What This Requirement Covers
U-values measure the rate of heat loss through a building element, expressed in watts per square metre per degree Kelvin (W/m²K). Lower U-values indicate better thermal performance. Approved Document L sets maximum U-values for building elements in both new and existing buildings.
Key Requirements
New Dwellings (2021 Standards)
- External walls: 0.26 W/m²K (typical construction: full-fill cavity insulation in masonry, or 120-140 mm rigid insulation in timber frame)
- Party walls (between dwellings): 0.20 W/m²K (using the edge-of-slab convention in SAP)
- Ground floors: 0.18 W/m²K (typically 100-120 mm rigid insulation below or above the slab)
- Pitched roofs (at ceiling level): 0.16 W/m²K (typically 300-400 mm mineral wool between and over joists)
- Flat roofs: 0.16 W/m²K (typically 130-160 mm rigid insulation above the deck)
- Windows: 1.6 W/m²K (whole window, including frame; typically double-glazed with low-E coating and argon fill)
- Doors: 1.6 W/m²K (whole door, including frame and glazing)
- Roof windows: 1.6 W/m²K
Existing Dwellings (Renovation/Replacement)
- Replacement windows: 1.4 W/m²K
- Extension walls: 0.28 W/m²K
- Extension roof: 0.16 W/m²K
- Extension floor: 0.22 W/m²K
- Replacement roof covering (with access to the void): 0.16 W/m²K
- Internal wall insulation (change of use): 0.30 W/m²K (or as low as reasonably practicable)
Common Insulation Materials and Typical Thicknesses
- Mineral wool (glass or rock wool): Thermal conductivity 0.032-0.044 W/mK; used in walls, lofts, and floors
- Rigid phenolic foam (e.g., Kingspan, Celotex): 0.018-0.022 W/mK; highest performance per thickness
- Expanded polystyrene (EPS): 0.030-0.038 W/mK; commonly used below floors and in cavity walls
- Polyisocyanurate (PIR): 0.022-0.025 W/mK; used in walls, roofs, and floors
- Blown fibre or bead: Various conductivities; used for cavity wall insulation in existing buildings
Thermal Bridging
- Thermal bridges (cold bridges) at junctions, around openings, and at structural elements reduce the overall thermal performance of the building envelope
- The impact of thermal bridges is accounted for in the SAP calculation using psi-values (linear thermal transmittance)
- Approved Document L requires the use of Accredited Construction Details (ACDs) or individually calculated psi-values
- Common thermal bridges: window reveals, lintels, ground floor edges, eaves, and corners
Practical Compliance Tips
- U-value calculations must be carried out to BS EN ISO 6946 (for walls and roofs) or BS EN ISO 13370 (for floors)
- Always use the manufacturer's declared thermal conductivity (lambda value) from their product certification
- Account for air gaps, fixings, and repeating thermal bridges (e.g., timber studs in a wall) in U-value calculations
- Insulation must be installed in full contact with the adjacent surfaces; gaps drastically reduce performance
- Photograph insulation installation at each stage for Building Control evidence
- Specify insulation retaining clips or mechanical fixings where there is a risk of the insulation slumping or falling out of position
- Pay particular attention to insulation continuity at junctions; a break in insulation at one point can undermine the performance of the entire element