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 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 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, or
- Partial-fill with PIR 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 between the studs instead.
The roof
Flat roof
Build it as a warm roof: PIR 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 and finish with insulated plasterboard below, the same way as a loft conversion.
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
- Insulated plasterboard and standard plasterboard
- Plywood 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 and Surrey, or check your region on 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.