Continuous Insulation Fastener Count Calculator

Frequently asked questions

Do the fasteners themselves cost the wall any R-value?

Yes, and this is the page where that gets decided. Continuous insulation earns its name by having no framing through it — every fastener you add puts a steel path back across the one layer that was clear. The effect is small per fastener and real in aggregate: published clip-and-rail assessments commonly show a clear-field assembly losing a meaningful fraction of its nominal R-value once the attachment pattern is counted, with the loss rising as clips get closer and as the fastener gets longer and thicker. Two consequences follow. Reducing the count on paper without checking the wind load is not a saving, it is a structural decision; and if you are claiming an assembly R-value to a code official or an energy model, claim the one that includes this attachment, not the one printed on the insulation board.

How is the total fastener count calculated?

Total fasteners = number of insulation boards × fasteners per board — the standard fastening-count method for mechanically-attached continuous insulation.

Where does the fasteners-per-board number come from?

It comes from the CI manufacturer's fastening schedule for your specific board size, which sets the required fastener count and pattern based on the project's wind/seismic load requirements — typically in the 3 to 12 fasteners per board range.

Does this total include fastener type or length selection?

No — this only calculates the fastener count. Fastener type, length, and embedment depth need to be selected separately based on the insulation thickness and the wall substrate you're fastening into.

Is one washer or plate needed for every fastener in this total?

Yes. Read the total as your washer count too, one for one, unless the fastener arrives with its washer already fitted. A screw on its own would pull straight through rigid foam, so each one is driven through a wide plastic or metal washer, called a plate, disc or cap depending on the maker, which spreads the load over the board face. Order the pair together and check it is the pair the fastening schedule was tested with: the holding values in a schedule belong to that screw with that washer, and a smaller washer from another supplier is a different fixing. Drive each one until the washer sits flush with the board and stop; a washer pulled below the surface has crushed the foam beneath it and given up part of the grip the count relies on.

Does the fasteners-per-board figure still hold if I switch to a different board size?

No. A schedule sets a pattern for one board size, and moving to another size changes both the number of boards and the number each one takes. Rigid boards for continuous insulation are commonly sold in North America as 4 × 8 ft (1.2 × 2.4 m) sheets, with longer sheets from some makers, while UK and European boards come in metric sizes such as 1200 × 2400 mm (about 4 × 8 ft) and, on external wall insulation, the smaller 1200 × 600 mm (about 4 × 2 ft) board. A per-board figure taken from a schedule written for one size and multiplied by a count of another gives a fixing density nobody tested. Recount the boards in the size you are actually buying and take the per-board figure from the schedule line for that size.

Can the fasteners go anywhere on the board, or do they have to hit something behind it?

They have to reach whatever the schedule's holding values were measured in, and that decides where they go as much as how many there are. Over steel or timber framing behind a sheathing board, a schedule that requires the fastener to engage the framing ties the pattern to the stud centres, typically 16 in (406 mm) or 24 in (610 mm) in North America and 400 mm (16 in) or 600 mm (24 in) in the UK, so the boards have to be laid out with their fastener rows falling on stud lines. Into concrete or masonry the fixing is a drilled anchor or plug with a minimum embedment and a minimum distance from the edge of the block or slab, and a hole that lands in a mortar joint or a hollow core does not hold what the schedule assumes. Mark the framing or check the masonry before the first board goes up; this count is only right if every fastener finds what it was rated in.

What are these fixings called, and how are they approved, outside North America?

Continuous insulation is a North American energy-code term for insulation run unbroken across the outside of the structure. In the UK the same layer sits behind rainscreen cladding or forms part of external wall insulation (EWI, called ETICS in Europe), and its fixings are called insulation fixings or anchors: a plastic plug with a broad disc head and a plastic or steel pin, set into a drilled hole. For rendered ETICS in Europe those anchors were assessed under the European guideline ETAG 014, since replaced by a European Assessment Document, which gives each anchor a characteristic pull-out value for each category of substrate, from concrete and solid masonry to hollow block and aircrete. The per-board number still comes from the system holder's fixing pattern, set from the design wind suction at the building's location and height, so the multiplication here is the same but the figure you enter should come from that pattern, not from a North American schedule.