Understanding your result
The headline is the depth of insulation to add, with the R-value and the bags. The stats give the existing R-value, the R-value to add and the bags. The breakdown shows the Energy Star recommendation for your zone, the existing R-value, the difference, the depth, the final depth, the bags and the depth markers.
The number to take to the store is the R-value to add, because the bag’s coverage chart is organised by R-value. The baffles that keep the insulation off the soffit vents come from the rafter baffles calculator, and the ventilation that goes with it from the attic ventilation calculator.
How we calculate this
The bag estimate, when no chart figure is entered, scales a typical coverage at R-38 inversely with the R-value added; it is a planning figure, and the chart on the actual bag is the one to buy by. The insulation guide has the zone map and the R-values per inch of common materials.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Recommended attic R-values | R-30 to R-49 (zone 1), R-49 to R-60 (zones 2–3), R-60 (zones 4–8) | Energy Star recommended home insulation R-values, uninsulated attic |
| R per inch | 2.5 blown fiberglass, 3.5 cellulose, 3.1 batts | Typical manufacturer data |
| Existing insulation | 3½ in at R-3 per in | Old fiberglass batts |
| Bag coverage | about 40 sq ft (fiberglass) or 25 (cellulose) per bag at R-38 | Typical bag charts; use yours |
| Depth markers | 1 per 300 sq ft | IRC N1102.2.1 |
Assumptions last reviewed October 7, 2026.
The calculator does not model compressed or wet insulation, cathedral ceilings or knee walls, and it does not price the job. Old vermiculite insulation may contain asbestos: do not disturb it, and see the EPA’s guidance before any work.
Two worked examples
Topping up a zone 5 attic
1,200 square feet, zone 5, target R-60, 3½ inches of old fiberglass at R-3, adding blown fiberglass.
- Existing: R-10.5; to add: R-49.5
- Depth: 49.5 ÷ 2.5 = 19.8 in; final depth 23.3 in
- Bags: about 40 (estimate); depth markers: 4
An empty zone 3 attic in cellulose
1,600 square feet, zone 3, target R-49, nothing there, cellulose with a bag chart of 24 square feet per bag at R-49.
- To add: R-49; depth: 49 ÷ 3.5 = 14 in
- Bags: 1,600 ÷ 24 = 67; depth markers: 6
Where to find your inputs
Attic floor area. The ceiling area below the attic.
Climate zone. From the Energy Star or Department of Energy climate zone map.
Existing depth and type. Measure in several places with a ruler; batts are usually fiberglass, loose grey fibres cellulose.
Bag coverage. On the bag’s chart, at the R-value you are adding.
Common mistakes
- Ignoring what is there. Measure it.
- Buying by the R-value printed on the front. Use the chart.
- Blocking the soffits. Baffles first.
- Burying recessed lights. Only IC-rated fixtures.
- Skipping air-sealing. Seal first.
- Disturbing vermiculite. It may contain asbestos.
Questions people ask
- How much attic insulation do I need?
- Energy Star recommends, for an uninsulated attic, R-30 to R-49 in climate zone 1, R-49 to R-60 in zones 2 and 3, and R-60 in zones 4 to 8. Subtract what is there already: 3½ inches of old fiberglass at about R-3 per inch is R-10.5, so reaching R-60 means adding R-49.5.
- How many inches of blown insulation is R-60?
- About 24 inches of blown fiberglass at R-2.5 per inch, or 17 inches of cellulose at R-3.5. The settled depth after a year or two is what counts, so the bag chart gives both installed and settled thickness.
- How many bags of blown insulation do I need?
- Read the coverage chart on the bag: it gives the square feet per bag (or bags per 1,000 square feet) for each R-value. The calculator estimates from a typical chart if you leave coverage at zero; for a 1,200 square foot attic adding R-49.5 of fiberglass the estimate is 40 bags.
- Fiberglass or cellulose?
- Cellulose has a higher R-value per inch (about 3.5 against 2.5 for blown fiberglass) and resists air movement better; fiberglass is lighter, does not absorb water and does not settle as much. Both work; follow the bag chart for the one you buy.
- What are attic depth markers?
- Rulers stapled to the trusses showing the installed depth of blown insulation, one per 300 square feet, facing the attic access, as the International Residential Code (N1102.2.1) requires so inspectors can check the depth.
- Should I air-seal before insulating?
- Yes. Gaps around top plates, light fixtures, plumbing and wiring and the attic hatch leak more heat than missing inches of insulation, and insulation does not stop air. Seal first, then insulate, then fit baffles at the eaves.