Rockwool U-Value Calculator
Estimate thermal transmittance (U-value) for a construction layer that includes rockwool insulation.
How to use this rockwool u value calculator
This calculator helps you estimate how much heat passes through a building element (wall, roof, floor, or ceiling) when rockwool is included in the construction. Lower U-values mean lower heat loss and generally better energy performance.
- Select your building element to prefill common internal and external surface resistances.
- Enter rockwool thickness in millimeters.
- Enter lambda (λ), the insulation thermal conductivity from your product datasheet.
- Add other R-value if you already know the resistance of plasterboard, sheathing, air layers, etc.
- Click Calculate to get the resulting U-value instantly.
What is U-value?
U-value is the thermal transmittance of a building element. It tells you how much heat flows through 1 square meter of that element for every 1°C temperature difference between inside and outside. Units are W/m²·K.
A lower U-value means better insulation performance:
- High U-value = more heat loss = higher heating/cooling demand
- Low U-value = less heat loss = better comfort and lower energy bills
Calculation method used
This page uses the standard resistance-based approach:
Where:
- d = material thickness (m)
- λ = thermal conductivity (W/m·K)
- Rsi / Rse = internal and external surface resistances
For this tool, the rockwool layer is calculated as Rrockwool = thickness / λ, with thickness converted from mm to meters.
Typical rockwool lambda values
Always use your product datasheet, but common values are:
- 0.032 W/m·K (high performance mineral wool boards)
- 0.035–0.037 W/m·K (common wall and roof products)
- 0.040 W/m·K (some general-purpose products)
Quick reference table (wall defaults, λ = 0.037, no other layers)
| Rockwool Thickness | Rockwool R (m²·K/W) | Total R (m²·K/W) | Estimated U (W/m²·K) |
|---|---|---|---|
| 50 mm | 1.351 | 1.521 | 0.658 |
| 100 mm | 2.703 | 2.873 | 0.348 |
| 150 mm | 4.054 | 4.224 | 0.237 |
| 200 mm | 5.405 | 5.575 | 0.179 |
Choosing the right insulation thickness
If your goal is to comply with building regulations or hit a specific energy target, compare the calculator output to your required U-value limit. You can also use the optional Target U-value field to estimate required rockwool thickness under your assumptions.
Practical tips
- Use declared or design λ from manufacturer documents.
- Include all layers for more realistic results (sheathing, linings, air films).
- Account for framing effects and thermal bridges in detailed design.
- Verify with local standards or an accredited energy assessor for compliance submissions.
Common mistakes to avoid
- Mixing units (mm vs m) when calculating resistance.
- Using nominal λ instead of declared/design λ for the exact product.
- Ignoring surface resistances entirely.
- Assuming layer-only U-value equals full assembly U-value.
- Forgetting thermal bridging from studs, rafters, junctions, and fixings.
Frequently asked questions
Is a lower U-value always better?
For heat loss, yes. Lower U-values reduce transmission losses. In real projects, balance this with moisture control, cost, and buildability.
Can I use this for roofs and floors too?
Yes. Choose the element type to load suitable starting values for Rsi and Rse, then adjust if your standard requires specific values.
Does this calculator replace full building energy modeling?
No. This is a fast estimation tool. For compliance, certification, or detailed retrofit planning, use a full method based on your local code and construction details.