House Insulation Thickness Calculator
The house insulation calculator helps you quickly determine the required thickness of insulation material to achieve the target thermal resistance of a building element. Simply enter the thermal conductivity coefficient λ (lambda) of your chosen material, the target thermal resistance R, and the insulated area — the calculator will instantly show the insulation thickness in centimetres, the thermal transmittance U-value, and estimated heat losses through the element. In Poland, building regulations (Technical Conditions) require external walls to achieve U ≤ 0.20 W/m²·K for new buildings as of 2023, which corresponds to an insulation thermal resistance of roughly 4.5–5.5 m²·K/W. For roofs and flat roofs the limit is U ≤ 0.15 W/m²·K. When choosing insulation material, pay attention to the λ value: mineral wool has λ ≈ 0.033–0.040 W/m·K, EPS polystyrene λ ≈ 0.031–0.040 W/m·K, and XPS extruded polystyrene λ ≈ 0.028–0.036 W/m·K. The lower the λ value, the thinner the insulation layer needed to meet the thermal resistance target.
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How we calculate insulation thickness
Insulation thickness d [m] = λ [W/m·K] × R [m²·K/W]. The U-value = 1 / R_total, where R_total includes surface resistances: Rsi = 0.13 m²·K/W (internal surface) and Rse = 0.04 m²·K/W (external surface). Heat loss Q = U × A × ΔT, where A is the area [m²] and ΔT is the temperature difference [°C]. All results are rounded to 2–3 decimal places.
Example: external wall, mineral wool λ=0.035, R=5 m²·K/W
Insulation thickness = λ × R = 0.035 × 5 = 0.175 m = 17.5 cm. Total thermal resistance including surface resistances: R_total = 5 + 0.13 + 0.04 = 5.17 m²·K/W. U-value = 1 / 5.17 ≈ 0.193 W/m²·K — meets the requirement of U ≤ 0.20. For an area of 100 m² and ΔT = 40°C, the estimated heat loss is approximately 772 W.
Frequently asked questions
How do I calculate the required insulation thickness?
Multiply the thermal conductivity λ of the insulation material (W/m·K) by the target thermal resistance R (m²·K/W). For example, mineral wool with λ = 0.035 W/m·K and a target R = 5 m²·K/W gives: d = 0.035 × 5 = 0.175 m = 17.5 cm.
What is the λ (lambda) coefficient and where do I find it?
Lambda (λ) is the thermal conductivity of the insulation material expressed in W/m·K. Lower values mean better insulation. It is listed on the product's technical data sheet and on the packaging. Typical values: mineral wool 0.033–0.040, EPS polystyrene 0.031–0.040, XPS 0.028–0.036, polyurethane foam (PUR) 0.022–0.028.
What is thermal resistance R and what value should I target?
Thermal resistance R (m²·K/W) measures how well a building element resists heat flow. For external walls in Poland, a minimum R of approximately 4.5–5.5 m²·K/W is needed to meet the U ≤ 0.20 W/m²·K requirement. For roofs, target R ≥ 6.0–7.0 m²·K/W to achieve U ≤ 0.15 W/m²·K.
What is the U-value (thermal transmittance)?
The U-value (W/m²·K) is the reciprocal of the total thermal resistance including surface resistances (Rsi = 0.13, Rse = 0.04 m²·K/W). A lower U-value means less heat escapes. Polish regulations for new buildings since 2023 require walls U ≤ 0.20, roofs U ≤ 0.15, and floors U ≤ 0.30 W/m²·K.
Which insulation material is best for external walls?
Mineral wool and EPS polystyrene are the most popular choices. Mineral wool offers good fire resistance and acoustic insulation, while EPS is more moisture-resistant. For very thin insulation layers with high performance, polyurethane boards (PUR/PIR) with λ ≈ 0.022–0.028 W/m·K are ideal. The choice also depends on the wall system (ETICS, ventilated facade, cavity wall).
Does thicker insulation always mean better energy savings?
Yes, but with diminishing returns. Doubling insulation thickness halves heat loss, but the absolute gain becomes smaller each time. An economic optimum exists — typically 15–25 cm for walls and 25–35 cm for roofs in Polish climate. Beyond that, savings do not justify the extra cost, though passive house standards go further (30–40 cm).
What are thermal bridges and should I account for them?
Thermal bridges are spots where heat escapes faster than through the main insulated area — for example at window frames, balcony connections, or structural columns. This calculator assumes a uniform insulation layer. In detailed building design, thermal bridges must be assessed separately and can increase effective U-values by 0.01–0.05 W/m²·K or more.
How does insulation thickness affect heating costs?
Heat loss through walls is proportional to the U-value. Reducing U from 0.30 to 0.20 W/m²·K on 100 m² of wall with ΔT = 40°C saves: (0.30 − 0.20) × 100 × 40 = 400 W continuously, or about 3 500 kWh per heating season (5 months). At 0.30 PLN/kWh (gas equivalent), this saves roughly 1 050 PLN per year.
Can I install insulation myself?
Simple tasks like insulating a flat roof or loft with loose-fill or roll insulation can be DIY. However, ETICS (External Thermal Insulation Composite System) for external walls requires proper adhesive application, anchoring, base coat, and render finish — mistakes cause moisture problems and loss of warranty. For structural elements, hire a qualified contractor.
Does this calculator account for all wall layers?
This calculator computes insulation thickness based on the thermal resistance of the insulation layer alone, plus standard surface resistances (Rsi + Rse). It does not include the thermal resistance of brickwork, plaster, or other structural layers. For the full U-value of a multi-layer wall, add the R-values of each layer (d/λ for each material) before calculating U.
Results are estimates for informational purposes only. The insulation design of a building should be prepared by a qualified designer, taking into account all wall layers, thermal bridges and local climatic conditions.