Retrofitting airtightness is a different job to building it in. The envelope already exists, much of it is inaccessible, and the sequence that makes new construction straightforward is not available.
It is still worth doing, and the return per hour is often better than in new work, because existing buildings leak in a small number of predictable places.
Blower door testing on Australian housing stock finds the same paths repeatedly.
The ceiling plane. Almost always the worst. Downlights, exhaust fans, ceiling hatches, plumbing vents, wiring penetrations. The ceiling also has the highest stack pressure in the building, so a hole there moves more air than the same hole in a wall.
Wall to ceiling junctions. The top plate junction is rarely sealed in older construction.
Service penetrations. Plumbing, electrical, gas, air conditioning.
Windows and doors. Perimeter gaps between frame and structure, distinct from weatherstripping between sash and frame.
Subfloor. In suspended floor construction, gaps around penetrations and at the perimeter.
Chimneys and flues. Both the flue itself and the gap around it.
Retrofit work should follow leakage volume, not accessibility.
1. Ceiling plane. Highest leakage, and usually accessible from the roof space without disturbing finished surfaces. Sealing downlights, fans and penetrations from above is the highest-value work available in most existing houses.
2. Service penetrations throughout. Under sinks, behind appliances, at meter boxes, wherever a pipe or cable enters.
3. Window and door perimeters. Where the architrave can be removed, the gap between frame and structure can be sealed properly.
4. Subfloor and perimeter. Lower priority but worth addressing during other work.
Sealing penetrations and services
Sealing an existing building changes how it manages moisture, and doing it without addressing ventilation can cause problems that did not previously exist.
An old leaky house ventilates itself continuously. That is why it is uncomfortable and expensive to heat, and it is also why it usually does not accumulate moisture. Remove the leakage without providing controlled ventilation and the moisture load that was leaving through the gaps has nowhere to go.
The result is condensation on cold surfaces and, in time, mould. This is a well documented outcome of energy retrofit programmes overseas and it is entirely avoidable.
The rule: if you seal it, ventilate it. Mechanical extract in wet areas at minimum, discharging outdoors, sized for the actual moisture load.
Ventilation and condensation control
New construction with attention to detail reaches 3 to 5 air changes per hour at 50 pascals without exotic measures. Passive House requires 0.6.
Existing housing typically starts somewhere between 15 and 25 ACH50. Targeted retrofit work focusing on the ceiling plane and penetrations can often improve that substantially without touching finished surfaces.
Getting an existing building to Passive House levels generally requires stripping back to structure, which puts it in the category of major renovation rather than retrofit. That is a legitimate approach, and the EnerPHit standard exists for exactly that work, with a less demanding airtightness target than new-build Passive House.
The best retrofit opportunity is during work that is happening anyway.
If linings are coming off a wall, the air barrier and insulation can be addressed properly at a fraction of the cost of doing it as a standalone job. Same for reroofing, recladding, or any work that opens the envelope.
The practical advice for a renovation: when you open an assembly, ask what the control layers are doing in it, and fix what you can reach while it is open. You will not get another chance without paying for the access twice.
Retrofit work benefits from measurement more than new work does, because the leakage paths are unknown at the start.
A blower door test with smoke or thermal imaging identifies where the air is actually moving, which is frequently not where people assume. Testing before and after also quantifies what the work achieved, which matters if the client is paying for a performance outcome.
