Insulation calculator for batts, rolls and boards

How the insulation calculator works

Take the area to insulate, add a margin for cuts and divide by the coverage of one bag, roll or board, rounding up. Batts and rolls are counted by the bag, using the square feet printed on it. Rigid foam boards are counted board by board, then by the package if they come bundled.

1. Area. Length × height of each wall, added up, minus doors and windows.
2. Extra for cuts. We add 10%, the minimum the UK merchant Materials Market recommends for both Knauf rolls and Recticel boards, to cover errors, damage and obstacles.
3. Batts and rolls. Area with the extra ÷ coverage of one bag, rounded up.
4. Boards. Area with the extra ÷ area of one board, rounded up. If the boards come in packs, divide by the boards per pack and round up again.
5. Fasteners. Owens Corning estimates 64 cap nails per 4' × 8' FOAMULAR board. For external wall insulation in Europe, Fassa Bortolo specifies 6 fixings per m² on its EPS boards, which is 3 per 100 × 50 cm board. Fasteners are counted on the installed area, without the extra.

The thickness does not change how many boards cover a wall, but it decides the R-value and how many boards fit in a pack, so the calculator asks for it.

Worked example: a 32 × 8 ft wall

These are the values the calculator opens with in feet: one 32 × 8 ft wall, 2 in foam boards of 4' × 8' bought by the sheet, 64 cap nails per board, 10% extra.

• Area: 32 × 8 = 256 sq ft
• With the extra: 256 × 1.10 = 281.6 sq ft
• Boards: 281.6 ÷ 32 = 8.8, so 9 boards
• Cap nails: 256 ÷ 32 = 8 boards installed, × 64 = 512 cap nails

With batts, a 16 × 8 ft wall is 128 sq ft, 140.8 sq ft with the extra. R-13 batts at 106.56 sq ft per bag give 1.32, so 2 bags.

In the UK, with metres: a 40 m² loft floor is 44 m² with 10% extra. Knauf Loft Roll 44 in 100 mm covers 13.89 m² per roll, so you need 4 rolls.

How many bags of batts for a wall

10% extra for cuts, Johns Manville coverage per bag.

Wall area	R-13, 15 in wide (106.56 sq ft)	R-13, 23 in wide (163.39 sq ft)	R-19, 15 in wide (87.18 sq ft)
96 sq ft (12 × 8 ft)	1 bag	1 bag	2 bags
128 sq ft (16 × 8 ft)	2 bags	1 bag	2 bags
200 sq ft	3 bags	2 bags	3 bags
320 sq ft (40 × 8 ft)	4 bags	3 bags	5 bags

The R-value comes with the thickness. In the Johns Manville range R-13 and R-15 batts are 3 1/2 in thick, R-21 is 5 1/2 in, R-19 is 6 1/2 in, R-30 is 10 1/4 in and R-38 is 12 in. Foam boards are rated per inch: FOAMULAR XPS gives R-5 per inch, so a 2 in board is R-10. ENERGY STAR recommends R30 to R60 in the attic depending on the climate zone, and R13 or R19 batts on basement walls.

Bags, rolls and boards on sale

Fiberglass batts come in bags with the square footage on the label. Johns Manville R-13 unfaced batts, 15 × 93 in, hold 11 pieces and cover 106.56 sq ft; the 23 in wide version covers 163.39 sq ft. Measure the gap between two studs and pick the width that matches, then type the bag coverage in the advanced options. Faced batts carry a kraft paper facing and unfaced batts have none; the coverage per bag is printed on both.

Rigid foam boards are sold by the sheet, usually 4' × 8', so the calculator starts with 1 board per package. Owens Corning lists 1 in cap nails long enough to reach the framing.

In Europe, EPS boards for external insulation are 100 × 50 cm and come in packs. Austrotherm packs 6 boards of 100 mm (3 m²), 5 of 120 mm and 3 of 160 mm. Mineral wool rolls carry the m² on the label: the Knauf Naturoll Plus 100 mm roll is 8.2 × 1.2 m, 9.84 m².

Common mistakes

• Using the coverage of another thickness. An R-19 bag covers less than an R-13 bag of the same size: 87.18 against 106.56 sq ft.
• Forgetting the window reveals. On external insulation the boards wrap into the reveals around windows: add them as extra areas.
• Counting fasteners on the extra. Fasteners go on the installed boards, so the calculator counts them on the net area.
• Buying to the exact square foot. Boards break and corners need cuts: keep the 10% and buy everything at once.