One account unlocks every calculator on the site — try it free for 5 days, no card required.
Partition templates
Add the first partition layer or choose a template
Heat transmittance calculations available after logging in
Heat transmittance calculations available after logging in
Add a calculated partition to the set to export a summary table.
Professional thermal transmittance calculator compliant with PN-EN ISO 6946:2017. Supports six envelope types: external and internal walls, ground and basement floors, roofs and ceilings above unheated attics. Lets you define any number of material layers from a built-in materials database (brick, aerated concrete, EPS/XPS, mineral wool, PIR, wood, panels and plasters) or enter your own λ values. Automatically checks compliance with current Polish technical regulations (WT 2021/2026) and indicates how much additional insulation is needed to meet the requirement. Results can be downloaded as a PDF report with the envelope cross-section and the table of thermal resistances — ready to be attached to the design documentation or energy performance certificate.
Pick the envelope type in the top tabs — external wall, internal wall, ground floor, basement floor, roof or ceiling above unheated attic. Each type has its own surface resistances and its own maximum U value allowed by Polish technical regulations.
Add layers in order from inside to outside. Pick a material from the database (the calculator will insert the default λ value) or type your own, and enter the thickness in centimetres. You can also use ready-made templates for typical assemblies (ETICS on aerated concrete, three-layer wall with klinker, pitched roof, ground floor etc.).
Read the results: the U value [W/(m²·K)], the total thermal resistance R, the table of resistances of individual layers and the WT compliance status. If the envelope does not meet the requirement, the calculator shows how many extra centimetres of insulation with a given λ are needed to reach the target. Results can be downloaded as a PDF report with the envelope cross-section.
The thermal transmittance U describes the heat flux through 1 m² of envelope per 1 K of temperature difference between the two sides:
U = 1 / R
where U [W/(m²·K)] is the thermal transmittance and R [m²·K/W] is the total thermal resistance of the envelope.
The total thermal resistance of a multi-layer envelope is the sum of the internal surface resistance, the resistances of the individual material layers and the external surface resistance:
R = Rsi + Σ(dᵢ / λᵢ) + Rse
where Rsi — internal surface heat-transfer resistance [m²·K/W], dᵢ — thickness of the i-th layer [m], λᵢ — thermal conductivity of the i-th layer [W/(m·K)], Rse — external surface heat-transfer resistance [m²·K/W]. The lower the λ of a material, the better its thermal insulation.
Maximum U-values for residential and public-use buildings, introduced by the technical conditions regulation in 2021 and still binding after the 2026 (WT 2026) amendment:
These limits apply to standard buildings. For energy-efficient and passive buildings (NF40, NF15, PHI standard) requirements are noticeably lower — typically U ≤ 0.15 W/(m²·K) for walls and U ≤ 0.10 W/(m²·K) for roofs.
The 2026 amendment to the technical regulations (WT 2026) does not change the U-value limits, but it tightens the primary energy (EP) requirements — in practice it pays to design partitions with a margin below the limits above.
The thermal conductivity λ (lambda) describes how easily a material conducts heat. The lower the λ, the better the insulation:
Declared values per PN-EN ISO 10456 and PN-EN ISO 6946. The manufacturer may publish design values that differ slightly from the declared ones — design calculations typically use values increased by a moisture correction factor.
The table below contains U values for typical building envelope assemblies in Polish construction. All calculations follow PN-EN ISO 6946:2017 with declared λ values from PN-EN ISO 10456. Examples #1–9 meet WT 2021 requirements; example #10 shows a typical wall in buildings before thermal retrofitting (1960s–1980s).
| # | Envelope type | Assembly (from inside) | U [W/(m²·K)] | Meets WT? |
|---|---|---|---|---|
| 1 | External wall | Gypsum plaster 1.5 cm + ceramic block 25 cm + EPS Graphite λ=0.031 thickness 15 cm + cement plaster 1.5 cm | 0.17 | yes |
| 2 | External wall | Gypsum plaster 1.5 cm + sand-lime brick 18 cm + mineral wool λ=0.035 thickness 18 cm + cement plaster 1.5 cm | 0.18 | yes |
| 3 | External wall | Gypsum plaster 1.5 cm + aerated concrete 600 kg/m³ thickness 36.5 cm + mineral wool λ=0.035 thickness 10 cm + cement plaster 1.5 cm | 0.20 | yes (at the limit) |
| 4 | Three-layer external wall | Gypsum plaster 1.5 cm + sand-lime brick 18 cm + mineral wool λ=0.035 thickness 16 cm + klinker brick 12 cm | 0.20 | yes (at the limit) |
| 5 | Pitched roof | GK board + vapour barrier + mineral wool λ=0.035 thickness 30 cm between and below rafters + breathable membrane | 0.11 | yes |
| 6 | Pitched roof | GK board + vapour barrier + mineral wool λ=0.035 thickness 25 cm + breathable membrane | 0.14 | yes |
| 7 | Flat roof | Gypsum plaster + reinforced concrete slab 18 cm + vapour barrier + PIR λ=0.022 thickness 18 cm + roofing membrane | 0.12 | yes |
| 8 | Floor on ground | Cement screed 5 cm + foil + XPS λ=0.033 thickness 12 cm + lean concrete 10 cm + ground | 0.26 | yes |
| 9 | Floor above unheated basement | Floor finish 1.5 cm + reinforced concrete slab 20 cm + mineral wool λ=0.035 thickness 15 cm + GK ceiling 1.25 cm (basement side) | 0.21 | yes |
| 10 | Old external wall (before thermal retrofit) | Lime plaster 1.5 cm + solid ceramic brick 51 cm + cement-lime plaster 1.5 cm — typical residential building from the 1960s–1970s | 1.13 | no |
Simplified calculations — they ignore linear thermal bridges (rafters, window frames, studs). In real projects the influence of bridges is taken into account separately per PN-EN ISO 14683 or via a ΔU correction. All assemblies above can be reproduced and adjusted to your project in the calculator above.
Current Polish technical regulations (WT 2021) require U ≤ 0.20 W/(m²·K) for external walls in rooms at ≥ 16 °C. This value does not change in 2026. For energy-efficient and passive buildings the target is U ≤ 0.15 W/(m²·K).
Thermal resistance R [m²·K/W] measures how well an envelope limits heat loss — the higher, the better. The U value [W/(m²·K)] is the reciprocal of the total thermal resistance (U = 1/Rt) — the lower, the better. Resistances of individual layers add up; the U value describes the whole envelope as a single number.
ISO 6946 itself assumes a one-dimensional, plane-homogeneous envelope. The influence of linear (Ψ) and point (χ) thermal bridges is taken into account separately — under PN-EN ISO 14683 or in the building's energy performance calculation. In typical envelopes thermal bridges add 5–15% to heat losses, so the actual design U (Uc) is usually slightly higher than the ISO 6946 result.
Add a layer of thermal insulation. Each 5 cm of EPS (λ ≈ 0.038) or mineral wool (λ ≈ 0.035) adds about 1.3–1.4 m²·K/W of thermal resistance. In practice: for an old brick wall with U ≈ 1.1 W/(m²·K), adding 15 cm of mineral wool gives U ≈ 0.20 W/(m²·K) — exactly the WT 2021 minimum.
Practically no. A bare wall of ceramic or sand-lime blocks has U = 0.4–0.6 W/(m²·K), and a solid brick wall about 1.1 W/(m²·K). The WT 2021 requirement is U ≤ 0.20 for an external wall, so any newly built envelope needs at least 12–18 cm of EPS or mineral wool — exact thickness depends on the wall material.
From the manufacturer's declaration of performance (CE, ETA) or from PN-EN ISO 10456 (table of typical design values). When in doubt, use the declared value with a safety margin. The calculator has a built-in database of 100+ materials with current values from standards; you can also enter your own λ value from a specific product's technical sheet.
Yes. Thermal transmittance is the formal name for the U-value [W/(m²·K)] — the lower the U, the better the thermal insulation. The calculator reports both U and the thermal resistance R of the whole building partition. It computes U for any multi-layer partition, including timber- or steel-frame walls — just define each layer with its thickness and λ value. In frame walls, the area containing the load-bearing structure (studs) has a higher U than the insulation between the studs, so a representative partition is assumed in the simplified calculation.
Calculators:
Knowledge base articles:
