Glossary · Insulation & windows

Blown-in insulation (cellulose, fiberglass)

Also called: blow-in insulation · loose-fill insulation · loose fill · blown cellulose · cellulose · cellulose insulation

Loose fibers of cellulose, fiberglass or mineral wool blown into attics or wall cavities by machine; fast, and good at filling around obstacles.

What it means

Cellulose is made mostly from recycled paper treated with borates for fire and pest resistance; fiberglass and mineral wool loose-fill are spun glass or rock fibers. In attics it's spread in an even layer to the target R-value, and in closed walls it's blown densely through small holes (see dense-pack insulation). Air sealing should be done first, and baffles installed at the eaves.

Why it matters in an audit

Topping up attic insulation with blown-in material is a common audit recommendation and qualifies for rebates in many programs.

Typical numbers

Reference values with their sources. Your auditor’s report gives the figure for your own house.

What	Value	Source
R per inch (NRCan design values)	Blown cellulose R-3.6 (range R-3.0–3.8); blown fiberglass R-2.9 (R-2.7–3.6); blown mineral wool R-3.0	natural-resources.canada.ca ↗

Sources

Learn more here

Attic and insulation inspection →

Related terms

R-value How well a material resists heat flow; higher is better, and insulation is rated with it, such as R-20 for walls or R-50 for attics.
Air sealing Blocking the hidden gaps where air leaks in and out of a house, which cuts drafts, heating and cooling costs, and moisture problems.
Soffit vent and baffle Soffit vents let outside air into the attic at the eaves, and baffles keep insulation from blocking that airflow.
Vermiculite insulation (Zonolite) A pebbly, brownish or silvery loose-fill attic insulation that may contain asbestos from a Montana mine, so it shouldn't be disturbed.
Spray foam (open-cell and closed-cell) Liquid foam sprayed in place that expands to insulate and air seal in one step; closed-cell is denser and higher R, open-cell lighter.
Attic bypass A hidden opening from the living space into the attic, around chimneys, plumbing stacks, wiring, pot lights or wall tops, that leaks warm air.
Dense-pack insulation Loose-fill insulation, usually cellulose, blown tightly into closed wall cavities through small holes so it fills them, resists settling and slows air movement.
Knob-and-tube wiring An early type of house wiring that runs separate wires on porcelain knobs and through porcelain tubes, with no ground wire; insulation must not cover it.

← Every term, A to Z