Does external wall insulation cause damp?
The short answer
Well-designed external wall insulation usually keeps walls warmer and drier, because the wall sits inside the insulation and is protected from rain. Damp problems come from poor detailing that lets water behind the system, unsuitable vapour-tight systems on old lime-built walls, or poor ventilation inside.
Why EWI usually helps
Condensation forms on cold surfaces. Insulating outside keeps the masonry and the inside wall surface warm, so condensation and mould on outside walls usually reduce. The render also stops driving rain soaking the masonry.
How it goes wrong
- Roof overhangs too short, so rain runs behind the top of the system.
- Window sills not extended, so water runs into the system below.
- The base of the system at or below ground level, wicking water up.
- Gaps around pipes, cables and vents left unsealed.
- Vapour-tight systems on old walls that need to dry outwards.
- Draughts sealed without improving ventilation inside.
Ventilation matters too
Insulating makes a house more airtight. Trickle vents in windows and working extract fans in kitchens and bathrooms remove the moisture daily life produces. We check ventilation at survey and recommend improvements where they are needed.
Related services
Questions people ask
Can EWI go on a damp wall?
Not until the cause is fixed. Rising damp, leaking gutters or high ground levels are dealt with first.
Which systems are most breathable?
Mineral wool and wood fibre with mineral or lime renders; insulating lime render on its own.