Cellulose or fibreglass: picking blown-in insulation

What are cellulose and fibreglass insulation?

Cellulose is made mostly from recycled paper, shredded and treated with fire-retardant chemicals such as borates. It is blown loose into attics or dense-packed into closed wall cavities. Fibreglass is made from spun glass fibres and comes as batts and rolls, or as loose-fill blown into attics and walls. Both are sold in Canada through installers and building suppliers, and both come in forms for do-it-yourself and contractor work.

How do they compare on R-value?

Per inch, the figures for the two products overlap, with cellulose often a little higher than loose fibreglass and some high-density batts higher still. Because the numbers vary by product and density, rely on the labelled R-value at the installed thickness rather than a rule of thumb. What matters more is how closely the material fills the space. Gaps, compression and voids cut real performance, a point developed in R-values by region. In a blown attic, thickness markers and coverage give a more reliable check than the brand name.

Which resists air movement and moisture better?

Dense cellulose slows air movement through the material better than light fibreglass, which is one reason it is popular in wall retrofits. It can absorb some moisture and release it again, although a persistent leak will damage either product. Fibreglass does not absorb water but loses performance when wet or compressed. Neither replaces air sealing. Warm air rising through gaps will bypass both, so the sealing work in the air sealing guide comes first.

What about fire, settling and health?

Fibreglass will not burn, while cellulose is treated to resist fire and flame spread and has to meet product standards. Cellulose settles over time, so installers blow it a little deeper to reach the target, and it should be checked after some years. Fibreglass can irritate skin, eyes and lungs during handling; gloves, long sleeves, goggles and a suitable mask are sensible for either product. Keep insulation clear of recessed lights not rated for contact and of chimneys and flues, and keep ventilation paths open at the eaves.

Which one should you choose?

Choose by job. For an open attic, either blown product works, and the lower quote from a competent installer who seals first is often the best choice, as laid out in attic insulation in Canada. For existing walls, dense-packed cellulose is widely used. For rim joists, basements and thin roof sections, foam may suit better, which is explored in spray foam insulation. If you are replacing old material, read when to replace attic insulation.

Prices change with region, season and product, and only written quotes give a number. Compare the same R-value, coverage area and warranty, and check what the quote says about preparation. The upgrade priorities guide helps put the spend in the right order.

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