U-value and insulation

How the calculation works

Choose the element (external wall, flat roof or ceiling to an unheated space), then enter the layers from inside to outside with their thickness and λ or a known resistance R. Mark the new layers as “new”. The calculator works out R for each layer, the total R and U before and after the works, draws the cross-section with the temperature profile, compares up to three insulation variants, calculates the thickness for your target U, the heat flow for the given temperatures and checks the dew point on the inner surface.

Formula and assumptions

Layer R = d (m) ÷ λ. Total R_T = Rsi + ΣR + Rse; U = 1 ÷ R_T. Rsi = 0.13 (wall), 0.10 (upward flow), 0.17 (downward flow); Rse = 0.04 to outside air and equal to Rsi towards an unheated space. An air layer has a tabulated resistance (e.g. 0.18 m²·K/W for 25 mm in a wall). Required thickness = (1 ÷ target U − R of the other layers and surfaces) × λ. Heat flow q = U × ΔT (W/m²), Φ = q × A (W).

Worked example

Wall: gypsum plaster 15 mm, clay block 250 mm (λ 0.30), lime-cement plaster 20 mm → R_T = 1.06 m²·K/W, U ≈ 0.94. With adhesive, 100 mm EPS (λD 0.038) and finish render R_T = 3.70, U ≈ 0.27 W/(m²·K). At 20 °C inside and −5 °C outside about 2360 W flows through 100 m² of wall before and 675 W after.

More worked examples

• Ceiling to an unheated attic: gypsum plaster 10 mm, RC slab 160 mm and stone wool 200 mm (λD 0.036) give R_T = 5.85 m²·K/W and U ≈ 0.17. At 20 °C below and 5 °C in the attic the heat flow is 2.6 W/m², i.e. 128 W through 50 m².
• The same clay block wall with a target U = 0.25: R without EPS is 1.07 m²·K/W, another 2.93 m²·K/W is needed, i.e. 2.93 × 0.038 = 111 mm of EPS. With available thicknesses of 100, 120 and 140 mm the suggestion is 120 mm (U ≈ 0.24).

EPS (λD 0.038) on a 25 cm clay block wall with plasters (R without EPS 1.07)

What affects real consumption

• The real λ of the product (declaration of performance) and the difference between declared λD and design λ.
• Thermal bridges, anchors and non-homogeneous layers that a plain cross-section does not cover.
• Moisture in the material, workmanship and airtightness.

Common mistakes

• Entering the λ of fired clay for blocks instead of the design λ of the masonry or the manufacturer's R.
• Entering an air layer as a solid material with the λ of air instead of the tabulated resistance.
• Reading the lower heat flow through the wall as a saving on the heating bill.

Frequently asked questions

What is the difference between λD and design λ?

λD is the product's declared value from laboratory conditions. Design λ takes installed conditions into account (moisture, temperature, ageing). The calculator does not silently treat them as equal: it adds a note for λD and you can set the difference with a conversion factor.

How does the calculator handle an air layer?

An unventilated layer gets a tabulated resistance by thickness and heat flow direction (up to 300 mm). For a well-ventilated layer it and the layers outside it are ignored and still air is assumed outside. A slightly ventilated layer is interpolated between the two cases.

What U does 10 cm of EPS give on a 25 cm clay block wall?

With inner and outer plaster, adhesive and finish about 0.27 W/(m²·K): R_T = 1.07 + 0.10 ÷ 0.038 = 3.70 m²·K/W. For U 0.25 you theoretically need 111 mm, so 120 mm from a catalogue.

Is the lower heat flow the same as the saving on the bill?

No. The percentage applies only to that element. The heating bill depends on the whole building, windows, ventilation, climate and heating system, so the calculator does not show an annual saving.

Related calculators

• Facade (ETICS)
• Anchors
• Plastering calculator
• Thermal Conductivity Conversion (λ)
• R-Value and U-Value Conversion

Tips

• Attic floor insulation: how many cm of wool