Insulation Calculator — batts, rolls and coverage

Bays, the batt width that is actually manufactured for them, and the bag count — with the part that catches people out stated next to the number, because a bag holds less square footage the higher the R-value goes, so an attic never prices like a wall.

Two things decide the answer and neither one is the wall area: how many bays there are, and which batt is actually manufactured for them. Enter the framing and this gives you both, then converts to bags at whatever coverage is printed on the bag you buy.

Bays	—
Pieces	—
Width to buy	—
Batt thickness	—
Depth check	—
Area	—
To order	—
Coverage used	—
Bag coverage vs R-13	—

Spacing — 12, 16, 19.2 and 24 in on centre are the columns of the IRC span tables for floor joists, ceiling joists and rafters (2021 IRC Tables R502.3.1(1)–(2), R802.4.1(1)–(8), R802.5.1). Walls are a different list: Table R602.3(5) gives maximum stud spacings of 16 and 24 in only, and Table R602.3(6) tabulates 12, 16 and 24 in. 19.2 in appears nowhere in IRC Chapter 6 as a stud spacing, which is why this page flags it on a wall. 19.2 is exact, not rounded — 96 ÷ 5 = 19.2, five bays to an 8 ft sheet.

Clear opening — spacing minus the member. 1-1/2 in is the minimum dressed dry thickness of nominal 2× lumber in US DOC PS 20-25, American Softwood Lumber Standard, Table 3; green surfaced stock is 1-9/16 in, which takes a 16 in o.c. bay to 14-7/16 in. Metric dressed timber is 38 mm, so 400 mm centres give 362 mm clear and 600 mm give 562 mm — those are computed on the metric module, not converted from the imperial answer. UK and European regularised carcassing is often 47 mm, which changes the opening again. An I-joist presents a 3/8 in web and a steel C-stud presents about 1/16 in, which is why their batts are wider at the same spacing.

Batt widths — Owens Corning Thermal Batt FIBERGLAS Product Data Sheet, Pub. No. 17931-J, “Thermal Batt Insulation Technical Data”, which separates the rows and prints the metric equivalents itself: Wood Frame Construction / Walls 15 in (381 mm) and 23 in (584 mm); Wood Frame Roof/Floor/Ceiling R-30 at 16, 19-1/4 (489 mm) and 24 in and R-38 at 16 and 24 in; Metal Frame Construction 16 in (406 mm) and 24 in (609 mm). Cross-checked against the Knauf EcoBatt data sheet (“Forms Available — Wood Frame Construction”, which also lists an 11 in width) and CertainTeed Sustainable Insulation product data (“Available Sizes”: 15, 15-1/4, 19, 23, 23-1/4 in). The quarter-inch-over sizes are the flangeless friction-fit products, faced or not — CertainTeed sells kraft-faced SpeedyR Tabless batts at 15-1/4 in, and Owens Corning stocks no 15-1/4 in wall batt at all. Steel-stud batts run wider still: ROCKWOOL ComfortBatt (technical data sheet, 05-2025) lists 15-1/4 and 23 in for wood but 16-1/4 and 24-1/4 in for steel.

Thicknesses are the standard products in the same data sheets. High-density and high-performance lines reach the same R in less depth — R-21 in 5-1/2 in, R-30C in 8-1/4 in, R-38C in 10-1/4 in — so check the sheet for the product you actually buy. RSI = R × 0.1761 m²·K/W.

Bag coverage is a model here, not a specification. Bags are packed to a roughly fixed compressed volume, so the square footage in a bag falls about in proportion to batt thickness. The default is anchored on one real package — CertainTeed unfaced R-13, 15-1/4 in × 93 in, 16 pieces, which is 15.25 × 93 × 16 ÷ 144 = 157.58 ft² and matches the catalogue figure exactly — then scaled by 3-1/2 in ÷ the batt thickness. Real bags vary by maker, R-value and retail pack, and every bag prints its own coverage. Type that number in and the model gets out of the way.

When the run is not a whole number of batts the piece count is given twice: the tight figure assumes the offcut from one bay fills the short end of the next, which a fibreglass offcut will do, and the whole-batt figure assumes it does not. The bag count is worked from the tight figure, so raise the waste allowance if nobody on site is going to splice.

None of this is a takeoff. Corners, king-stud packs, headers, blocking, recessed lights and knee walls all need cutting, the first and last bay of a wall are never the field dimension, and any of it can be overridden by the drawings in front of you.