What is a U-value for SIP panels?

A U-value measures how well a SIP panel transmits heat. It's measured in W/m²K (watts per square meter kelvin). Lower U-values indicate better insulation performance. For example, a 122mm SIP panel with 100mm Celotex GA4000 core achieves a U-value of 0.209 W/m²K, which exceeds UK Building Regulations requirements.

What U-value do SIP panels need to meet UK Building Regulations?

UK Building Regulations Part L 2021 require: Walls ≤ 0.18 W/m²K, Pitched Roofs ≤ 0.16 W/m²K, and Flat Roofs ≤ 0.18 W/m²K. UltraSIPs panels with 100mm+ Celotex insulation typically meet or exceed these requirements.

How is the U-value of a SIP panel calculated?

U-value is calculated using the formula: U = 1 / R-total, where R-total is the sum of thermal resistances. For a SIP panel, this includes: external surface resistance (0.04), external OSB layer (thickness/λ), Celotex PIR core (thickness/0.022), internal OSB layer (thickness/λ), and internal surface resistance (0.13). The calculation follows BS EN ISO 6946 standard.

What is the difference between Celotex GA4000 and XR4000?

Both Celotex GA4000 and XR4000 have identical thermal performance with a thermal conductivity (λ) of 0.022 W/mK. The main difference is thickness range: GA4000 is available in 50mm-100mm thicknesses for general applications, while XR4000 extends from 110mm-200mm for applications requiring higher insulation levels.

What thickness SIP panel do I need for my project?

Panel thickness depends on your application and Building Regulations requirements. For walls, a 122mm panel (11mm OSB + 100mm Celotex + 11mm OSB) typically achieves U-value 0.209 W/m²K. For pitched roofs requiring U ≤ 0.16 W/m²K, you'll need approximately 140mm+ total thickness. Use our U-Value Calculator to determine the exact specification for your project.

Do I need 11mm or 18mm OSB for my SIP panels?

OSB thickness choice depends on structural requirements rather than thermal performance. 11mm Sterling OSB is suitable for most applications including garden rooms and light residential. 18mm OSB provides greater structural strength for larger spans, commercial buildings, or where building regulations require enhanced structural capacity. Both achieve similar thermal performance per unit thickness.

Are SIP panels better than traditional insulation?

SIP panels typically outperform traditional insulation methods because they eliminate thermal bridging, provide continuous insulation, and achieve superior airtightness. A 122mm SIP panel (U-value 0.209) provides equivalent thermal performance to a much thicker traditional cavity wall, while reducing construction time by up to 50%.

How accurate is the UltraSIPs U-Value Calculator?

Our calculator uses official thermal conductivity values: Celotex GA4000/XR4000 (λ = 0.022 W/mK) and Sterling OSB (λ = 0.13 W/mK). Calculations follow BS EN ISO 6946 standard with standard surface resistances (Rse = 0.04, Rsi = 0.13 m²K/W). Results are accurate for specification purposes and building regulations compliance verification.

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OSB Thickness (Both Sides) Select the thickness of OSB board on each side of your panel

Celotex Insulation Type Both types have identical thermal performance (λ = 0.022 W/mK)

Insulation Core Thickness Thickness of Celotex PIR insulation core

Your SIP Panel Specification

Panel Composition

External OSB: 11mm Sterling OSB

Insulation Core: 100mm Celotex GA4000

Internal OSB: 11mm Sterling OSB

Total Panel Thickness: 122mm

U-Value: 0.209 W/m²K

Excellent Performance

Thermal Resistance Breakdown

Component	Thickness	R-Value (m²K/W)
External Surface	-	0.040
External OSB	11mm	0.085
Celotex PIR Core	100mm	4.545
Internal OSB	11mm	0.085
Internal Surface	-	0.130
Total Thermal Resistance (R-total)		4.885

Building Regulations Compliance

Walls: 0.18 W/m²K required

Pitched Roof: 0.16 W/m²K required

Flat Roof: 0.18 W/m²K required

Based on UK Building Regulations Part L 2021

About U-Values

The U-value measures how well a building element transmits heat. Lower U-values indicate better insulation performance.

Our SIP panels combine Sterling OSB structural boards with Celotex PIR insulation for exceptional thermal efficiency.

Technical Specifications

Celotex GA4000/XR4000: λ = 0.022 W/mK
Sterling OSB: λ = 0.13 W/mK
Calculation Standard: BS EN ISO 6946

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