That distinction — accidental air versus designed air — is the whole subject. Get it right and an old house becomes warm, quiet and healthy. Get it wrong in either direction and you have either a leaky house or a condensation problem.
Measured, not assumed: a real as-found baseline
Most talk about draughty old houses is anecdote. On our whole-house projects, leakage is measured before anything is designed.
At Orchard House, a 1970 stone house near Banbury, a pre-works pulse test recorded air changes per hour at 4 Pa ranging from 1.4 in the large sitting room to 5.6 in one bedroom — a fourfold variation between rooms of the same house. These numbers are the as-found baseline, measured before the works, and they told us something the anecdotes never do: the house was leaky in the wrong places. Some rooms leaked heavily; others were surprisingly tight — yet the building had no deliberate background ventilation anywhere. The result was the worst of both worlds: high heat loss in the leaky rooms, and condensation and mould at the cold stone window surrounds where moist air met an uninsulated bridge.
That is what “draughts are ventilation” actually looks like when you measure it: neither efficient nor healthy, just random.
Where houses leak: junctions, not walls
Air does not leak through the middle of a brick wall. It leaks where things meet: window frame to masonry, joist end to wall, ceiling to eaves, pipe to plaster, floor to skirting. Airtightness is therefore not a product but a line — a continuous air-control layer you should be able to trace around the whole building section with a pencil, never lifting it off the paper.
These are the real details from our project record:
Walls: lime plaster as the air-control layer. On our internally insulated solid walls, the airtight layer is the continuous, mesh-reinforced lime plaster over the insulation — repairable, vapour-open, and unbroken across the wall face. Beneath it, a lime parge coat of at least 8 mm levels the masonry and closes the air paths through old joints, and boards are bedded with no air pocket behind them: a void behind an insulation board is a highway for moist air.
Windows: taped, not masticked. Every frame-to-wall junction is sealed with airtightness tape lapping frame to masonry — on Orchard House, pre-compressed impregnated sealing tape (Iso-Chemie Bloco One is named in the specification) links weather seal, insulation and airtight layer in a single detail. On a North Oxford Victorian semi, each of thirty-four replacement sashes was set on insulating structural blocks with compressible expanding tape in the frame gap and an internal airtightness tape sealing the junction — a thermal-bridge-free, airtight install repeated window by window.
Reveals sealed before plaster. At Orchard House, airtightness tapes sealed reveals, perimeters and every service penetration before the lime plaster went on. The tape is buried in the finished wall — invisible, and permanent.
Joist ends: the leak nobody sees. Where floor joists bed into an external wall, warm air can leak past the timber into the masonry. On Howard Street in East Oxford the joist ends were bedded in a lime parge coat, and the wall insulation carried down through the intermediate floor void between the joists — closing a leak path that runs around the entire house at every floor level.
Roof slopes: taped vapour-control membranes. In every insulated roof slope — the nine skeiling zones at Orchard House, the attic storey of the St Margaret’s Road semi — a vapour-control layer runs across the rafters with every junction and lap taped airtight, while a ventilation gap above the insulation keeps the cold side of the roof breathing.
Chimneys and old penetrations. Redundant flues are draught-sealed and ventilated appropriately, and redundant extract holes are insulated and sealed airtight rather than left as permanent leaks.
Airtight is not the same as vapour-tight
The persistent fear about sealing old houses — “the walls need to breathe” — confuses two different kinds of movement.
- Air leakage carries heat and large amounts of moisture through gaps, fast. It should be stopped.
- Vapour diffusion moves moisture slowly through materials. In solid-walled houses it should be preserved.
Our assemblies do both at once: lime plaster, wood fibre and vapour-open paints (specified at Sd ≤ 0.1 m) let the wall keep absorbing, buffering and releasing moisture — while the taped junctions and continuous plaster stop the bulk air movement. The house is sealed against draughts and open to vapour. That combination, not one or the other, is what modern retrofit practice for traditional buildings demands.
The other half of the bargain: designed ventilation
Every airtightness detail above was installed alongside a ventilation plan, never instead of one. At Orchard House, the same programme that taped the reveals also installed humidity-controlled whole-house extract ventilation — moisture removed at source in the wet rooms, planned background air to the habitable rooms, decided before the windows went into production so the inlets were designed into the fabric rather than bored through it afterwards.
Seal a house without that plan and you trade draughts for condensation. Seal it with the plan and you get the real prize: even warmth, lower bills, no cold streams of air across the floor — and better air quality than the draughty house ever had, because the fresh air now arrives where and when it is needed. The full logic is in our ventilation after insulation guide.
What this means for your house
If your home is pre-war and draughty, the sequence that works is the one we run on every whole-house project: measure the leakage as found, design the air-control line and the ventilation system together, build the airtightness into every junction while the fabric is open — and commission the ventilation before calling the job done. Draughts are not a feature of old houses. They are just the leaks nobody has fixed yet.
This article is part of our whole-house retrofit guide. Related reading: internal wall insulation for solid brick and the full Orchard House project.