HEATLOSSPROU-values of walls, windows, floors and roofs
The U-value is the number at the heart of every heat loss calculation. Here are typical values by construction, and a calculator that works one out from the layers of a wall.
In short: a U-value (W/m²K) is the heat that passes through 1 m² for each degree between inside and outside. Lower is better. Calculate it as U = 1 ÷ (Rsi + Σ thickness ÷ λ + Rse). An empty cavity wall is about 1.4–1.6, the same wall filled with insulation about 0.5, and a new-build wall about 0.18.
Typical U-values
These are the values the HeatLoss Pro designer uses as presets. For an existing building, use them when the actual construction is unknown.
| Walls | U (W/m²K) |
|---|---|
| Solid brick, 9 inch, uninsulated | 2.10 |
| Solid stone or masonry, uninsulated | 1.90 |
| Cavity wall, empty (1930–1970) | 1.40–1.45 |
| Cavity, partly insulated (40–50 mm) | 0.55 |
| Cavity, fully insulated | 0.35 |
| Solid brick with external insulation | 0.29 |
| Modern wall, about 100 mm PIR | 0.22 |
| Passive house, 200 mm+ insulation | 0.12 |
| Windows | U (W/m²K) |
|---|---|
| Single glazing | 5.0 |
| Old double glazing, no low-e | 2.8 |
| Modern double glazing, argon and low-e | 1.3 |
| Triple glazing | 0.8 |
| Floors and roofs | U (W/m²K) |
|---|---|
| Concrete floor on the ground, uninsulated | 1.10 |
| Ground floor, 50 mm insulation | 0.40 |
| Ground floor, 100 mm+ PIR | 0.18 |
| Roof or loft, uninsulated | 2.0 |
| Loft, 100 mm insulation | 0.40 |
| Loft, 300 mm insulation | 0.14 |
How to calculate a U-value
Each layer resists heat by its thickness divided by its conductivity. Insulation has a λ of 0.02–0.04, brick about 0.77 and concrete 1–2, so 100 mm of insulation does more than a metre of brick. An unventilated air cavity counts as a fixed R of about 0.18. Insulation boards often print their R (or Rd) on the label: you can use that directly.
U-value calculator
Enter the layers from outside to inside. The example is an empty cavity wall.
From U-value to heat loss
Multiply each surface's U-value by its area and by the temperature difference, and add the ventilation loss. That is a room's heat loss; see how to calculate heat loss for a worked example. In the designer you pick a construction for each wall, window, floor and roof, or build one layer by layer, and the areas come from your drawing, so each room's heat loss is ready as you draw. Drawing is free; a full calculation with a PDF report costs €1.
Frequently asked questions
What is a U-value?
A U-value is how many watts of heat pass through one square metre of a wall, window, floor or roof for every degree of temperature difference between inside and outside, in W/m²K. The lower the U-value, the better it insulates. Heat loss through a surface is its U-value × its area × the temperature difference.
How do I calculate a U-value?
Divide the thickness of each layer (in metres) by its thermal conductivity λ to get its R-value. Add up the R-values of all layers, plus the inside and outside surface resistances (0.13 and 0.04 m²K/W for a wall). The U-value is 1 divided by that total.
What is the difference between a U-value and an R-value?
An R-value is a resistance: how well a layer holds heat back, in m²K/W. R-values of layers add up. A U-value is the opposite, the heat that gets through the whole construction: U = 1 ÷ total R. In the Netherlands and Belgium insulation is often specified as an Rc or Rd value instead of a U-value.
What is a good U-value for a wall?
For new houses in England, a wall of about 0.18 W/m²K, a roof of 0.11–0.15, a floor of 0.13–0.18 and windows of 1.2–1.4. In the Netherlands, new walls need Rc 4.7 m²K/W (about U 0.2), roofs Rc 6.3 and floors Rc 3.7. An uninsulated cavity wall is around 1.4–1.6, and a solid brick wall about 2.1.
What U-value do windows have?
Single glazing is about 5.0 W/m²K, older double glazing about 2.8, modern double glazing with low-e coating and argon 1.1–1.4, and triple glazing 0.6–0.9. The frame counts too: a whole window is usually a little worse than its glass alone.
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