Insulation calculator: batt bundles, boards, and the R-value check

How insulation is taken off

Building insulation is taken off by surface, not by volume. You measure the wall or ceiling area on the plan, deduct the openings, tag that area to the R-value and assembly type called out in the wall or roof schedule, and carry it as square feet. The square feet then convert into a purchase unit: bundles or bags for batt, boards for rigid, board feet for closed-cell spray foam.

The takeoff is organised by assembly rather than by room because one floor commonly carries three or four different insulation specs. R-13 or R-15 in the 2x4 metal stud partitions, R-21 in the 2x6 exterior walls, acoustic batt in the STC-rated partitions, and R-30 or better at the roof deck. Each of those is a separate line at a separate unit price, and each is checkable on its own. Roll them into one number and nobody can check anything.

Framing spacing does two jobs in the takeoff. It sets the batt width you buy, because 15 in nominal batts friction fit framing at 16 in on center and 23 in nominal batts fit 24 in on center. It also gives you the bay count, which is the fastest independent check on a batt quantity: if the bay count and the square footage disagree about the size of the wall, one of them is wrong.

One thing an insulation takeoff does not do is deduct the framing. Batts are sized to sit in the bay, and manufacturers publish coverage against wall area, so subtracting the studs from the area leaves you short of material. Framing area matters for thermal performance, which is a different calculation, not for the order.

The formulas

Gross area = dimension A x dimension B Net area = gross area minus openings Framing bays = floor(run in inches / spacing) Area to order = net area x (1 + waste factor) Batt bundles = ceiling(area to order / published bundle coverage) Rigid boards = ceiling(area to order / board area) Continuous rigid R = board thickness in inches x R per inch Insulation R = cavity batt R + continuous rigid R

That last line is a sum of insulation layers and nothing more. It is not an assembly U-factor. It leaves out thermal bridging through the studs and plates, interior and exterior air films, the sheathing, the gypsum, and the cladding. Code compliance is demonstrated either by U-factor calculation or by the prescriptive route, and the prescriptive wall tables in the IECC are written in exactly this cavity-plus-continuous format, with entries of the form R-20 plus R-5ci. That is why the tool reports the two layers separately instead of collapsing them.

Bundle coverage is a product number, not a constant

There is no single square footage per bag of batt insulation, which is why this calculator takes bundle coverage as an input instead of burying one manufacturer's figure in the code. Published coverage changes with the batt width, the batt length, the thickness, and how many pieces the plant puts in the bag. A thin R-13 3.5 in batt in a 15 in width can run past 100 sf per bag, while a thick high-R batt in the same width can come in under half of that, because fewer pieces fit in a compressed bag.

The default in the field above is a round 100 sf. It is a placeholder so the tool produces a number on first paint, not a product specification. The real figure is printed on the bag and on the product data sheet in the submittal package, and it is the only one you should order against. Publishing a single "typical" coverage rate would be the kind of number that gets an order short by a truckload on a big job.

Board sizes are more stable. Rigid boards come in 4 ft by 8 ft (32 sf) and 4 ft by 4 ft (16 sf) across most product lines, and semi-rigid mineral wool board is commonly 2 ft by 4 ft (8 sf). Those are geometry, so they can be constants.

Standard batt thicknesses and rigid board R-values

The batt list in the tool uses the nominal thicknesses and R-values published across the major batt manufacturers. Confirm the exact thickness on the product data sheet in the submittal, because it varies slightly by product line:

• R-11 fiberglass, 3.5 in (2x4 cavity)
• R-13 fiberglass, 3.5 in (2x4 cavity)
• R-15 high density fiberglass or mineral wool, 3.5 in (2x4 cavity)
• R-19 fiberglass, 6.25 in nominal
• R-21 high density fiberglass, 5.5 in (2x6 cavity)
• R-23 mineral wool, 5.5 in (2x6 cavity)
• R-30 fiberglass, 9.5 in
• R-38 fiberglass, 12 in

The rigid board figures are the published nominal R per inch for each material. XPS at R-5.0 per inch is the ASTM C578 Type IV value at 75 F mean temperature. Foil faced polyisocyanurate at R-5.7 per inch is the ASTM C1289 long-term thermal resistance figure, which is where the familiar R-11.4 for 2 in polyiso comes from. Expanded polystyrene is published across a range of roughly R-3.6 to R-4.2 per inch depending on density and type, so the tool uses R-4.0 as a mid value and you should substitute the value from your submittal. Semi-rigid mineral wool board is published at R-4.2 per inch.

Two cautions on those numbers. Polyiso R-value is temperature dependent and falls at low mean temperatures, so cold climate designers sometimes carry a derated value, which is a design decision that belongs to the architect and not to your takeoff. And the batt R-values only hold at the labelled thickness. A 6.25 in R-19 batt pushed into a 5.5 in 2x6 cavity is compressed, and manufacturer compressed-thickness tables put it around R-18 rather than R-19. The tool flags the compression so you know to go look at the table rather than quietly pricing R-19.

The R-value the cavity cannot hold

This is the classic insulation takeoff trap, and it is the reason the calculator asks for the cavity depth at all. The specified R-value may require a thickness the cavity physically cannot hold.

A 2x4 cavity is 3.5 in of actual depth. The highest standard batt that fits is R-15, and no product substitution changes that, because you cannot put 5.5 in of material in a 3.5 in space without compressing it and losing the R you were trying to buy. A 2x6 cavity at 5.5 in reaches R-21 in high density fiberglass or R-23 in mineral wool. So when a wall type is drawn as 2x4 studs and the energy sheet calls for R-21, those two documents do not agree, and neither does a 2x6 wall carrying a spec of R-26.

There are only three ways out. The framing gets deeper, which is a structural and dimensional change. Continuous exterior insulation is added over the sheathing to make up the difference, which changes the cladding attachment, the window detailing, and the trim depths. Or the assembly changes to a different insulation type entirely. Every one of those is a design question, and it belongs in an RFI before bid, not in your number.

The failure mode is an estimator who notices the gap, quietly prices the thickest batt that fits, and hopes. That is how you end up owning the difference in a change order argument during closeout, or owning a failed inspection when the energy compliance paperwork is checked against the built wall. Continuous exterior insulation over cavity batt is now a common assembly precisely because the cavity ran out of room, so when you see a spec written as cavity R plus continuous R, the design team has already made the call and your job is to quantify both layers.

What is not in your insulation scope

Two things get pulled into insulation bids that are not building insulation, and both are worth checking against the specification index before you agree to them.

Mechanical insulation is a separate specification section. Building thermal insulation sits in Division 07, under 07 21 00 Thermal Insulation. Pipe insulation sits in 22 07 00 Plumbing Insulation and duct insulation in 23 07 00 HVAC Insulation, both in the mechanical divisions. That is not a filing detail. Mechanical insulation lives on the plumbing and HVAC drawings rather than the architectural set, it is measured in linear feet by pipe size and insulation thickness with fittings counted as each, and it is very often awarded to a mechanical insulation specialist rather than to the building insulation sub. If a GC asks you to cover it, price it as its own scope from the mechanical drawings, or exclude it in writing.

The air barrier is frequently a different trade. Air barriers have their own section, 07 27 00, distinct from thermal insulation. Depending on the project, the air barrier can land with the sheathing installer, the waterproofing or building envelope contractor, or the insulation sub, and the specification will say which. The expensive part is not the field of the wall, it is continuity at the transitions: wall to roof, wall to foundation, at window and door openings, at floor lines, and at every penetration. Those transitions are exactly where two subs each assume the other has it. Read the air barrier section, look at the transition details, and state your assumption in your proposal.

Also keep thermal and acoustic batt on separate lines. They are measured the same way but priced differently and specified in different assemblies, and combining them is a reliable way to double count.

What the area math misses

The net area is the clean geometric quantity. Real jobs carry more material than the area shows:

• Cutting waste. Batt gets cut around blocking, fire blocking, bracing, electrical boxes, and non-standard bay widths, and some pieces get damaged. Shops commonly carry something in the 5 to 10 percent range, but the right figure is your own history on similar work, not a number off a web page.
• Continuous insulation runs past the cavity. The rigid layer covers the rim joist band, the plate line, floor lines, and spandrel areas where there is no cavity at all. This tool applies the same net area to both layers, so on a real building the rigid quantity comes out larger than the batt quantity and you should add those bands.
• Openings practice varies. Openings here are deducted at the rough opening area. Shops differ on whether small openings are deducted at all, since the cutting labour around them offsets the material saved, and the jamb and header cavities still get insulated. Pick a standard and apply it consistently.
• Fasteners for the rigid layer. Continuous insulation needs plate washers and fasteners long enough to reach the framing through the board, and the pattern comes from the manufacturer's installation instructions and the cladding attachment detail. That is a separate count, not a percentage.
• Attic ceilings above about R-49 are usually blown-in loose fill sold by bag coverage at a stated settled depth, which is a different unit than batt bundles. This tool covers batt and board.

Worked example

Take the calculator's own defaults. A wall run of 120 lf at 10 ft high, with 180 sf of openings, framed at 16 in on center, 5.5 in cavity depth, R-21 high density batt plus 1 in of XPS continuous, a 5 percent waste factor, bundle coverage set to the 100 sf placeholder, 4 ft by 8 ft boards, and a specified assembly R of 26.

Gross area is 120 x 10 = 1,200 sf. Deduct the 180 sf of openings and the net area is 1,020 sf. Stud bays are floor(1,440 / 16) = 90, and at 16 in on center you are buying 15 in nominal batts. Area to order is 1,020 x 1.05 = 1,071 sf. Bundles are 1,071 / 100 = 10.71, rounded up to 11 bags. The rigid layer takes the same 1,071 sf, and at 32 sf per 4 ft by 8 ft board that is 1,071 / 32 = 33.47, rounded up to 34 boards.

For the R check: the cavity batt gives R-21, and 1 in of XPS at R-5.0 per inch gives R-5.0, so the insulation total is R-26.0 against a specified R-26. The assembly meets the spec. Note what the tool also reports: the highest standard batt that fits a 5.5 in cavity is R-23, so the cavity on its own could never have reached R-26 no matter which batt you substituted. The continuous inch of XPS is not an upgrade, it is the only reason this wall works.

From quantity to a priced line

The square footage, the bundle count, and the board count are the quantity half of the bid. The priced line multiplies each measured quantity by your own installed unit rate for that assembly, and those rates differ sharply between R-13 in an open partition and R-21 in an exterior wall behind bracing and blocking.

That is the step Ruh automates. It reads the drawing set, measures the scope by assembly, and prices those measured quantities against your own price book, with every line traceable back to the sheet it came from, and your estimator reviewing the result before it leaves the office. For the trade workflow see insulation estimating software, and for the bid mechanics see the guide on how to bid insulation as a subcontractor.