The Ecologist's piece on decarbonising the UK's housing stock covers familiar ground on scale and cost. The part worth pulling out — because it is the part homeowners consistently underestimate — is what happens to moisture when you seal a leaky house.
Old houses breathe by accident
A Victorian terrace with a suspended timber floor, an open flue and single-glazed sashes has appalling heat loss and excellent unintentional ventilation. Every litre of water vapour produced by cooking, washing and breathing gets carried out through gaps.
Now insulate it, draught-proof it and fit modern glazing. The heat loss drops, which is the point. But the moisture has nowhere to go, and it will find the coldest surface in the building and condense there — typically a window reveal, a corner where two external walls meet, or behind a wardrobe on an external wall.
That is the mechanism behind a great deal of what gets reported as damp. It is not rising damp and it is usually not a leak. It is condensation caused by an improvement.
What good practice requires
This is precisely why PAS 2035 makes a ventilation assessment part of the retrofit process rather than an optional extra. Done properly, insulation work is specified alongside a ventilation strategy — extract fans that actually meet the required flow rates and are actually ducted to outside, background ventilators, or mechanical ventilation with heat recovery on deeper projects.
Done improperly, someone insulates the walls, the occupant closes the trickle vents because of draughts, and mould appears within two winters.
The regulatory backdrop is tightening
Damp and mould are no longer just a comfort issue. Awaab's Law brought enforceable timescales for social landlords in October 2025, and Phase 2 extends the regime from 30 November 2026 to a much broader set of hazards including excess cold. The direction of travel is that cold, damp homes become a legal problem, not merely an uncomfortable one.
If you are planning insulation work, get the ventilation looked at in the same breath. Our retrofit assessment covers condition, occupancy and ventilation alongside the energy modelling — and we do not sell the measures, so there is nothing we are steering you towards.