Guide · calculation
Calculate U-value for walls, roofs, floors and windows
You calculate the U-value of a building element as the inverse of the sum of all R-values and surface air resistances (U = 1 / ΣR). The lower the result, the better the element insulates. This guide shows the calculation for 4 element types, when to use default values, and how to type windows correctly.
Author: Dieter Vanwetter, certified Belgian Type A energy expert (VEKA EP18784) ~8 min read Updated 6 September 2026
Summary
- The U-value is the inverse of the sum of all R-values and surface air resistances: U = 1 / ΣR.
- A well-insulated cavity wall with 12 cm of PIR easily reaches U = 0.16 W/m²·K, well below the EPB standard of 0.24.
- For windows, use published U-values per frame and glazing combination, not the R = d/λ formula.
- VEKA allows conservative default values if you cannot demonstrate the construction, provided you document that choice in the file.
Contents
How do you go from R-value to U-value?
The U-value is the inverse of the R-value plus surface air resistances:
U = 1 / (R_air_inside + R_materials + R_air_outside)
Unit: W/m²·K. Lower is better.
Standard air resistances:
- Inside: 0.13 for vertical, 0.10 for roof, 0.17 for floor
- Outside: 0.04 m²·K/W
What is the U-value of an external wall?
Cavity wall with 12 cm PIR: R_total = 6.27 (inside air 0.13 + plaster 0.029 + brick 0.519 + PIR 5.455 + facing brick 0.100 + outside air 0.04).
U = 1 / 6.27 = 0.159 W/m²·K. EPB standard requires max 0.24, well below.
What is the U-value of a pitched roof?
Pitched roof with 22 cm rockwool: R = 6.72 → U = 0.149 W/m²·K.
Beware of rafters (λ=0.13) in the same layer as insulation: this creates a thermal bridge. Correct with a small surcharge (+0.02 to 0.05 W/m²·K).
What is the U-value of a slab on grade?
Floor with 10 cm PUR: R ≈ 5.87 → U ≈ 0.17 W/m²·K.
Watch the B-factor (area/perimeter). Larger area relative to perimeter means less edge losses. VEKA auto-corrects if dimensions are entered correctly.
How do you calculate the U-value of a window?
For windows, don't use R = d/λ, because it's a composite construction of frame, glazing and spacer. Formula: U_window = (A_frame·U_frame + A_glass·U_glass + L_spacer·ψ_spacer) / A_total.
In practice: published values.
| Type | Typical U |
|---|---|
| Single glazing + wood frame (1970s) | 5.0 W/m²·K |
| Double HR + PVC | 1.8 |
| Triple HR++ + PVC | 1.0 |
| Triple HR+++ + aluminium | 0.8 |
The U-value is often on the corner spacer sticker. Always photograph it during the survey: this photo is required under the inspection protocol (see the photo checklist).
When can you use default values?
VEKA allows default values in 3 cases:
- No view of the construction
- No product information
- Insulation not visually verifiable
Defaults are conservative (worse than reality). Document each default choice in the file annex, retain for 5 years minimum. EPC-Expert Pro's auto-calculations always show the source used.
FAQ about U-values
What is a good U-value for an external wall?
In Flanders, new-build must meet a maximum of U = 0.24 W/m²K for external walls. A well-insulated cavity wall with 12 cm of PIR easily reaches U = 0.16.
How do you account for a thermal bridge in the U-value?
A local interruption of the insulation, such as a rafter or concrete column, adds a small surcharge to the U-value. Usually 0.02 to 0.05 W/m²K, depending on the bridge's share of the element.
Why is a window's U-value printed on the glazing sticker?
Manufacturers print the glazing's U-value on the spacer in the corner of the glass. That's the fastest and most reliable source during a measurement, and photographing it is also required under the inspection protocol.
Let U-values calculate automatically
In EPC-Expert Pro you enter material + thickness and U-value is calculated instantly, thermal bridges included.