XPS vs PIR Insulation Cost Calculator

How this calculator works

This calculator works in ranges rather than single figures, because real quotes, yields and tariffs genuinely vary that much. It starts from typical UK price, output and savings ranges drawn from published 2026 industry guides and official scheme documentation, then scales them by the quantities you enter and adjusts them for each choice you make below. The result is a low-to-high band that a realistic outcome is likely to fall within, not a promise of the figure you will achieve.

XPS costs roughly to per square metre at 100 mm against to for PIR in 2026, overlapping ranges that make neither reliably cheaper, while PIR insulates better per millimetre at around 0.022 W/mK against XPS’s 0.028 to 0.035. On pure thermal value PIR wins comfortably; XPS survives and thrives because insulation sometimes has to do more than insulate.

Area to insulate (square metres)

Enter the area to be covered in the material. Both options are priced per square metre for the same area so the totals compare directly; allow around 10 percent extra for cuts and waste when ordering boards, and check coverage per pack for rolls and quilts.

Board thickness

Rigid insulation is priced largely by volume, so thickness moves the per square metre cost almost linearly: 50 mm boards cost roughly 60 percent of the 100 mm price and 150 mm around 40 percent more. The thickness you need depends on the U-value target for the element under Building Regulations Approved Document L and the material’s thermal conductivity, which is why higher-performing boards can hit the same target with less depth.

What else affects the figures

XPS’s closed-cell structure gives it two properties PIR cannot match: near-total moisture resistance and compressive strengths from 300 kPa upwards, which is why XPS owns the applications where insulation gets wet or loaded, inverted flat roofs where it sits above the waterproofing, below-ground and perimeter work, under heavy floor slabs, car park decks and terraces. Everywhere dry and unloaded, pitched roofs, walls, standard floors, PIR’s better conductivity means thinner build-ups for the same target and usually less money. The two are not really rivals so much as neighbours with a clear property line: if the board will see water or sustained load, XPS; if not, PIR, with the calculator above pricing whichever side of the line your project sits.

Certification, planning and grid connection

If a calculation here prompts a project, Building Regulations Approved Document L is the standard the work should meet, with U-value targets for each element, and installers registered with TrustMark or a competent person scheme can self-certify compliance. Check grant eligibility before paying for anything: ECO4 runs to 31 December 2026 and the Great British Insulation Scheme continues alongside it, both funding measures partly or fully for qualifying households, and domestic insulation carries 0 percent VAT until 31 March 2027 in any case. An EPC assessment before and after larger projects evidences the improvement for future buyers and lenders.