practical tips

• Buy all bags from the same product line so the bag chart applies across the attic.
• Keep bags dry before use.
• Blow from far to near.
• Check depth markers often.
• Keep the soffit baffles clear.

Mixed attics

Many attics end up with cellulose over old fiberglass batts or vice versa. That works as long as the existing layer is dry and safe. Estimate the old layer's R and add the new layer to reach your target with the attic insulation calculator.

Health and handling

Both release dust during installation. Fiberglass fibres can irritate skin, eyes and throat; cellulose dust is irritating too. Wear a fitted respirator, eye protection, gloves and long sleeves, and follow product safety data sheets. Once installed and undisturbed, both are generally stable. People with respiratory conditions should take extra care or hire a pro.

Moisture

Cellulose absorbs moisture and can hold it; prolonged wetting can cause clumping and mould. Fiberglass does not absorb water but loses R when wet. In both cases, fix leaks quickly and remove soaked insulation.

Cost comparison, illustrative

Illustrative example (2026-10-29): assume fiberglass loose-fill at $0.55 per sq ft for R-30 and cellulose at $0.50 per sq ft for R-30, materials only. For a 1,000 sq ft attic, that is $550 vs $500. These are invented numbers for showing the method; check current local prices and bag charts.

Bottom line

Both work well when installed to the right depth over a sealed ceiling. Pick by local price, availability, machine access and your preference on dust and settling.

Quick tip

Keep one product line per attic so bag charts stay consistent.

FAQ

Can I put cellulose over fiberglass?

Yes, if the existing layer is dry and clean.

Which is cheaper?

Prices vary by region; compare cost per R using your own prices.

Does cellulose burn?

It is treated with fire retardants and must meet fire standards.

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