# How to Insulate a House Extension: Floor, Walls and Roof

A new extension has to meet current Building Regulations for energy efficiency, which means insulating the floor, walls and roof to set U-values. Getting the build-up right at design stage avoids a failed inspection and makes the new room as comfortable as the rest of the house.

## U-values to aim for

For new elements in an existing home in England, Approved Document L sets these limiting U-values (lower is better):

-   **Walls:** 0.18 W/m²K
-   **Floors:** 0.18 W/m²K
-   **Roofs (pitched or flat):** 0.15 W/m²K

Wales and Scotland have their own requirements. Your designer or building control will confirm the figures for your project, and the large glazed areas common in extensions can affect the overall calculation.

## The floor

On a ground-bearing concrete slab, [PIR floor insulation](/collections/floor-insulation) is usually laid over the slab and damp-proof membrane, then covered with a separating layer and screed. Run a perimeter strip of insulation up the edge of the screed to stop heat escaping at the wall junction. Where boards will sit in damp conditions below the slab, [XPS](/collections/xps) is the better choice. For suspended timber floors, fit PIR boards between the joists, supported on battens.

## The walls

Most extensions are built with masonry cavity walls. You can either:

-   **Full-fill** the cavity with mineral wool [cavity batts](/collections/cavity-boards), or
-   **Partial-fill** with [PIR cavity boards](/collections/cavity-boards), leaving a clear residual cavity as set out in the manufacturer's guidance.

PIR gives more performance per millimetre, so it suits a narrower cavity. Mineral wool is cheaper and non-combustible. Either way, close the cavity around windows and doors with insulated cavity closers to avoid cold bridging. Timber-frame extensions use [timber frame boards](/collections/timber-steel-frame-boards) between the studs instead.

## The roof

### Flat roof

Build it as a warm roof: [PIR boards](/collections/pir-insulation-boards) on top of the timber deck with a vapour control layer beneath them, then the waterproofing on top. The structure stays warm, so condensation in the joists isn't a problem and no ventilation is needed. Cold flat roofs, with insulation between the joists, are more prone to condensation and are best avoided.

### Pitched or vaulted roof

Insulate between the rafters with [pitched roof boards](/collections/pitched-roof-boards) and finish with [insulated plasterboard](/collections/insulated-plasterboard) below, the same way as a [loft conversion](/pages/loft-conversion-insulation).

## Typical materials list

-   PIR floor boards and perimeter edge strip
-   Cavity batts or PIR cavity boards
-   PIR boards for a warm flat roof, or pitched roof boards
-   Damp-proof and vapour control [membranes](/collections/membranes)
-   Insulated plasterboard and standard [plasterboard](/collections/plasterboards)
-   [Plywood](/collections/ply) or OSB for the roof deck, plus foil tape and fixings

## Extensions near you

We deliver extension packages across most of England. See our local pages for [Birmingham](/pages/insulation-supply-birmingham) and [Surrey](/pages/next-day-insulation-surrey), or check your region on [Areas We Serve](/pages/areas-we-serve).

Send your drawings or target U-values to sales@nextdayinsulationltd.co.uk and we'll work out thicknesses, quantities and a delivered price.

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> Source: [Next Day Insulation](https://nextdayinsulations.com/pages/extension-insulation)