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How the insulation calculator works
Insulation is sold by the roll or the pack, and what a roll covers is not a fixed area: it changes with the make, the thickness and the format, and a loft roll, a wall slab and a rigid board are all different. That is why the calculator takes the coverage as its own figure rather than assuming one. The area tells it how much there is to cover; the coverage figure tells it how many rolls that takes. The m² is printed on every pack, so read it off the one you are buying and type it in — the 8 m² default here is a planning estimate, nothing more.
Start with the area. A loft, a floor or a wall is length × width, and an L-shaped space is two rectangles measured separately and added together. Then add the waste allowance: insulation is cut to fit around joists, rafters, pipes and the ends of a run, and offcuts are rarely reusable because every bay is a different size.
The waste allowance, and why whole rolls matter
Five per cent suits a clear loft with regular joists and a single straightforward run. A loft full of services, cross-battens and awkward hips wants nearer 10%, and a room with lots of pipework wants more again. You cannot buy part of a roll, so the order rounds up — and the spare that leaves is normal, because a repair or a top-up in a few years will want a matching roll.
There is a second reason to keep a spare roll. Insulation is a matched product, and a later top-up from a different make or batch may not sit the same way in the space. Buying one roll more than the arithmetic suggests is cheaper than a second delivery for a single roll.
What this page does not decide
This is a quantity estimate, not a specification. It works out how much insulation to order for the area you give it; it does not tell you the thickness to use, the U-value to reach, or how to ventilate the space. Those depend on the construction. In England they are set out in Approved Document L for energy and Approved Document C for damp and ventilation, and a build-up also has to keep its dew point out of the structure.
That last point matters most for a flat roof, a dormer, or a wall on a solid masonry build. Get the build-up wrong and the insulation traps moisture rather than keeping it out, and the damage appears years later. Use the area figure here to price the job, then take the build-up itself from a competent person.
Insulation FAQs
How many rolls of insulation do I need?
Divide the area by what one roll covers. A 5 m × 4 m area is 20 m²; add the waste allowance for the cuts at the edges and around the joists, which is 10% in this example, giving 22 m². If a roll covers 8 m², 22 ÷ 8 is 2.75, so you buy 3 rolls — a part roll is bought as a whole roll. Read the m² off the pack, because it changes with the make and the thickness.
What does a roll of insulation cover?
That is a product figure, not a fixed one. A roll of one thickness or make can cover several square metres more or less than a roll of another, and loft rolls, wall slabs and rigid boards are all different. This calculator uses 8 m² as a labelled planning estimate only — check the m² printed on the pack you are buying and type that in.
What waste allowance should I add for insulation?
Five to ten per cent is a sensible allowance for cutting insulation to fit around joists, rafters, pipes and the awkward ends of a run. A simple, clear loft needs less; a loft full of services and cross-battens needs more. Offcuts are rarely reusable, because each bay is a different size.
Does this calculator tell me what insulation thickness to use?
No. It works out how many rolls to order for the area you give it. The thickness, the U-value and the ventilation a build-up needs depend on the construction and, in England, are set out in Approved Document L for energy and Approved Document C for damp and ventilation. Take advice for anything beyond a simple loft top-up.
Can I use this for a flat roof or a wall?
You can use it for the area, but the build-up is not simply a matter of laying rolls. A flat roof or a wall has to be arranged so that the dew point does not fall inside the insulation, or it will trap moisture and rot the structure beneath it. That is a design question: use the area figure here, then get the build-up from a competent person.
Regs & planning for this calculator
Insulating a loft or a floor is not notifiable building work by itself, but the depth you fit and the ventilation you leave are both set by regulation in England. Two documents on the site's reference library are the ones that bite.
- Approved Document L — energy. The target U-values a new or altered thermal element must reach are set here, and they decide the thickness you need rather than the roll. Read the requirement.
- Approved Document C — damp and ventilation. A roof space has to be ventilated, and a build-up has to stop water vapour condensing inside it, which is the point that catches out a flat roof or a dormer. Read the requirement.