What is Part L?
Part L (Conservation of Fuel and Power) is the section of the UK Building Regulations that sets minimum energy efficiency standards. For pipework, it specifies minimum insulation thicknesses for heating, hot water, chilled water, and refrigeration systems — in both new builds (L1A/L2A) and existing buildings (L1B/L2B). L1 covers domestic buildings; L2 covers non-domestic (commercial, industrial, public).
Minimum Insulation Thicknesses
Heating Pipework (unheated spaces)
| Pipe OD | Minimum Thickness |
|---|---|
| Up to 28mm | 13mm |
| 28–42mm | 19mm |
| 42–60mm | 25mm |
| Over 60mm | 32mm |
Hot Water Pipework (primary circulation)
| Pipe OD | Minimum Thickness |
|---|---|
| Up to 28mm | 13mm |
| 28–42mm | 19mm |
| Over 42mm | 25mm |
Chilled Water & Refrigeration
| Application | Temperature | Material | Minimum Thickness |
|---|---|---|---|
| Chilled water | 6–12°C | Closed-cell elastomeric | 19–25mm |
| Refrigeration | -30 to +10°C | Closed-cell elastomeric | 19–32mm |
Closed-cell elastomeric insulation is mandatory for cold pipe applications — open-cell materials do not provide the vapour barrier required by Part L or by good practice.
Combined Thickness Reference (Part L Compliant)
| Pipe OD | Heating/Hot Water | Chilled Water | Refrigeration |
|---|---|---|---|
| 15–22mm | 13mm | 19mm | 19–25mm |
| 28–35mm | 19mm | 25mm | 25–32mm |
| 42–54mm | 25mm | 25–32mm | 32mm |
| 60mm+ | 32mm | 32mm | 32mm+ |
Material Requirements
Cold pipes (chilled water, refrigeration): K-FLEX ST/SK closed-cell elastomeric — built-in vapour barrier, all joints sealed with K-420 adhesive.
Heating and hot water: K-FLEX ST elastomeric or K-Rock mineral wool where non-combustible A1/A2 rating is required.
Fire-critical areas (plant rooms, commercial risers, escape routes): K-Rock mineral wool — non-combustible A1/A2 rating, compliant with Building Regulations Part B.
Compliance Checklist
- Insulation thickness meets or exceeds Part L minimums for pipe size and application
- Closed-cell elastomeric used on all cold pipes
- All joints sealed with adhesive — no gaps
- All valves, fittings, and elbows insulated
- Outdoor insulation protected with UV-resistant cladding or Solar HT grade
- Fire rating appropriate for location (Class O/1 minimum; A1/A2 in fire-critical areas)
- Product data sheets retained for Building Control inspection
Documentation
Building Control may inspect insulation on new builds and major refurbishments. Retain product data sheets showing thermal conductivity (λ-value), fire rating, and temperature range. For non-domestic (Part L2) projects, commissioning documentation is also required. Photographs of installed insulation before any cladding or ceiling closure are useful evidence of compliance.
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