Phenolic Insulation vs PIR 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.

Phenolic board costs roughly to per square metre at 100 mm against to for PIR in 2026, a premium of 60 to 90 percent for the best thermal conductivity in mainstream insulation, around 0.019 W/mK against PIR’s 0.022. That gap sounds small and is: like-for-like, phenolic buys roughly 10 to 15 percent more performance per millimetre, or the same performance around 15 mm thinner at 100 mm class thicknesses.

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

Phenolic earns its premium only where every millimetre is expensive: internal wall insulation where room depth is being lost, reveals and window details, dormers, floors with fixed threshold heights, and any build-up chasing a demanding U-value inside a fixed dimension, which is exactly the Kooltherm K118 and K106 territory. Where the build-up has room, over rafters, in wide floors, standard PIR delivers the same U-value for much less money by simply being slightly thicker, which is why PIR outsells phenolic many times over. Phenolic also brings genuinely better fire behaviour and lower smoke, worth specifying in some junctions and buildings. The honest rule: specify the U-value first, then price both materials at the thicknesses that achieve it, and let the constrained dimension decide.

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.