Home Insulation in London: Types, Costs and What Works in Victorian Houses

Loft insulation is almost always the highest-priority insulation measure for London houses with accessible loft spaces, because heat rises and an uninsulated loft allows a significant proportion of the home's heat to escape. Standard cold loft insulation (insulating the loft floor between and over the joists) is straightforward and cost-effective. The current UK standard for loft insulation is 270mm total depth. This is achieved by laying 100mm between the joists (filling the joist depth) and then 170mm of mineral wool laid at right angles over the joists. Material cost for 270mm mineral wool for a typical London terrace loft floor (25-35m2): £150-£300. Professional installation of loft floor insulation: £200-£500 for a standard terraced house loft. Government-funded insulation schemes (such as the Great British Insulation Scheme or ECO4) may fund some or all of loft insulation costs for eligible homeowners — check eligibility on the government website. If the loft is being converted, the insulation position moves from the loft floor to the roof slope (warm roof insulation between and below the rafters) — this is covered in the loft conversion costs guide.

Victorian and Edwardian London houses have solid brick walls (typically 225mm of brickwork), not modern cavity walls. There is no cavity to inject insulation into. The two main options for insulating solid brick walls are external wall insulation (EWI) and internal wall insulation (IWI). External wall insulation (EWI) involves fixing rigid insulation boards (typically EPS expanded polystyrene, mineral wool, or phenolic board) to the outside of the wall, then applying a reinforced render system over them. EWI achieves good U-values (0.3 W/m2K or better) without reducing internal floor area, but: it changes the external appearance of the building significantly; it requires planning permission in conservation areas and for listed buildings; it must wrap around reveals and soffits to avoid cold bridging; and the render system must be compatible with the existing substrate. Cost of EWI in London: £80-£150/m2 supply and install including render system and any scaffolding. For a typical end-of-terrace London house with 3 exposed elevations (approximately 80m2 of external wall), total EWI cost: £6,500-£12,000. EWI is particularly effective for end-of-terrace and detached houses where multiple elevations can be treated.

Internal wall insulation (IWI) installs insulation on the inside face of external walls, reducing the internal room dimensions but not changing the external appearance. This makes it suitable for conservation areas and properties where EWI is not appropriate. Options for internal wall insulation in Victorian houses: Rigid insulation board (PIR or phenolic) with plasterboard finish: highly effective (25-50mm of PIR gives a significant improvement in U-value) and relatively thin, but impermeable — must not be applied to walls with residual dampness or rising damp. Woodfibre, hemp-lime, or cork board insulation boards: breathable (vapour-open) insulation materials compatible with solid brick walls and lime plasterwork. Slower to install and with a lower thermal performance per mm than PIR, but appropriate for Victorian buildings where maintaining breathability is important. Insulated plasterboard (PIR bonded to the back of plasterboard): the thinnest practical IWI option. Typical costs for IWI in London: PIR-based systems £50-£90/m2 supply and install; breathable insulation systems £70-£120/m2. Key planning consideration: IWI on solid brick walls in Victorian houses requires careful assessment of the building's moisture dynamics to avoid creating interstitial condensation problems. Seek specialist advice before specifying IWI on solid Victorian walls.

Victorian London houses typically have suspended timber ground floors over a sub-floor void. Ground floors are a significant source of heat loss and cold draughts. The options for ground floor insulation in London terraces: Insulation beneath floorboards: the most effective approach for suspended timber floors. Rigid insulation (PIR boards or mineral wool batts) is fitted between and supported below the floor joists from underneath (via the sub-floor void where accessible through an airbrick) or from above (by lifting the floorboards). R-value improvement is significant. Cost of lifting and refitting floorboards with 100mm PIR floor insulation below: £60-£100/m2 supply and install. Draught-sealing: a lower-cost step before full floor insulation, sealing gaps between boards and around pipes and service penetrations significantly reduces draughts and heat loss. Draughtproofing compounds and draught-excluding tapes: £5-£15/m2 DIY. New insulated slab: if the ground floor is being dug out (as part of an extension, damp remediation, or UFH installation), a new concrete slab with integral insulation below it is the most effective outcome. 100mm of PIR below a 65mm screed: approximately £40-£70/m2 for the insulation and slab (excludes excavation costs).

Windows are a significant source of heat loss in older London properties, particularly those still fitted with original single-glazed timber sash windows. Modern double-glazed windows (argon-filled low-e glass) have a U-value of approximately 1.4-1.6 W/m2K versus 5.8 W/m2K for single glazing. Secondary glazing (fitting an inner pane of glass or clear acrylic inside the existing window frame) is a popular option in conservation areas and listed buildings where planning restrictions prevent replacement of original windows — it achieves a U-value of 1.6-2.0 W/m2K and preserves the original window. Options for London properties: double glazing replacement in standard UPVC or aluminium: £400-£900 per window supply and install. Timber double-glazed windows matching the original profile (required for conservation area and listed building projects): £1,200-£3,000 per window. Secondary glazing for existing single-glazed sash windows: £250-£600 per window. Draught-sealing existing timber sash windows with brush seals: £80-£200 per window DIY or professionally done — a very cost-effective first step.