Insulation choices that don't cause damp in an older home

Internal wall insulation on solid walls

Where solid walls need internal insulation, breathable options such as woodfibre board with a lime plaster finish, or calcium silicate board, allow the wall to continue managing moisture while adding thermal performance. These systems cost more than standard foil-backed foam board but avoid trapping moisture at the cold face of the original wall, which is where interstitial condensation causes hidden rot in timber joist ends built into the wall.

Foam and foil-backed insulated plasterboard is faster and cheaper to fit and can work on cavity walls without issue, but on a solid wall it risks pushing the dew point to a location where condensation forms inside the wall structure rather than on its surface, out of sight until timber decay or damp patches appear.

External wall insulation

Where the elevation and planning situation allow it, external wall insulation avoids the internal moisture risk entirely by keeping the original wall warm and dry on the inside while adding insulation and a new render finish outside. This is rarely appropriate on a listed building or in a conservation area where the external appearance is protected, but can be a strong option on a rendered 1930s semi or a rear elevation not visible from the street.

It does change the building's external dimensions slightly and needs careful detailing around window reveals, eaves and door thresholds to avoid creating cold bridges or trip hazards at thresholds.

Loft and roof insulation

Loft insulation is usually the best value thermal upgrade available in an older home, but it must be installed without blocking eaves ventilation, since a cold, unventilated roof space is prone to condensation forming on the underside of the roof covering. Insulation should stop short of the eaves or be paired with proprietary ventilation baffles that maintain airflow.

Where a loft is being converted rather than just insulated, the insulation strategy changes entirely, since the roof structure itself needs insulating between and often below the rafters, with careful attention to ventilation and vapour control layers to avoid trapping moisture within the roof build-up.

• Use breathable insulation systems on solid walls, not foil-backed foam boards
• Keep eaves ventilation clear when adding loft insulation
• Consider external wall insulation on unlisted, non-visible elevations
• Get any wall build-up modelled for condensation risk before insulating

Floor insulation

Suspended timber ground floors in older properties can be insulated between the joists using a breathable insulation material, provided under-floor ventilation through airbricks is maintained to stop timber decay from trapped moisture. Sealing up airbricks to reduce draughts while insulating is a common mistake that leads to rotten joists within a few years.

Solid concrete ground floors are harder to insulate retroactively without raising floor levels, and any attempt should include a proper damp-proof membrane detail to avoid trapping ground moisture beneath the new insulation layer.

Specifying insulation properly

Ask whether the proposed insulation system has been chosen with the wall or roof's existing moisture behaviour in mind, not just its U-value. For solid wall properties, ask specifically whether the specification is breathable and what ventilation is maintained where relevant.

On a whole-house retrofit, it's worth having the insulation strategy considered as one system across walls, floor and roof, since improving one element without the others can shift moisture problems to whichever part of the building is weakest.

Common questions