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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Cellulose is sold by the bag, and each bag covers less area the deeper you go — a 19 lb bag covers about 37 sq ft at R-19 but only 12 sq ft at R-49. This uses the manufacturer's coverage chart to count bags for your attic, tells you how deep to blow it, and sizes dense-pack cellulose for closed walls.
With these inputs:
| Result | Value |
|---|---|
| Bags of cellulose (19 lb) | 92 bags |
| Coverage per bag | 12 sq ft at R-49 |
| Blow it to (installed depth) | 15.2 in |
| Settled depth | 13.7 in |
| Weight on the ceiling | 1,753 lb |
| Installed cost | $1,796 – $2,848 |
Why cellulose needs more bags than people expect. Cellulose is dense. At R-49 the chart calls for about 1.6 lb per square foot, so a 19 lb bag covers only about 12 sq ft; at R-30 it covers about 22. A 1,000 sq ft attic taken from bare to R-49 needs 84 bags by the chart — about 92 once you allow 10% for uneven depth — and weighs about 1,750 lb. Many online rules of thumb assume older 25–30 lb bags or much lighter fiberglass and come out at half that. The chart gives two thicknesses. Blow to the initial installed depth (15.2 in for R-49); within a few months it settles to the minimum settled depth (13.7 in), which is what delivers the rated R-value. Staple depth rulers to trusses across the attic before you start so you can see the depth from the hatch. Dense-pack walls. In closed walls, cellulose is blown through 1–2 in holes at about 3.5 lb per cubic foot so it cannot settle and leave a gap at the top of the cavity. A 2×4 wall takes a little over 1 lb per square foot of wall, so 1,000 sq ft of wall needs about 52 bags. Dense-packing needs a high-pressure blower and a fill tube; the free big-box machines are made for loose fill in attics.
Coverage per bag = interpolated from the 19 lb bag chart (R-19 36.7 · R-30 21.7 · R-38 16.4 · R-49 12.0 · R-60 9.3 sq ft) · Bags = area ÷ coverage per bag × (1 + extra %) · Depth = chart settled and installed thickness at your R-value · Walls: bags = wall area × 0.88 × cavity depth ÷ 12 × 3.5 lb ÷ 19.05 lb × (1 + extra %)
| R-value | Sq ft per bag | Bags (chart) | Bags +10% | Settled depth | Blow to |
|---|---|---|---|---|---|
| R-19 | 36.7 | 27.2 | 30 | 5.6 in | 6.2 in |
| R-30 | 21.7 | 46.1 | 51 | 8.6 in | 9.6 in |
| R-38 | 16.4 | 61 | 68 | 10.8 in | 12 in |
| R-49 | 12 | 83.3 | 92 | 13.7 in | 15.2 in |
| R-60 | 9.3 | 107.5 | 119 | 16.5 in | 18.4 in |
From GreenFiber's attic card for its 19.05 lb bag (INS541LD). Other brands and bag sizes differ — the chart on your bag decides.
Divide the attic area by the bag's coverage at your R-value. For a 19 lb bag that is about 36.7 sq ft per bag at R-19, 21.7 at R-30, 16.4 at R-38, 12.0 at R-49 and 9.3 at R-60. So 1,000 sq ft at R-49 is 84 bags, or about 92 with 10% extra.
At R-38, about 92 bags (101 with 10% extra); at R-49, 125 bags (138 with extra); at R-60, about 162 bags (178). That is for common 19 lb bags; check the coverage chart on your bag.
Blow it to about 15 inches; it settles to about 13.7 inches, which is the depth that delivers R-49. For R-38 blow about 12 inches (10.8 settled), and for R-60 about 18.4 inches (16.5 settled).
It reaches a given R-value in about 30% less depth and resists air movement better; fiberglass is lighter, does not absorb water and settles less. Both perform well installed to the chart. See the fiberglass vs cellulose guide for cost and moisture trade-offs.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.