Building thermal insulation · Stone wool
ROCKWOOL Cool 'n' Comfort RL
- Thickness
- 25–50 mm
- Density
- 40–60 kg/m³
- Thermal conductivity (λ)
- 0.034–0.035 W/m·K
What is U-value, and how is it different from R-value?
In short
U-value (thermal transmittance) shows how much heat passes through a whole building element, such as a wall, roof or window. Its unit is W/m²·K, and the lower it is, the less heat gets through. The R-value is the resistance of each layer, where higher is better. U is the inverse of the total R of all the layers, including the resistance of the air surfaces on both sides.
- 1λ (thermal conductivity, W/m·K): a property of the material; lower means less heat conducted — independent of thickness
- 2R (thermal resistance, m²·K/W): for a layer of given thickness, R = thickness ÷ λ; the same material in a thicker layer gives a higher R
- 3U (W/m²·K): the heat transfer rate through a whole wall or roof, all layers and surface air films included; lower means less heat passes
How do U and R differ?
| R-value | U-value | |
|---|---|---|
| --- | --- | --- |
| What it describes | Thermal resistance of one layer | Heat passing through a whole building element |
| SI unit | m²·K/W | W/m²·K |
| Better value | Higher | Lower |
| Applies to | Insulation, ceiling board, wall board, etc., layer by layer | Walls, roofs, ceilings, windows as a system |
Put simply: use R to choose an insulation layer, and U to see the result for the whole wall or roof.
How is U calculated (in principle)?
Following the ISO 6946 method for elements made of layers:
- Find the R of each layer (for a uniform layer, thickness ÷ λ).
- Add the internal surface resistance (Rsi) and external surface resistance (Rse); the standard sets these according to the direction of heat flow.
- The total is R_T = Rsi + R₁ + R₂ + … + Rse.
- U = 1 ÷ R_T.
A hypothetical example that only explains the method (not data for any system): if Rsi + Rse together are 0.20 and all the layers together are 2.30 m²·K/W, then R_T = 2.50 and U = 1 ÷ 2.50 = 0.40 W/m²·K.
What is U-value used for?
U gives a rough estimate of the heat passing through a building element under steady conditions: heat flow (W) ≈ U × area (m²) × temperature difference between the two sides (K).
For example (hypothetical), U 0.40 × 100 m² × 10 K ≈ 400 W. Halving the U-value halves the heat flow under the same conditions.
Why can the real U-value differ from the calculation?
- Thermal bridges — steel frames, purlins, battens or places where the insulation stops are short cuts for heat. A simple layer-by-layer calculation can therefore look better than reality. ISO 6946 also gives a method for elements with non-uniform layers.
- Installation quality — gaps, joints and compressed insulation make the real result worse.
- Windows and doors are outside the scope of ISO 6946; they have their own methods, and window makers usually state a U-value (or U-factor) for the whole unit.
- U is a steady-state value — a real roof in the sun also involves solar radiation and heat stored in the materials. U is a comparison tool, not a figure that predicts room temperature at every hour.
U-values a building must meet
Some types of building have legal energy requirements. The values to meet and how to calculate them follow the law and standards that apply to that project, and should be checked by the designer or engineer. This article does not set U-values for design.
If you need to choose insulation so that a wall or roof reaches a U-value set by your designer, send us the build-up of layers and we can help. Read more: thermal conductivity (λ) and how to choose insulation thickness.
Sources
- ISO 6946:2017 — Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods
- ISO 7345:2018 — Thermal performance of buildings and building components — Physical quantities and definitions
- U.S. Department of Energy, Energy Saver — Energy Performance Ratings for Windows, Doors, and Skylights (window U-factor)
- U.S. Department of Energy / ENERGY STAR — Insulation Fact Sheet (R-values add up; effect of gaps)
Related products
Categories and example products for this topic
- ROCKWOOL Cool 'n' Comfort RL — The insulation layer of a single-skin metal roof: R 1.43–1.47 m²·K/W at 50 mm by grade
- ROCKWOOL Cool 'n' Comfort SL — The insulation layer of a double-skin metal roof: R 1.43–1.47 m²·K/W at 50 mm by grade
- SCG SUPER COOL-G — A glass wool roof layer: λ 0.035–0.042 W/m·K at 24 °C depending on density
Building thermal insulation · Stone wool
ROCKWOOL Cool 'n' Comfort SL
- Thermal conductivity (λ)
- 0.034–0.035 W/m·K
- Thermal resistance (R)
- 1.43–1.47 m²·K/W
- Sound absorption (NRC)
- 1.00–1.05
Building thermal insulation · Glass wool
SCG SUPER COOL-G
- Thickness
- 25–75 mm
- Density
- 12–24 kg/m³
- Thermal conductivity (λ)
- 0.035–0.042 W/m·K