Insulation Calculator

Every new extension has to meet minimum U-value targets under Part L of the building regulations: 0.18 W/m²K for external walls, 0.15 W/m²K for the roof, and 0.18 W/m²K for the ground floor. Lower is better, because the number is how much heat the element lets out. This calculator works out how thick your insulation has to be to hit those targets, how many boards or rolls that comes to, and what the material will cost.

Enter each element you are insulating, its area, and the material you plan to use, then read the thickness, the unit count and the cost. The thickness changes with the material, so it pays to try more than one before you commit.

Why the material changes the thickness

Insulation is rated by its lambda value, the rate at which it conducts heat. The lower the lambda, the thinner the board can be for the same U-value. PIR board sits at 0.022 W/mK, the best of the common options. Mineral wool runs 0.032-0.044 W/mK and EPS 0.032-0.038 W/mK, so both need to be noticeably thicker than PIR to reach the same target.

That trade-off drives the choice. PIR goes where depth is tight, such as a cavity or a shallow floor build-up. Mineral wool suits places with room to spare, where the acoustic benefit and lower price per board matter more than saving millimetres. EPS is the standard pick under a floor slab, where extra thickness is cheap to add.

Whatever you choose, the install matters as much as the thickness. Every board joint needs taping, the vapour control layer has to be continuous, and the insulation must carry through the junction where the extension meets the existing house, or you leave a permanent cold bridge that shows up as condensation.

What it costs

The result is supply-only material cost. For a 100mm PIR board that works out at around £10 – £25 per m². Fitted, with the damp-proof membrane and builder's labour, floor insulation comes to roughly £20 – £50 per m². Across all three elements a typical single-storey extension runs about £3,000 – £5,000, though that assumes the specification on your drawings holds. If building control calls for a thicker board at the pre-plasterboard inspection, you fit the upgrade before any plasterboard goes up, so keep a little contingency back for it.

A worked example

Take the external walls of a single-storey rear extension. To meet the 0.18 W/m²K wall target with PIR at a lambda of 0.022 W/mK, the calculator lands on a board thickness, then divides your wall area by the coverage of a standard board, 2.88 m², adds an allowance for cutting waste, and rounds up to whole boards.

Switch that same wall to mineral wool and the required thickness rises, because its lambda of 0.032-0.044 W/mK is higher, so you either accept a deeper build-up or stay with PIR. Tick walls, roof and floor and the tool prices each one separately, which lets you check the numbers line by line against your builder's quote instead of trusting a single lump sum.