As an Amazon Associate I earn from qualifying purchases.
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.
Every other insulation number starts with square footage. Enter your room dimensions and this figures the flat (attic/floor) area, the wall area, and the net area after you deduct doors and windows.
With these inputs:
| Result | Value |
|---|---|
| Flat area (attic / floor) | 384 sq ft |
| Gross wall area | 640 sq ft |
| Net wall area after openings | 580 sq ft |
| Order quantity with waste | 638 sq ft of wall |
| Wall perimeter | 80 ft |
Flat area = length × width · Gross wall area = 2 × (length + width) × wall height · Net wall area = gross wall area − door & window area · Order quantity = net area × (1 + waste %)
Deduct them from the amount you install, but it is normal to still buy the gross figure when the openings are small — offcuts get used in narrow bays and around boxes.
10% is standard for batts in regular 16-inch bays. Go to 15% for irregular framing, cathedral ceilings or lots of obstructions.
Both, separately, because they are different numbers and take different products. Attic and floor insulation is sized on the flat area, which is simply length times width. Walls are sized on perimeter times height, which for a long narrow room is far more than the floor area. A 30 by 10 room has 300 sq ft of floor and 640 sq ft of wall at 8 ft.
Measure along the slope rather than across the floor beneath it. A pitched surface is longer than its horizontal run, so using the footprint under-orders — on a 6 in 12 pitch by about 12 percent. Measure one rafter from wall to ridge and multiply by the length of the room.
Only if the attic is conditioned space. In a vented attic the insulation goes on the flat ceiling and the gable walls stay outside the thermal envelope, so they are not part of the area. In a conditioned attic or a roofline assembly the gables are inside and must be counted, which is one reason roofline insulation costs noticeably more.
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.