1. 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
2. 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.
3. 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 (+0.02-0.05 W/m²·K).
4. 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.
5. U-value of windows: frame + glazing + spacer
For windows, don't use R = d/λ. 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 |
U-value is often on the corner spacer sticker. Always photograph at measurement (IP2025 requirement).
6. When to 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 file annex, retain for 5 years minimum.