Insulation Calculator

An attic or a 2x4 wall is a bag count only after you know how many square metres one bag covers at the thickness you are installing. This page starts from the Owens Corning R-13 unfaced 15 by 93 inch batt, which the Home Depot listing says covers 106.56 square feet, about 9.9 square metres. The R-value is the label of that product. Changing the label without changing the coverage does not change the bags.

Insulation bags are a coverage problem, and the coverage depends on the product and the installed thickness. A higher R-value usually means a thicker blanket and fewer square feet per bag (square feet per bag on the metric toggle), but that relationship is the manufacturer's, not a formula this page is allowed to invent. You type the square metres from the bag that matches the R-value you are installing. The R-value field is a reminder. Change it from 13 to 38 and, if the coverage field stays at 4, the bags stay at 5. If the bag says the higher R-value covers only 2 square metres, type 2. The honesty is in that edit.

Area is yours as well. An attic is the plan area minus the hatch, or the net of the joist bays if you are fitting batts between joists. A wall is the net wall from the room-addition or drywall thinking, minus openings you will not fill. Waste starts at 0. Raise it for angled cuts in a roof slope. Do not raise it to compensate for a coverage number you do not have. Blown-in insulation is often sold by the bag against a coverage chart that also states the settled thickness. Use that line. Do not use the open-blow coverage for a dense-pack wall.

There is no stored chart for any brand, and there is no conversion between US R-value and metric RSI inside the maths. If your bag is marked in RSI, you can still type that number in the label field as a note. It will not be converted, and it will not be checked. Bags round up. Twenty square metres at 4.5 square feet per bag (square feet per bag on the metric toggle) is 5 bags because 4.44 rounds up. Buy the same product you typed. A leftover bag of a different R-value is not half a bag of this one.

Drywall goes over the insulation and does not change this count. Paint and wallpaper cover the board, not the insulation. A dormer's sloping ceiling should be measured as slope area, which is larger than the floor of the dormer. The roofing calculator's slope factor is one way to grow a plan area. There is no price per bag.

Area starts at 1000 sq ft. Area to insulate in square feet. R-value label starts at 13. A label for the Owens Corning R-13 batt this coverage belongs to. Changing the label does not change the bags. Waste allowance starts at 0 %. Optional offcut allowance. It does not stand in for a missing coverage number. Coverage per bag starts at 107 sq ft. 107 sq ft is about 9.9 m². Owens Corning prints 106.56 sq ft for one bag of R-13 unfaced 15 in × 93 in batt, E51, checked 27 October 2026. A different batt has a different coverage.

An attic bay is often narrower than the joist spacing on the drawing. Measure the clear opening. A 400 mm centre with a 45 mm joist is not 400 mm of batt. Type the area you will actually fill, which is the clear width times the length times the number of bays. Rim joists and the hatch are separate small areas. Add them. If the bag chart lists coverage at several thicknesses, use the row for the thickness you will install, not the row for the maximum R-value the product can reach when blown deeper. The label field can hold the R-value you chose so the bag count stays attached to that row. It still will not do the multiplication. Blown-in coverage and batt coverage are different rows even on the same brand. Type the row you are installing.