Insulation Area Calculator

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.