Bags of cellulose or fiberglass from area & R-value
This blow-in insulation calculator tells you how many bags of loose-fill insulation to buy for your attic. Enter the square footage, your target R-value, and the product's bag coverage, and it returns bags needed, depth to blow, and estimated material cost — adjusted for any insulation you already have.
Blown-in insulation is the standard retrofit for attics because it fills irregular joist bays and around obstructions without dismantling anything. Cellulose insulates at about R-3.7 per inch, and loose fiberglass at R-2.9–3.1 per inch, so hitting a target like R-49 takes roughly 13.2 inches of cellulose or more of fiberglass. Bag counts come from the coverage chart printed on each manufacturer's bag, which is legally required to state settled coverage.
Depth = target R ÷ R-per-inch for your material, so R-49 with cellulose means 49 ÷ 3.7 ≈ 13.2 inches. If existing insulation is present, its equivalent R-value is subtracted and the calculator sizes only the top-up. Bags = area ÷ coverage per bag at the target R — coverage at R-49 typically runs 40–67 sq ft per bag depending on brand and settling characteristics.
As a rule-of-thumb cross-check, bags ≈ area × depth ÷ bag yield, where a typical cellulose bag settles to about 6.5 cubic feet. If your chart-based and rule-of-thumb numbers disagree wildly, re-read the coverage chart row — charts list multiple thicknesses and it is easy to read the wrong line.
The table below shows typical settled R-per-inch values and the approximate depth each target R-value needs. Always use your specific product's label for the final number — the chart on the bag is the legally binding coverage statement.
| Material | R per inch | Depth for R-49 (approx.) | Depth for R-60 (approx.) |
|---|---|---|---|
| Cellulose (blown) | R-3.7 | 13.2" | 17–18" |
| Fiberglass (blown) | R-2.9–3.1 | 16–17" | 21–22" |
Blowing is the last step. Air-seal first: top plates, wire and pipe penetrations, and the chimney chase leak more heat than missing insulation ever will, and insulation does not stop air movement. Mark attic rulers or joist markers every few feet so you can verify settled depth from above; cellulose settles about 20% and the manufacturer's chart already accounts for it.
Keep insulation away from heat-producing fixtures unless they are IC-rated, maintain clearances at flues using metal dams, and confirm the attic has open vent chutes at the eaves so airflow from soffit to ridge stays unblocked. One extra bag on hand beats stopping mid-job, and any leftover unopened bag is returnable.
R-value targets depend on climate zone, and building codes set minimums for new construction. Most US zones call for attic insulation in the R-38 to R-60 range, with the coldest zones at the top of that band. Diminishing returns apply: going from an uninsulated attic to R-30 captures the large majority of the possible benefit, and each additional R-value beyond that saves progressively less. R-49 is a common deep-retrofit target that balances cost and performance.
Ventilation and insulation have to be designed together. A properly vented attic stays near outdoor temperature, with insulation on the attic floor keeping the living space below warm; baffle chutes at the eaves keep the air path open over the new insulation. Alternatively, a conditioned attic moves the thermal boundary to the roof deck, but that requires air-impermeable foam or dense-packing under the sheathing and is a bigger project than blowing the floor.
Bag economics reward accurate measurement. Blown cellulose typically costs a fraction of sprayed foam per square foot at equivalent R-value, and renting a blower machine is commonly free or cheap with a large-enough material order. Dense-packing walls or cathedral ceilings is a professional job with different density targets — roughly 3.5 lb/ft³ — whereas attic loose-fill is well within a careful DIYer's reach with two people: one feeding the hopper, one directing the hose.
Safety basics for the DIY blower: wear a dust mask and eye protection (cellulose dust is fine but persistent), lay walking boards across the joists rather than trusting the ceiling drywall, and keep the hose moving for an even blanket. Check depth at the rulers from several points before closing up, and photograph the attic for future reference.
How many bags of insulation do I need for 1,000 sq ft at R-49?
Roughly 17–25 bags of cellulose, since coverage runs 40–67 sq ft per bag at R-49 depending on product. Check the coverage chart printed on the bag — it is legally required to state settled coverage.
How deep is R-49 blown insulation?
About 13.2 inches of cellulose (49 ÷ 3.7) or 16–17 inches of loose fiberglass (R-2.9–3.1 per inch).
Does blown insulation settle?
Yes — cellulose settles about 20%, and the manufacturer's coverage chart already accounts for it. Blow to the chart's minimum thickness, marked on attic rulers stapled to the joists.
Can I blow new insulation over existing?
Yes, if the existing layer is dry and unfaced (no vapor barrier in the middle). Subtract its R-value from your target and blow the difference on top.
Cellulose or fiberglass — which should I choose?
Cellulose insulates better per inch (about R-3.7 vs R-2.9–3.1) and denser, so it resists air movement better; fiberglass is lighter and often cheaper per bag. Both perform well when installed to the labeled depth.