Batt vs Blown-In Insulation

Attic framing is full of odd bays, cross-bracing, wires and truss webs. Batts leave gaps around all of it, and gaps are what kill performance. Loose fill flows around everything and self-levels.

In clean, uniform framing — new construction walls, standard joist bays — batts installed carefully perform just as well.

DIY material for a 1,500 sq ft attic at R-49 is about 125 bags of cellulose or 38 bags of blown fiberglass before waste — roughly $0.90–$1.60 per sq ft at retail bag prices, and the blower is usually free with the bag order. Two people blow it in 4–6 hours. Batts for the same attic cost more in material — about $2 per sq ft at R-49 on this site's fiberglass batt prices — and take a full weekend of cutting and fitting, and they need two layers (for example R-30 between the joists and R-19 across them) to reach R-49.

Hired out, the gap is small. The national baseline below is what the cost calculators on this site use before your state's labour index.

Blown-in is sold by the bag, and each bag covers less area the deeper you blow it. Divide your attic floor area by the square feet per bag that the bag chart lists for your target R-value — the table below does it for 1,000 sq ft. Fiberglass needs fewer, lighter bags but a deeper layer, because it is about R-2.5 per inch against cellulose's R-3.6 to R-3.7.

Batts are counted by area, not depth: area ÷ the square feet printed on the bag. An R-13 bag for 2x4 walls typically covers 88–106 sq ft and an R-30 attic bag 40–58 sq ft, so 400 sq ft of open 2x4 wall takes 5 bags of 88 sq ft (42 batts of 15 × 93 in) before waste.

Blown-in for walls means dense-pack: cellulose blown at about 3.5 lb per cubic foot into closed cavities through drilled holes. The same 400 sq ft of 2x4 wall is about 103 cu ft of cavity (88% of the wall is cavity, 3.5 in deep), or 359 lb — 19 bags of 19 lb cellulose for roughly R-13. It needs a high-pressure blower and a fill tube, not the free big-box attic machine.

If you need attic storage, batts between joists plus a raised platform keeps things tidy. Deep loose fill and storage do not mix — compressing loose fill with plywood destroys its value.

Many good attics use both: batts laid between joists, then blown insulation over the top to bury the thermal bridging of the framing. That combination is hard to beat.