Insulating a Wall Without Thermal Bridges

What this calculation does not cover

• This counts sheets to cover an area. It does not choose a thickness or R-value, check your climate zone's requirement for continuous insulation, or confirm the board's compressive strength grade suits a load-bearing position under a slab or screed.
• The count is for a single layer. A staggered two- or three-layer build-up is a separate full-area order for each layer, so run the area through once per layer rather than entering the combined thickness.
• The waste allowance is a flat uplift on area, not a cutting layout. Rafter bays, hips, curved walls and dense penetrations throw off more offcuts than that, and offcuts from one bay are often unusable in the next. A wall whose window and door openings you did not deduct is over-ordered by roughly their area.
• It assumes every board is the market's standard full sheet. It does not cover the 2 ft wide XPS or 1200 x 600 mm half boards some ranges are sold in, and it does not deduct the overlap on tongue-and-groove or shiplap edges, which cover less than the board's nominal face area.
• Nothing beyond the boards is counted: seam tape, adhesive or foam, fixings and washers sized to the board thickness, furring, and any separate vapour or air control layer. Rigid foam is also combustible, and building codes generally require a thermal or ignition barrier between it and an occupied space. This calculator neither sizes nor includes that.

What this calculation does not cover

• The figure is the raw geometric volume of the space — area times thickness — with nothing added for overspray, foam trimmed back flush, hose purge, or material stranded in a spent tank. Published kit yields are ideal-condition maximums; cold tanks or a cold substrate deliver less than the rating.
• Area is taken as gross surface. Studs, joists, rafters and blocking are not deducted, and nothing is allowed for working around wiring, boxes and pipework, so a clear bay and a congested one produce the same number.
• This is a quantity take-off, not a thermal or code check. It returns no R-value, and it does not test the thickness you enter against a code requirement or against the depth of the cavity you are filling. Open-cell and closed-cell foam are nowhere near the same R per inch, so the same thickness is not the same performance.
• It does not cover how the foam goes on. Closed-cell has to be built up in passes rather than one deep lift, cured foam left exposed generally needs a thermal or ignition barrier over it to satisfy code, and two-part kits require respiratory protection and a re-occupancy period before the space is used.
• Where foam belongs in the assembly is a separate decision. Whether a roof can be unvented, whether a rim joist or wall needs vapour control, and where the dew point lands in the finished build-up are not questions this take-off answers.

What this calculation does not cover

• Air leakage, not diffusion, is what usually wets a wall, and there is nothing about it here. The gradient gives the temperature at the plane; it cannot say how much moisture arrives there. A wall that passes this check and leaks warm interior air through a top plate, a service penetration or an unsealed electrical box deposits far more water at that plane than vapor diffusion through an intact assembly ever could.
• The R-values entered describe the clear field of the wall, between the framing. The path through every stud, plate and header is colder than this straight line says, and at a steel stud or an uninsulated slab edge much colder — so an assembly that passes in the middle of a bay can be sitting below dew point on the back of the sheathing at every framing member, which is exactly where mold turns up.
• It is one snapshot at one pair of temperatures, and it totals nothing. What damages an assembly is how many hours a year it spends below dew point and whether it dries out in between. A wall that dips below on a few cold nights and recovers is not the same wall as one that stays below for a month, and this returns the identical verdict for both.
• The obvious fix for a failing result can produce a wall that cannot dry. Moving the barrier inboard is right only if the outboard side is open to vapor. Where a low-perm layer already sits outside — exterior foam, a self-adhered membrane, an impermeable sheathing — a second one inside traps whatever gets past either of them, and the assembly then passes this temperature check with no drying path in either direction.

What this calculation does not cover

• The parallel-path split flatters metal. Treating the girt and the insulation as two independent one-dimensional paths ignores heat funnelling sideways out of the insulation into the steel, which is why the zone and modified-zone methods exist and why they return a lower effective R for the same wall. Read this figure as the optimistic end of the range, not the middle, whenever it lands close to a required value.
• Only the layer the furring passes through is modeled. Sheathing, any cavity insulation, gypsum, the cladding and the inside and outside air films all still have to be added in series afterwards, so this number is not yet the whole-wall R or U that a code table or an energy model is asking for.
• Repeating girts are not the only bridge on the facade. Shelf angles, slab edges, parapets, balcony penetrations and the metal around every window opening carry linear and point heat flow that no area fraction can represent, and on a real building they routinely cost as much again as the furring counted here.

What this calculation does not cover

• Board thickness is not an input, so the count is the same for a 25 mm board as for a 100 mm one, and it covers a single layer. Nothing here tests the resulting R-value against what the rim joist is expected to achieve, and building up two layers to reach a target doubles the sheet count.
• Thickness is also what keeps the rim board dry, and that is not checked either. Foam too thin for the climate leaves the inner face of the rim board cold enough for indoor air to condense against it, so the penalty for getting thickness wrong here is damp timber rather than a slightly higher heating bill, and the thickness needed to hold that face above the dew point depends on the climate zone.
• The 10% allowance covers ordinary offcuts, not the yield lost to cutting a whole sheet down into bay-sized pieces. A 4x8 ft sheet cut into 9.25 in by 14.5 in rectangles — 2x10 joists, the default here, at 16 in centres — gives 30 pieces, about 87% of the sheet, with the rip and end strips discarded, so a long band worked out from area alone can come up a sheet short.
• Only the boards are counted. The canned foam that seals the perimeter of every cut piece, the sealant at the sill plate, and any covering the foam needs where it is left exposed are all outside the number — foam plastic inside a dwelling normally has to sit behind a thermal or ignition barrier, and whether the rim joist band is granted an exception is jurisdictional.
• Perimeter times joist height treats the band as an unbroken rectangle. Nothing is deducted for basement windows, walkout doors or beam pockets, and nothing is added where a cantilever, bay window or bump-out carries the band past the foundation line that the help text asks you to measure.
• The band is assumed to be exactly as tall as the joist. Many rim joist details are insulated from the underside of the subfloor down over the sill plate to the top of the foundation wall, which is taller than the joist depth entered here.