Insulation Requirements for Existing Buildings

What This Requirement Covers

When building work is carried out on an existing dwelling, Approved Document L Volume 2 requires that the energy performance of the affected elements is improved to meet minimum standards. This applies to extensions, renovations, replacement of thermal elements (walls, roofs, floors), and replacement of controlled fittings (windows, doors, boilers).

Key Requirements

Thermal Element Upgrades

When a thermal element (wall, roof, or floor) is renovated to the extent that it constitutes a major renovation, the element must be upgraded to at least the following U-values

1. Wall: 0.30 W/m²K (or as low as reasonably practicable if this cannot be achieved)
2. Pitched roof (at ceiling level): 0.16 W/m²K
3. Flat roof: 0.18 W/m²K
4. Floor: 0.25 W/m²K

A thermal element is considered to be undergoing major renovation when more than 50% of the surface is being renewed or replaced.

Consequential Improvements

For extensions over 100 m² to existing dwellings, consequential improvements may be required

1. Upgrading the heating system to a condensing boiler (if the existing boiler is non-condensing)
2. Adding loft insulation to 270 mm if the existing loft has less than the recommended level
3. Adding cavity wall insulation if the existing cavities are unfilled
4. Improving hot water cylinder insulation

Retained Thermal Elements in Extensions

1. New walls, roof, and floor in an extension must meet the following minimum U-values:

- Walls: 0.28 W/m²K - Roof: 0.16 W/m²K - Floor: 0.22 W/m²K

1. Windows and doors in extensions: 1.4 W/m²K

Practical Upgrade Solutions

1. External wall insulation (EWI): 50-100 mm rigid insulation fixed to the outside of the wall with a render or cladding finish; effective but changes the building's appearance
2. Internal wall insulation (IWI): 50-100 mm insulation fixed to the inside of external walls; reduces room sizes but does not affect the external appearance
3. Loft insulation top-up: Adding mineral wool to reach 270 mm total depth (typically costs under GBP 500 for a standard house)
4. Cavity wall insulation: Injected insulation (beads, fibre, or foam) for unfilled cavities
5. Floor insulation: Rigid insulation above or below the floor structure

Practical Compliance Tips

1. Assess the existing building's thermal performance before designing the upgrade; a thermographic survey can identify heat loss areas
2. Prioritise upgrades that provide the best cost-to-benefit ratio: loft insulation, cavity wall insulation, and boiler replacement are typically the most cost-effective
3. When adding internal wall insulation, consider the impact on room sizes, window reveals, and electrical outlets
4. Ensure continuity of insulation at junctions; gaps between wall insulation and roof insulation, or wall insulation and floor insulation, create thermal bridges
5. Check for condensation risk when adding insulation to existing walls; a condensation risk analysis may be needed
6. Listed buildings may be exempt from some upgrade requirements if the improvements would unacceptably alter the building's character
7. Keep records of all thermal upgrades for the EPC and for future property sales