SIP Panel U-Values Explained: Meeting UK Building Regulations 2025
SIP Panel U-Values Explained: Meeting UK Building Regulations 2025
Confused about U-values? This straightforward guide explains what U-values mean, how they're calculated, and exactly which SIP panels meet UK Building Regulations Part L 2021.
U-values determine whether your building complies with UK Building Regulations. Get them wrong, and Building Control won't sign off your project. Get them right, and you'll enjoy low energy bills for decades.
If you're planning an extension, new build, or garden room with SIP panels, understanding U-values is essential. This guide cuts through the jargon and gives you the practical information you need.
By the end, you'll know exactly which SIP panel specification meets regulations—and why lower U-values save you money long-term.
What Is a U-Value? (Simple Explanation)
A U-value measures how well a building element (wall, roof, floor) resists heat flow. It's expressed in W/m²K (Watts per square meter per Kelvin).
The Simple Rule:
Lower U-value = Better insulation = Lower heating bills
What The Numbers Actually Mean
A U-value of 0.20 W/m²K means:
- For every 1m² of wall/roof area
- For every 1°C temperature difference between inside and outside
- 0.20 Watts of heat energy escapes per second
0.15
Excellent
Passive house standard. Minimal heat loss.
0.18-0.20
Very Good
Meets/exceeds UK Building Regs. Low energy bills.
0.25-0.30
Adequate
May not meet regulations. Higher bills.
0.40+
Poor
Fails regulations. Very high heating costs.
U-Value vs R-Value: What's The Difference?
| U-Value | R-Value | |
|---|---|---|
| Measures | Heat transmission (how much escapes) | Thermal resistance (how much it stops heat) |
| Units | W/m²K | m²K/W |
| Better value | Lower is better | Higher is better |
| Relationship | U = 1 ÷ R (they're reciprocals) | |
| Used in UK? | ✅ Yes (Building Regs use U-values) | Sometimes (technical calculations) |
In the UK, Building Regulations specify maximum U-values. That's what you need to focus on.
UK Building Regulations Part L 2021: The Requirements
Building Regulations Part L (Conservation of Fuel and Power) sets maximum U-values for new builds and extensions in England and Wales.
Maximum U-Values (England & Wales 2021)
| Building Element | Maximum U-Value (W/m²K) | SIP Panel Recommendation |
|---|---|---|
| Walls | ≤ 0.18 | 122mm minimum (with additional measures*) |
| Pitched Roofs (insulation at ceiling level) | ≤ 0.16 | 142mm or 172mm |
| Pitched Roofs (insulation at rafter level) | ≤ 0.18 | 142mm or 172mm |
| Flat Roofs | ≤ 0.18 | 142mm or 172mm |
| Floors | ≤ 0.18 | 122mm or 142mm |
*Additional measures may include: high-performance windows/doors, minimized thermal bridging, or compensating via other elements.
⚠️ Important: These are MAXIMUM values (worst allowed)
Building Regulations also require whole-building SAP or SBEM calculations. Individual elements might exceed these U-values if other parts of the building compensate. Always consult a qualified energy assessor or building control officer for your specific project.
Do Garden Rooms Need to Meet Building Regulations?
It depends:
- Permitted Development garden rooms (under 2.5m height, away from boundaries): Generally NO Building Regs required for thermal performance
- Larger garden rooms or those needing planning permission: YES, must meet Part L
- Extensions: YES, always must meet Part L
- New builds: YES, must meet Part L and achieve target SAP rating
Even if not legally required, better U-values = lower running costs forever. Don't skip good insulation just because you legally can!
SIP Panel U-Values: Complete Specification Guide
Here are the exact U-values for all UltraSIPs panel specifications, calculated according to BS EN ISO 6946:
| Panel Type | Construction | U-Value (W/m²K) | Building Regs Compliance |
|---|---|---|---|
| 72mm SIP Panel | 11mm OSB + 50mm Celotex GA4000 + 11mm OSB | 0.396 | ❌ Does not meet requirements |
| 97mm SIP Panel | 11mm OSB + 75mm Celotex GA4000 + 11mm OSB | 0.264 | ❌ Below standard (close) |
| 122mm SIP Panel | 11mm OSB + 100mm Celotex GA4000 + 11mm OSB | 0.209 | ⚠️ Walls/floors with additional measures* |
| 142mm SIP Panel | 11mm OSB + 120mm Celotex XR4000 + 11mm OSB | 0.179 | ✅ Meets all requirements |
| 172mm SIP Panel | 11mm OSB + 150mm Celotex XR4000 + 11mm OSB | 0.144 | ✅ Exceeds all requirements |
Calculations include surface resistances (Rse = 0.04, Rsi = 0.13 m²K/W) per BS EN ISO 6946. OSB λ = 0.13 W/mK, Celotex PIR λ = 0.022 W/mK.
Calculate U-values for any configuration:
Use Our Free U-Value Calculator →How SIP Panel U-Values Are Calculated
Want to understand the math? Here's the calculation for a 122mm SIP panel:
Step-by-Step Calculation (122mm Panel)
Panel construction:
- External OSB facing: 11mm (λ = 0.13 W/mK)
- Celotex GA4000 core: 100mm (λ = 0.022 W/mK)
- Internal OSB facing: 11mm (λ = 0.13 W/mK)
Step 1: Calculate R-value for each layer
R = thickness (m) ÷ λ (W/mK)
- External surface resistance (Rse) = 0.04 m²K/W
- External OSB: R = 0.011 ÷ 0.13 = 0.085 m²K/W
- Celotex core: R = 0.100 ÷ 0.022 = 4.545 m²K/W
- Internal OSB: R = 0.011 ÷ 0.13 = 0.085 m²K/W
- Internal surface resistance (Rsi) = 0.13 m²K/W
Step 2: Add all R-values
Rtotal = 0.04 + 0.085 + 4.545 + 0.085 + 0.13 = 4.885 m²K/W
Step 3: Calculate U-value
U = 1 ÷ Rtotal
U = 1 ÷ 4.885 = 0.209 W/m²K
Why are SIP panel U-values so good? The thick PIR insulation core (R = 4.545) dominates the calculation. OSB contributes very little thermal resistance, but that's fine—the PIR does the heavy lifting.
💡 Pro Tip: Want to improve U-value? Increase insulation thickness. Doubling OSB thickness from 11mm to 22mm only improves U-value by ~0.01 W/m²K. Increasing Celotex from 100mm to 120mm improves it by ~0.03 W/m²K. Insulation thickness matters most.
Should You Exceed Building Regulations?
Building Regulations are minimum standards. Here's why smart builders often specify better:
1️⃣ Rising Energy Costs
UK energy prices have doubled in recent years. Better insulation today = protection against future price rises. Extra £1,000 spent now saves £3,000-5,000 over 20 years.
2️⃣ Regulations Get Stricter
Part L requirements tighten every 5-7 years. Buildings that barely meet 2021 standards may be considered "inefficient" by 2030. Build to tomorrow's standards today.
3️⃣ Resale Value
Energy Performance Certificates (EPCs) affect property values. Higher EPC ratings (A/B) command 5-15% price premium vs lower ratings (C/D). Better U-values = better EPC.
4️⃣ Comfort
Better insulation = more stable temperatures. No cold spots, less temperature swing between day/night. Noticeably more comfortable, especially in bedrooms.
5️⃣ Environmental Impact
Lower U-values = reduced carbon footprint. A house with U=0.15 walls vs U=0.30 walls saves ~2-3 tonnes CO₂/year. Makes a real difference.
6️⃣ It's Cheap To Upgrade Now
Upgrading from 122mm to 142mm panels costs ~£15/m². For a 100m² house = £1,500. Retrofitting better insulation later costs £10,000+. Do it right the first time.
Our recommendation: If budget allows, choose 142mm or 172mm panels even if Building Regulations don't require it. The marginal cost increase delivers significant long-term value.
U-Values FAQ
Can I use 97mm panels for my extension?
Not without additional insulation. 97mm panels achieve U=0.264, which doesn't meet the 0.18 W/m²K requirement for walls. You'd need 122mm minimum (with additional measures like high-performance windows) or 142mm for straightforward compliance.
Do U-values include finishes (plasterboard, cladding)?
The U-values listed above are for SIP panels only. Adding plasterboard (12.5mm) and external cladding (timber/render) improves U-values by ~0.01-0.02 W/m²K. Not a huge difference, but every bit helps. Our U-value calculator includes options for finishes.
What's the difference between GA4000 and XR4000 Celotex?
Both have the same thermal conductivity (λ = 0.022 W/mK), so performance per mm is identical. GA4000 is available in 50-100mm thicknesses, XR4000 in 110-200mm. We use GA4000 in our 72-122mm panels, XR4000 in 142-172mm panels. Choice is based on thickness availability, not performance difference.
Does using 18mm OSB instead of 11mm improve U-value?
Marginally—by about 0.008 W/m²K (e.g., 122mm panel goes from 0.209 to 0.201). OSB has relatively poor thermal resistance compared to PIR insulation. If you want better U-values, increase insulation thickness (much bigger impact) rather than OSB thickness.
My Building Control officer says I need U=0.15. How do I achieve this?
U=0.15 requires 172mm+ panels, or 142mm panels with additional external insulation (40-50mm PIR board + render). This is becoming common for new builds aiming for very low energy consumption or near-passive house standards. Contact us for specification advice.
Need Help Choosing The Right SIP Panel Specification?
Our technical team can advise on U-values, Building Regulations compliance, and the best panel specification for your project.