Blown-in insulation for walls: how much you need (dense pack)

Blowing insulation into an existing wall is a different job from topping up an attic. In the attic, loose fill lies on the floor at a low density and you buy it by depth. In a closed wall it has to be packed in hard, at about 3.5 pounds per cubic foot, or it settles and leaves a cold gap at the top of every stud bay. That changes the maths: you work out the cavity volume, multiply by the packing density, and buy by weight. Here is the formula, two worked examples and a quick way to check you are packing it tight enough.

The short answer

• 2x4 walls (3.5-inch cavity): about 1.0 lb of dense-pack cellulose per square foot of cavity, roughly R-13.
• 2x6 walls (5.5-inch cavity): about 1.6 lb per square foot, roughly R-20.
• A standard 2x4 stud bay (14.5 inches wide, about 93 inches tall) takes about 9.5 lb, so a 25 lb bag fills about 2.6 bays.

These are planning figures for dense-pack cellulose. Always read the coverage chart on the bag you buy: bag weights and recommended wall densities differ between brands.

The formula

cavity area (sq ft) = net wall area × (1 − framing fraction)

pounds = cavity area × (cavity depth in inches ÷ 12) × density (lb per cu ft)

bags = pounds × 1.10 ÷ bag weight, rounded up

• Net wall area is length × height minus windows and doors.
• Framing fraction is the share of the wall taken up by studs, plates, headers and corners. Studs alone at 16 inches on centre are 1.5 ÷ 16 ≈ 9% of the wall; with plates and headers, 15% is a sensible planning number for an ordinary house wall.
• Density: dense-pack cellulose is commonly installed at around 3.5 lb per cubic foot. That is more than twice the density of loose fill on an attic floor, and it is what stops the material settling inside the wall.
• The extra 10% covers spillage, hose clear-out and the odd bay that takes more than you expect.

Example 1 — one exterior wall

A 40 ft long, 8 ft high wall of 2x4 framing, with two 3 x 5 ft windows and a 3 x 7 ft door:

• Gross area: 40 × 8 = 320 sq ft
• Openings: 2 × 15 + 21 = 51 sq ft, so the net wall is 269 sq ft
• Cavity area: 269 × 0.85 = 228.7 sq ft
• Volume: 228.7 × (3.5 ÷ 12) = 66.7 cu ft
• Weight: 66.7 × 3.5 = 233 lb
• With 10% extra: 257 lb ÷ 25 lb bags = 10.3, so 11 bags

Example 2 — a whole single-storey house

A house with 150 ft of exterior wall at 8 ft high, where windows and doors make up about 15% of the wall:

• Gross area: 150 × 8 = 1,200 sq ft; net of openings: 1,200 × 0.85 = 1,020 sq ft
• Cavity area: 1,020 × 0.85 = 867 sq ft
• 2x4 walls: 867 × 0.2917 × 3.5 = 885 lb; with 10% extra, 974 lb, or 39 bags of 25 lb
• 2x6 walls: 867 × 0.4583 × 3.5 = 1,391 lb; with 10% extra, 1,530 lb, or 62 bags

Notice how much the wall depth matters: the deeper 2x6 cavity takes 57% more material for the same wall. Measure the actual cavity depth through one of your drill holes before you order.

Check you are packing it dense enough

The most common DIY mistake is filling walls at attic density. It looks full on the day, then slumps over the next year or two and leaves an uninsulated strip under the top plate. Two checks:

1. Count bays per bag. A 2x4 bay of about 9.4 sq ft holds 2.7 cu ft of cavity, which is about 9.5 lb at 3.5 lb per cubic foot. If a 25 lb bag is filling four or five bays, you are packing far too loosely.
2. Feel the wall. A properly dense-packed bay feels firm when you push on the drill hole, and the blower note changes as the cavity fills and back-pressure builds.

Dense packing needs a strong blower and a fill tube that reaches to the top and bottom of each bay. Many home centres lend a blowing machine when you buy a minimum number of bags, but some light-duty machines struggle to reach dense-pack pressure. If the bags-per-bay count stays high, a contractor with a dense-pack rig is often the better value.

Before you drill

• Wiring: have an electrician look at any old knob-and-tube wiring before you insulate around it. Many local codes do not allow insulation in contact with it.
• Fire blocking and braces: horizontal blocks part-way up a bay split it in two, so each half needs its own hole or a tube that can get past.
• Moisture: fix leaks and failed flashing first. Insulation in a wall that gets wet does not dry out quickly.
• Hole size: fill-tube methods typically use holes of about 1 to 3 inches, drilled from inside through drywall or plaster, or from outside behind a lifted course of siding.

Cellulose or fiberglass in walls?

Both are blown into existing walls. Dense-pack cellulose is the more common retrofit choice because the packed fibres also slow air movement through gaps. Loose-fill fiberglass is far lighter per cubic foot, so the pounds per square foot above do not apply to it; use the wall chart on the fiberglass bag instead. Either way, the R-value depends on filling every bay completely, top to bottom.

What R-value should your walls have?

A full 2x4 cavity of dense-pack cellulose gives roughly R-13 and a 2x6 cavity roughly R-20, which is about the most a cavity can hold without adding continuous insulation outside the framing. Our guide to what R-value insulation you need lists the targets by climate zone, and how much insulation do I need covers batts, rolls and attic depths.

Our app: Smart Insulation Calculator

Smart Insulation Calculator is free on Android and in any browser at smartinsulationcalculator.com. It covers attics, walls, crawlspaces, basements and rim joists, works out batts, blown-in bags and spray foam board feet, and adds up the project cost. Doing the attic as well? The blown-in insulation calculator guide works through attic depth, bag counts and settling.