Mineral Wool vs Foam 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.

Mineral wool costs roughly to per square metre at 100 mm against to for PIR-class rigid foam in 2026, roughly half the price per millimetre, while foam insulates roughly half again better per millimetre at 0.022 W/mK against wool’s 0.035 to 0.044. Price the two at equal performance rather than equal thickness and wool still usually wins on pure cost, needing around 150 mm to match 100 mm of foam and remaining cheaper even so.

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

Space is what converts foam’s premium into value: wherever the build-up dimension is fixed or expensive, between rafters, inside solid walls, under thresholds, foam’s thinness wins regardless of price, while open lofts and generous floor voids let wool’s economics run. Beyond the headline trade, each brings properties the other lacks: wool is non-combustible, acoustically absorbent, forgiving of imperfect cutting since it compresses into gaps, and vapour-open for breathable build-ups; foam boards bring rigidity, integral vapour control in foil facings, moisture tolerance, and joints that tape airtight. Spray-applied foam adds its own considerations covered by our dedicated spray foam comparisons, chiefly the mortgage-lender caution attached to roof applications that no wool product attracts.

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.