Celotex U-Value Calculator (Wall / Roof / Floor)
Enter your insulation details to estimate thermal performance in W/m²K. Lower U-values mean better insulation.
What this Celotex U-value calculator does
This calculator estimates the thermal transmittance (U-value) of a building element using Celotex PIR insulation. It is designed for quick early-stage checks for walls, roofs, and floors. You can include a base construction resistance, surface resistances, and an optional plasterboard layer to get a more realistic result.
If you enter an area and a temperature difference, the tool also estimates steady-state heat loss in watts, which is useful for comparing options before final design work.
How U-value is calculated
Core formula
The calculator uses the standard method:
U = 1 / (Rsi + Rse + Σ(d / λ))
- U = U-value (W/m²K)
- Rsi = internal surface resistance
- Rse = external surface resistance
- d = layer thickness in meters
- λ = thermal conductivity (W/m·K)
For Celotex boards, λ is often around 0.022 W/m·K, but always confirm from the exact product datasheet because values vary by range and certification.
Typical targets used in UK projects
Targets vary with project type (new build vs retrofit), element type, and local compliance route. As a rough planning guide:
| Element | Common target range (W/m²K) | Better performance |
|---|---|---|
| External Wall | 0.18 to 0.30 | ≤ 0.18 |
| Roof | 0.16 to 0.20 | ≤ 0.16 |
| Floor | 0.13 to 0.25 | ≤ 0.13 |
Use these numbers as orientation only. Final compliance should be checked against current regulations and approved calculation methods.
Worked example
Example setup
- Element: Wall
- Celotex thickness: 100 mm
- Celotex λ: 0.022 W/m·K
- Existing structure R-value: 0.18 m²K/W
- Plasterboard: 12.5 mm at λ = 0.25 W/m·K
This typically gives a U-value around 0.20 W/m²K (exact value depends on assumptions). Increasing insulation thickness or reducing thermal bridging can improve performance further.
Practical tips for more accurate design
- Check exact product data: different Celotex boards can have different declared lambda values.
- Model thermal bridges: junctions, studs, and fixings can increase heat flow and worsen real-world U-values.
- Control air leakage: airtightness and proper taping are essential for delivered performance.
- Protect continuity: avoid gaps, compression, and poorly aligned boards.
- Validate build-up: ensure dew point and condensation risk assessments are carried out where required.
FAQ
Is lower U-value always better?
For heat loss, yes. But design decisions should also consider moisture control, ventilation strategy, cost, and available space.
Can I use this for Building Control submission?
This calculator is for estimation and option testing. Formal submissions usually require an approved method and professional verification.
Why include Rsi and Rse?
Surface resistances represent the thermal resistance at internal and external surfaces and are part of standard U-value methodology.
Does this include thermal bridging by default?
No. This is a simplified plane-element estimate. Junction and repeating bridge effects should be added in detailed design stages.
Important note
Always check current local standards, manufacturer guidance, and project-specific specifications before procurement or installation. For high-stakes projects, consult a qualified building physicist or energy assessor.