Insulation, Ventilation And The Trade-Off Between Them

Improving a building's thermal performance and keeping it dry are related problems that pull in opposite directions. Work that ignores the link creates the second problem while solving the first.

Older buildings were designed to leak

Draughty windows, open chimneys and suspended floors gave older houses continuous background air movement. That movement carried away the moisture produced by cooking, washing and breathing.

The heat loss was substantial and the ventilation was accidental rather than designed. Nobody specified it, but the building depended on it.

Removing those paths without replacing the airflow leaves the moisture with nowhere to go, and it condenses on whatever surface in the house is coldest.

Where the moisture goes instead

Warm air holds more water vapour than cold air. When it meets a cold surface it cools, reaches saturation and deposits water on that surface.

The cold surfaces in a partly insulated house are the parts that were missed: window reveals, junctions between wall and roof, and the corners of external walls.

Mould follows persistent surface moisture rather than a plumbing failure, which is why the pattern of black growth in a corner looks nothing like a leak.

Insulation moves the dew point

Insulating on the inside of a solid wall keeps the wall itself cold, because the heat is now stopped before it reaches the masonry.

If warm internal air can reach that cold masonry through gaps in the insulation, condensation forms inside the construction where nobody can see it and timber sits wet.

This is why detailing at edges, service penetrations and floor junctions matters more than the thickness of the material, and why poorly executed work performs worse than none.

Ventilation has to be deliberate

A sealed building needs a designed route for moist air to leave: extraction at the sources, controllable background vents, or a mechanical system that recovers heat as it exchanges air.

Extraction only works if it is used and if replacement air can enter. A sealed room with a fan and no air path simply runs the fan against a vacuum.

Occupant behaviour affects the load but cannot substitute for the route. A household producing ordinary amounts of moisture in a building with no ventilation will still see condensation.

What a surveyor is looking for

Evidence of recent insulation work alongside blocked chimneys, sealed vents and replacement windows suggests a building whose moisture balance has been changed.

Mould patterns at thermal bridges, staining on window reveals and condensation between panes tell a different story from rising ground moisture or a defective gutter.

Distinguishing the causes matters because the remedies are unrelated. Treating a ventilation problem as a plumbing one produces expense and no improvement.