Insulation being installed between framing

Reference

Batt vs blown-in insulation

Batts are pre-cut blankets you lay between framing; blown-in (loose-fill) is fiberglass or cellulose blown into place with a machine. Per inch they are close on the brand charts (generic loose fiberglass runs lower), so the choice is mostly about the space: an open wall or a clear joist bay suits a batt, a cluttered attic floor or a closed wall suits loose fill.

Which is better? For an unfinished attic floor, blown-in — it fills around wiring, bracing and odd joist spacing with no seams, and reaches R-49 or R-60 in one pass. For open walls and floors, batts. Per inch: fiberglass batts R-3.1, blown fiberglass R-2.9, blown cellulose R-3.5.

Batt vs blown-in insulation in an attic: depth and bags

Per 1,000 sq ft of attic floor. Blown-in bags are the manufacturer coverage charts (a 27.5 lb fiberglass bag, a 19 lb cellulose bag); cellulose is blown deeper and settles to about 90% of the installed depth. Batts assume joists 16 in apart: the first layer goes between the joists in 15 in packs, and above R-38 a second, unfaced layer crosses on top in 24 in attic packs. The R-values add.

Target	Blown fiberglass	Blown cellulose (installed → settled)	Fiberglass batts
R-30	10.75 in · 15 bags	9.6 → 8.6 in · 47 bags	10 in (R-30) · 18 packs
R-38	13.5 in · 19 bags	12 → 10.8 in · 62 bags	12 in (R-38) · 24 packs
R-49	17 in · 25 bags	15.2 → 13.7 in · 84 bags	16.25 in (R-19 between joists + R-30 across) · 26 packs, both layers
R-60	20.5 in · 32 bags	18.4 → 16.6 in · 109 bags	20 in (R-30 between joists + R-30 across) · 31 packs, both layers

Batt packs: 15 in batts cover 77.5 sq ft (R-19), 58.7 sq ft (R-30) and 42.7 sq ft (R-38) a pack; 24 in attic batts 80 sq ft (R-30). With joists 24 in apart the first layer uses the 24 in packs too. Blown-in bags are the charts' minimum counts — install to both the bag count and the depth.

R-value per inch: batts vs loose fill

Insulation	Form	R per inch	Notes
Fiberglass batts and rolls	Batt	R-3.1	Sold in 15 in and 23 in widths for 16 and 24 in framing.
Mineral wool batts	Batt	R-4.2	Sold in 15.25 in and 23 in widths for 16 and 24 in framing.
Blown-in fiberglass (loose fill)	Blown-in	R-2.9	From the Owens Corning AttiCat chart; generic loose fiberglass runs R-2.2–2.7.
Blown-in cellulose	Blown-in	R-3.5	Generic range R-3.2–3.8 settled; settles 10–20%.

The per-inch figures are the ones the insulation calculator uses. The full chart, foam boards and spray foam included, and the R-value your climate zone needs are in the R-value guide.

Where batts work and where blown-in works

Type	Where and how it goes in (U.S. Department of Energy)
Batts and rolls	Unfinished walls (including foundation walls), floors and ceilings, fitted between studs, joists and beams
Blown-in (loose fill)	Enclosed existing wall cavities or open new wall cavities, unfinished attic floors and hard-to-reach places

A batt only performs at its rated R-value when it fills the bay: cut around every wire and pipe, and don't squash it under boards. That is easy in an open stud wall and tedious across an attic full of wiring, which is where loose fill earns its keep — it flows around obstructions and leaves no seams. Batts laid over existing insulation, or as a second layer, must be unfaced (or have the facing slashed), and any kraft facing goes toward the heated side. Blown fiberglass settling is negligible; the chart depth is the finished depth; cellulose should be blown to the installed depth, because it settles to about 90% of it.

Whichever you choose, the DOE attic prep steps come first: seal the attic floor's air leaks; dam off or replace live knob-and-tube wiring; flag junction boxes; build dams taller than the finished depth around the hatch and every heat source — recessed lights, flues, chimneys, equipment — with the clearance your inspector requires; and fit eave baffles with at least a 1 in air gap. Mark the depth with a ruler every 300 sq ft when blowing. Pebbly gray-brown or gold granules are vermiculite, which may contain asbestos. Do not disturb it, vacuum it or blow new insulation over it until it has been tested — the EPA advises leaving it alone and calling a trained professional.

Count bags for your attic →

Figures: Where and how each type is installed: U.S. Department of Energy, Energy Saver "Types of Insulation" table (blanket batts and rolls: unfinished walls, including foundation walls, floors and ceilings, fitted between studs, joists and beams; loose-fill: enclosed existing wall or open new wall cavities, unfinished attic floors and hard-to-reach places). Bags per 1,000 sq ft straight from the manufacturer charts: Owens Corning AttiCat (15, 19, 25 and 32 bags at R-30, 38, 49 and 60) and Greenfiber INS541LD attic card (46.1, 61.1, 83.5 and 108.1 bags minimum). Attic prep: DOE SCEP job aids 4-1 Prepare Attic Floor and 6-1 Insulate an Unfloored Attic (June 2024): knob-and-tube wiring, junction boxes flagged, dams around all heat sources, baffles with at least 1 in air gap, one depth ruler per 300 sq ft, install to both the bag count and the depth. R-values per inch, depths, batt sizes and cellulose settling: the site's insulation data (resources/data/insulation.json), sourced there from Owens Corning, Johns Manville, Greenfiber, NREL, PNNL and DOE. Accessed 2026-10-31.

Good to know

Batt vs blown-in questions

For an open attic floor, usually yes: it covers wiring and odd joist spacing without gaps and reaches R-49 or R-60 in one layer. In open walls and floors, batts are simpler and cleaner. Per inch they are close — R-3.1 for fiberglass batts, R-2.9 for blown fiberglass, R-3.5 for cellulose.

17 in of blown fiberglass, 15.2 in of cellulose (settling to 13.7 in), or 16.25 in of batts laid as R-19 between joists + R-30 across.

Yes, if the old layer is dry. Existing batts count toward the total — R-values add — so measure what is there and blow enough on top to reach your target. If you add batts instead, use unfaced ones.

Cellulose settles 10–20%, so it is blown deeper than its rated depth. Blown fiberglass settling is negligible.

At R-49, 25 bags of blown fiberglass (Owens Corning AttiCat) or 84 bags of cellulose (Greenfiber), the charts' minimums.

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