How much polystyrene do you need to insulate a house?
Choose what the wall is made of and the insulation material. The calculator shows the minimum thickness that brings U down to 0.20 W/(m²·K), the board area in m² and the volume in m³, plus the yearly saving.
Example: 180 m², solid brick, 38 cm, 15 cm
Minimum thickness: 18 cm
EPS, λ 0.040
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
How many cm of polystyrene a wall needs
The thickness depends on how much resistance the wall lacks to reach U 0.20 W/(m²·K): d = λ × (1 ÷ 0.20 - R of the wall). We round up to a whole centimetre, because boards are sold in such thicknesses.
A 38 cm solid brick wall needs 18 cm of white EPS λ 0.040, 16 cm of wool λ 0.036 or 14 cm of graphite EPS λ 0.031. A 51 cm brick wall and a 24 cm aerated concrete wall each need 17 cm of white EPS. If the wall already has 5 cm of old polystyrene, another 13 cm is enough.
How much polystyrene to buy
You need as many m² of boards as the wall has without windows and doors, plus an allowance for offcuts that your contractor will give you. The volume is area times thickness: 180 m² of wall with 18 cm of white EPS is 32.4 m³, and with 14 cm of graphite EPS 25.2 m³.
The thinner graphite layer cuts the volume by about 22%. So compare offers in zloty per m² of wall at the required thickness, not in zloty per m³ of boards.
Frequently asked questions
How many cm of polystyrene for a brick house?
A 38 cm solid brick wall needs 18 cm of white EPS λ 0.040 to bring U down to about 0.19 W/(m²·K) and meet WT 2021. With graphite EPS λ 0.031, 14 cm is enough.
Is 15 cm of polystyrene enough?
Not on a 38 cm brick wall: U will be about 0.23 W/(m²·K), more than the 0.20 required by WT 2021. On a wall that already has 5 cm of old polystyrene, 15 cm is enough: U drops to about 0.18.
How many m³ of polystyrene for 180 m² of wall?
With 18 cm of white EPS it is 32.4 m³, with 15 cm 27 m³. Add an allowance for offcuts.
Graphite or white polystyrene?
Graphite EPS has a lower λ (0.031 against 0.040), so a thinner board gives the same U: 14 cm instead of 18 cm on a 38 cm brick wall. The choice comes down to the price of the whole job in your contractor's quote.