Insulation Explained: A Contractor's Guide

1Why Insulation Matters

Building Regulations Part L (Conservation of Fuel and Power) sets minimum thermal performance standards for all new-build and renovation work in England and Wales. Choosing the right insulation is critical to passing SAP calculations, achieving energy performance targets, and reducing running costs for your clients.

2Understanding U-Values

A U-value measures how quickly heat passes through a building element — the lower the number, the better the insulation. Current Part L targets are approximately 0.18 W/m²K for roofs, 0.26 W/m²K for walls, and 0.18 W/m²K for ground floors. Your building control officer or SAP assessor will confirm the specific requirements for your project.

3Mineral Wool (Glass & Rock Wool)

Mineral wool is the most widely used insulation product in the UK — available as rolls, slabs, and loose fill. It is non-combustible (Class A1), vapour permeable, and provides good acoustic performance in addition to thermal insulation.

• Glass wool rolls — cost-effective solution for loft insulation laid between and over joists.
• Rock wool slabs — preferred for cavity walls, party walls, and between timber studs due to better rigidity.
• Acoustic mineral wool — higher-density products specifically designed to reduce airborne and impact sound.

4Rigid Foam Boards

Rigid insulation boards offer much higher thermal resistance per mm than mineral wool, making them ideal where space is limited. The three main types are:

• EPS (Expanded Polystyrene) — lowest cost rigid board. Used for floor insulation, cavity fill, and external wall insulation (EWI) systems.
• XPS (Extruded Polystyrene) — higher compressive strength than EPS; preferred for below-ground and inverted flat roof applications.
• PIR / PUR (Polyisocyanurate / Polyurethane) — the highest thermal performance per mm. Used in warm roof build-ups, floors, and wall cavities where thickness is critical.

5Spray Foam Insulation

Spray polyurethane foam (SPF) is applied as a liquid that expands and hardens, filling voids and bonding to substrates. It provides an excellent air seal in addition to thermal insulation. However, spray foam in lofts has attracted significant controversy because it can make roofing timbers difficult to inspect and may reduce mortgage valuations — always discuss this with your client before specifying.

6Vapour Control & Membranes

All insulation systems must manage moisture correctly. In warm roof and floor constructions, a vapour control layer (VCL) on the warm side of the insulation prevents moisture-laden air from reaching the cold structure and condensing. Breathable membranes on the cold side allow any trapped vapour to escape. Always check the manufacturer guide for the correct membrane pairing.

7Acoustic Insulation

Meeting Approved Document E (sound) requirements in flats, conversions, and party-wall situations requires insulation with specific acoustic credentials. Look for products with a tested Rw (weighted sound reduction index) value and ensure your build-up has been tested as a complete system rather than relying on individual product data.