Wall Insulation Calculator — m, R-value, cost
How to use this calculator
Pick a measurement method: single wall (length x height), room perimeter (2 x (W + L) x height), or a known gross area in m². Choose the unit (m, cm, or mm) per field. Subtract door and window openings to get the net insulated area, then choose the insulation type (batt/roll, blown-in, spray foam, or rigid board), stud spacing (400 or 600 mm o.c.), and cavity depth (89, 140, or 200 mm).
Cost — enter price per batt package, blown-in bag, spray kit, or rigid sheet and select your currency (EUR, PLN, CZK, etc.).
Vapour barrier — optional PE vapour control layer roll count from net wall area.
U-value — computed as U = 1/(Rsi + Rins + Rse) per EN ISO 6946, with country target U-values shown for reference.
U = 1/Rtotal (includes surface resistances Rsi=0.13, Rse=0.04 m2K/W per EN ISO 6946)
Country examples: DE: U ≤ 0.24 | FR: U ≤ 0.28 | PL: U ≤ 0.20 | CZ: U ≤ 0.30 | SE: U ≤ 0.18
Saved Calculations
| Time | Method | Net m2 | Type | R-value | U-value | Qty | Ins | VB | Total |
|---|
How to Calculate Wall Insulation (Metric)
Enter wall length and height in m, cm, or mm (or a known gross area in m²) and the calculator returns the package, bag, kit, or sheet count and the resulting R / U-value for a stud-cavity wall. Doors are deducted at 1.89 m² each (standard 0.9 × 2.1 m) and windows at the dimensions you enter, so quantities reflect the cavity actually filled. Apply 5% waste on plain walls and 8–10% where openings, dormers, or short studs force many trim cuts.
Thermal Resistance and U-value
Thermal resistance R (m²K/W) = thickness ÷ thermal conductivity λ. The assembly U-value (W/m²K) = 1 / (Rsi + Rins + Rse) with surface resistances Rsi = 0.13 and Rse = 0.04 m²K/W per EN ISO 6946. Typical cavity values: ~2.3 m²K/W for 89 mm mineral wool, ~3.7 for 140 mm, ~5.3 for 200 mm. Polyiso (λ ≈ 0.022 W/mK) gives the most resistance per millimetre. For roof build-ups use the roof insulation calculator; for spray-applied work the spray foam calculator sizes kits by area.
Formulas
Net area = gross wall area − (doors × 1.89 m²) − (windows × avg W × avg H). Packages = ceil(net area × (1 + waste%) ÷ coverage per package). R = t / λ; U = 1 / (Rsi + R + Rse). Vapour barrier rolls = ceil(net area × (1 + waste%) ÷ roll coverage).
FAQ
Do I need a vapour barrier? In cold and mixed climates a PE vapour control layer on the warm-in-winter side limits interstitial condensation per EN 13788; size it with the vapour barrier calculator and add an exterior breather membrane via the house wrap calculator. Can I mix insulation types? Yes — continuous rigid board over a filled cavity reduces thermal bridging per EN ISO 10211. For below-grade walls, see the waterproofing calculator.
On install day
Three things estimators forget on a wall job. Friction-fit, not crushed — batts compressed behind cables or pipes lose 25–40% of rated R; split the batt around the obstruction rather than tucking it in. Rim joists and top plates — the rim joist band at each floor and the gap between top plate and sheathing are usually skipped on a stud-count takeoff; add ~10% extra material per floor for those areas. Framing factor — 400 mm o.c. timber framing is roughly 23% wood (15% with 600 mm advanced framing), so the whole-wall U-value is 10–25% higher than cavity-only per EN ISO 6946 §6 and ASHRAE 90.1 Appendix A; a 30–50 mm continuous rigid board over the sheathing is the cleanest fix. 2025 EU installed prices: mineral wool batts €8–18/m², blown cellulose €10–22/m², closed-cell spray foam €25–55/m² at 50 mm.
