Where air leaks hide in an old house
Applies to: Assumes a masonry-walled house with timber floor joists built into the walls, suspended timber ground floor, and a later-added heating system.
Last worked on 2015-03-11
Air leakage is not spread evenly across a building. It concentrates at junctions, and in an older masonry house the junctions are nearly all horizontal.
Why junctions and not walls
A solid masonry wall is a poor insulator but a fairly good air barrier. Brick and lime are not airtight in the laboratory sense, but a plastered wall with no penetrations moves very little air compared with the gaps around it. The leaks are at the edges, where one material stops and another starts, and where the builder had to leave tolerance to fit a component.
This matters because it changes what you do first. Insulating a wall you have not sealed will disappoint you: the insulation slows conduction, but air moving past it carries heat away regardless. Sealing first is usually cheaper, less disruptive and more noticeable.
The floor-to-wall junction
Where a suspended timber floor meets a masonry wall there is a gap by design. The joist ends sit in pockets, the boards stop short of the plaster, and the skirting covers the difference. Underneath the floor is a ventilated void that is deliberately open to outside air, so the gap at the skirting is a direct path from outside to the room.
You can usually feel this on a windy day by holding the back of your hand at the skirting line. It reads as a cold draught at ankle height rather than as cold air falling from a window, which is the usual confusion.
The ceiling-to-wall junction and the stair
At the top of the house the same gap runs the other way. Ceiling joists are built into the wall, the lath or board stops short, and above it is a roof space that is also ventilated on purpose. Warm air that has risen up the stair leaves there.
In a tall narrow house the stair is the mechanism, not the culprit. Every leak at the top of the building pulls air in at the bottom, which is why a draughty ground floor and a warm-but-leaky top floor are the same problem seen from two ends.
Service penetrations
Every pipe, cable and duct added after construction was fitted through a hole cut oversize. Almost none of them were sealed, because in a house with open fires the leakage was part of the ventilation strategy. Once the fires are gone the holes remain.
Redundant flues are the largest single one. A disused chimney with an open throat and an open pot is a continuously running extract fan with no off switch. Capping it needs care: a flue that is sealed at both ends without any through-ventilation will trap moisture in the stack.
Order to work in
- Find and treat the largest single opening first, which is nearly always a disused flue or an unsealed loft hatch.
- Seal the ceiling-to-wall junction on the top floor, because that is what drives the stack effect for the whole house.
- Seal service penetrations, working from the top of the building downward.
- Seal the floor-to-wall junction behind the skirting on the ground floor.
- Only then add insulation, so that you are insulating a building that is no longer pumping air through the new layer.
Air sealing an old house is not the same as making it airtight. The aim is to replace uncontrolled leakage with ventilation you choose - trickle openings, extract where moisture is made, and openable windows - so that the house dries itself without also emptying itself of heat.