Blown-In Insulation Cost: Fiberglass vs. Cellulose Loose Fill

Blown-in (loose-fill) insulation is machine-blown into an attic floor or wall cavity, where it fills around framing and irregular spaces that batts struggle with. The two common materials are loose-fill fiberglass and cellulose (recycled paper fiber). Cost is almost always thought of per square foot, but the real driver underneath that number is the depth you blow to reach your target R-value.

Because cellulose delivers more R per inch than fiberglass, it typically needs less depth — and sometimes fewer bags — to hit the same R-target. All unit prices here are editable planning assumptions, not verified national averages. Replace them with your own local quotes before you commit a budget.

Two homes with the same square footage can land at very different numbers depending on material, target R-value, and whether a pro is involved. The table below shows the relative cost drivers as planning ranges — not fixed prices:

The biggest swing factors are: the R-target (a colder climate zone pushes it higher, which means more material), the area covered, access (a cramped or obstructed attic slows a crew), and whether you pay for a DIY blower rental or a professional crew. Prep such as air sealing is a separate line item — price it on its own rather than folding it into the per-square-foot figure.

Your target R-value is not a single universal number — it depends on your climate zone and local building code. Attic targets commonly fall somewhere between roughly R-38 and R-60 depending on region, but you should confirm the figure with your local building department rather than assuming one value.

Once you know the target, the required depth follows from the material's R-per-inch. As a rough planning illustration, hitting a given R-target takes noticeably more depth with fiberglass (~2.5 per inch) than with cellulose (~3.5 per inch). Bag labels list a coverage chart — square feet per bag at a stated installed thickness and minimum bag count per 1,000 sq ft to reach a specific R. Follow that chart; blowing too shallow quietly short-changes your R-value, and compressing loose fill reduces its rated performance.

Cellulose settles after installation, so coverage charts tell you to blow to a higher initial depth than the final settled depth. Plan for that so your settled R still meets the target.

Loose-fill is one of the more DIY-approachable insulation jobs: many home centers rent or loan a blowing machine with a bulk bag purchase. A DIY run can lower the per-square-foot figure, but it is a two-person job — one feeds the hopper, one works the hose in the attic — and the labor is hot, dusty, and awkward in a confined space.

A professional crew brings a commercial blower, hits the coverage chart consistently, and finishes an attic quickly. Pay for a pro when the attic is hard to access, when old insulation must be removed first, when air sealing and depth markers need to be done correctly, or when you simply do not want to work in the space. Get the coverage/bag-count math right either way — it is what determines whether you actually reach your R-target.

Adding insulation itself rarely needs a permit, but related conditions can stop a DIY job cold. Before you blow anything, check for these and bring in a qualified professional if you find them:

• Pre-1978 homes: older materials and finishes can contain asbestos or lead; disturbing them is a job for licensed specialists.
• Knob-and-tube wiring: this older wiring relies on open air to shed heat. Burying it in loose fill is a recognized fire hazard — have an electrician evaluate or replace it first.
• Moisture or mold: blowing insulation over a damp deck, a roof leak, or existing mold traps the problem and can make it worse. Find and fix the water source first.
• Recessed lighting and flues: non-IC-rated fixtures and heat-producing flues need proper clearance or shields before you bury the area.

Treat these as safety gates, not optional upgrades. When in doubt, have a professional inspect the attic before any material goes in.

• Pricing by the bag instead of by coverage — the depth needed for your R-target is what sets the real bag count.
• Assuming fiberglass and cellulose cover the same depth; they do not, because R-per-inch differs.
• Ignoring settling with cellulose and blowing to the final depth instead of the higher initial depth.
• Skipping air sealing, so blown-in material sits over gaps that undercut its performance.
• Treating a national "average" as your quote. Always confirm locally.