Wall U-value calculator: heat transfer through a wall
Choose what the wall is made of and add insulation. The calculator works out the U-value before and after, shows the formula with your numbers and checks whether the wall meets the 0.20 W/(m²·K) limit from WT 2021.
Example: 180 m², solid brick, 38 cm, 15 cm
U after insulation about 0.23 W/(m²·K), before about 1.51
about 5,596 PLN a yearabout 17,149 kWh less heat per seasonpays back in about 9.0 years
- Before1.51 W/(m²·K)
- After0.23 W/(m²·K)
U 0.23 exceeds 0.20 W/(m²·K). 18 cm of white polystyrene (EPS) is enough.
| Item | Value |
|---|---|
| U before insulation | 1.51 W/(m²·K) |
| U after insulation | 0.227 W/(m²·K) |
| Heat loss through the element before | 20,184 kWh/yr |
| Heat loss through the element after | 3,034 kWh/yr |
| Heat saved | 17,149 kWh/yr |
| Energy or fuel you buy less | 18,052 kWh |
| Yearly saving | 5,596 PLN/yr |
| Insulation cost | 50,400 PLN |
| Grant or tax relief | 0 PLN |
| Cost after grant or relief | 50,400 PLN |
| Simple payback | about 9.0 years |
| Minimum thickness under WT 2021 | 18 cm |
| Boards or rolls, no offcut allowance | 180 m² |
| Insulation volume | 27.0 m³ |
Assumptions and formula11 October 2026
Thermal resistance before insulation
R = 0.13 + 0.38 ÷ 0.77 + 0.04 = 0.664 m²·K/W; U = 1.51
Resistance and U after insulation
R = 0.664 + 0.15 ÷ 0.040 = 4.414 m²·K/W; U = 0.227
Heat saved per season
Q = 1.281 × 180 m² × 3,100 × 24 ÷ 1000 = 17,149 kWh/yr
Saving and payback
saving = Q ÷ efficiency × energy price
17,149 ÷ 0.95 × 0.31 = 5,596 PLN/yr
payback = (cost - grant) ÷ saving
(50,400 - 0) ÷ 5,596 = 9.0
Layer resistances are calculated to three digits and U is shown with two decimal places. The WT 2021 limit is compared with the unrounded value.
- Rsi, internal surface resistance
- 0.13 m²·K/WPN-EN ISO 6946:2008, surface resistance table (ProAkademia summary)
- Rse, external surface resistance
- 0.04 m²·K/WPN-EN ISO 6946:2008, surface resistance table (ProAkademia summary)
- U(max) under WT 2021
- 0.20 W/(m²·K)WT 2021, annex 2 (Journal of Laws 2022 item 1225); may be applied until March 2028 (art. 102a of the Construction Law)
- λ of the element: Solid brick, 38 cm
- 0.770ITB Building Physics Department data and PN-EN 12524:2003 (ProAkademia summary)
- λ of insulation: EPS
- 0.040 W/(m·K)Holcim Biały FASADA PRO 040, declaration of performance BFP 040-01/26 of 8 June 2026
- Degree days (base 18°C)
- 3,100Eurostat nrg_chdd_a, Poland, 2016-2025 average (about 3119)
- Efficiency: natural gas
- 0.95
- Price: natural gas
- 0.31 PLN/kWhmyORLEN (formerly PGNiG OD) W-3.6 tariff, Oct 2026, with PSG distribution
- Insulation cost per m²
- 280 PLN/m²KB.pl, wall insulation prices 2026: 15 cm polystyrene with materials
An estimate for one building element, without thermal bridges, windows and ventilation. The real saving depends on how you heat the house.
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Ready-made calculations
What the U-value tells you
U is the number of watts passing through 1 m² of an element for each degree of temperature difference. The lower the U-value, the less heat escapes. A square metre of 38 cm brick wall (U about 1.51) loses about 30 W with 20°C inside and 0°C outside, and the same wall with 15 cm of white EPS (U about 0.23) about 4.5 W.
U-values of typical walls
Without insulation, using λ values from ITB data: 38 cm solid brick has a U-value of about 1.51, 51 cm solid brick about 1.20, 24 cm aerated concrete on ordinary mortar about 1.03, and 38 cm brick with 5 cm of old polystyrene about 0.56 W/(m²·K).
With 15 cm of polystyrene a 38 cm brick wall has a U-value of about 0.23 with white EPS λ 0.040 and about 0.18 with graphite EPS λ 0.031. The WT 2021 limit for walls is 0.20 W/(m²·K).
Frequently asked questions
How do I calculate the U-value of an insulated wall?
Divide the thickness of each layer in metres by its λ, add the results and the surface resistances 0.13 and 0.04, then take the inverse of the sum. A 38 cm brick wall with 15 cm of EPS λ 0.040: R = 0.13 + 0.494 + 3.75 + 0.04 = about 4.414 m²·K/W, so U = about 0.23 W/(m²·K).
What U-value should an external wall have in Poland?
WT 2021 required external walls of rooms heated to at least 16°C to have a U-value of at most 0.20 W/(m²·K), and roofs and ceilings below an unheated attic 0.15. The regulation expired in September 2026, but these requirements may be applied until March 2028.
What is the U-value of a brick wall?
A 38 cm solid brick wall without insulation has a U-value of about 1.51 W/(m²·K), and a 51 cm one about 1.20. That is about 7.5 and 6 times the 0.20 limit from WT 2021.