How to Improve Your Home's Insulation During a Renovation
A renovation or extension project is the best opportunity to improve your home's insulation — because the disruption of opening up walls, floors, and ceilings is already happening for other reasons. Adding insulation at marginal cost during this disruption avoids the much greater cost and disruption of retrofitting insulation separately. Here's where the biggest gains are and how to approach each element.
Roof and Loft Insulation
Heat rises — and a poorly insulated roof is the single biggest source of heat loss in most homes. Where a loft conversion is being carried out, the roof insulation is designed and installed as part of the project to meet current Part L standards. Where a roof is being replaced or the loft is being disturbed for other reasons, this is the time to upgrade insulation. Current Part L requires a U-value of 0.15 W/m²K for roofs — achievable with 270mm of mineral wool in a cold roof, or 120mm of PIR board in a warm roof.
Where a loft is not being converted and is accessible, upgrading the loft insulation (a cold loft, with insulation at ceiling level) is the most cost-effective insulation measure available — typically £300–£600 for a standard terraced house and frequently grant-funded through energy efficiency schemes.
Wall Insulation
External walls account for approximately 30–35% of heat loss in a typical Victorian terrace. The appropriate insulation solution depends on the construction type:
Solid brick walls (Victorian and Edwardian properties): Internal wall insulation (warm wall system or PIR board) or external wall insulation (where the exterior is being re-rendered). Current standard: 0.18 W/m²K. A renovation involving replastering or re-rendering is the opportunity to add insulation at marginal cost.
Cavity walls (1920s+): Cavity wall insulation (blown mineral fibre, polystyrene bead, or foam) is the most cost-effective solution for uninsulated cavity walls. This can be installed without any internal disruption and is frequently grant-funded. Check whether the cavity is already insulated before assuming it isn't.
Floor Insulation
Ground floors account for approximately 15% of heat loss in an uninsulated property. A renovation that involves disturbing the ground floor — for a new extension, a knock-through, or a ground-floor refurbishment — is the opportunity to add insulation beneath the new screed or floor finish.
For suspended timber floors, insulating between the joists from below (where accessible via a crawl space) or above (where the floor is being relaid) is effective and significantly improves comfort in what are otherwise cold floors in winter.
Windows and Doors
Upgrading to A-rated double or triple glazed windows when windows are being replaced as part of a renovation is a cost-effective improvement — the marginal cost over standard replacement windows is modest. Draught-proofing existing windows and external doors (where replacement isn't planned) is the lowest-cost improvement of all and often overlooked.
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