Airtightness in NZ Homes: What It Is and Why It Matters

It measures how much air moves through a building envelope when you do not want it to. The test is a blower door: a calibrated fan is sealed into a doorway and used to hold the house at 50 pascals of pressure, and the airflow required to maintain that tells you how leaky the building is. The result is reported as air changes per hour at that pressure, written ACH50, which lets you compare houses of different sizes honestly.

The Passive House standard sets the bar at 0.6 air changes per hour at 50 pascals, which is a demanding target reached by detailing and workmanship rather than by any single product (PHINZ). Most older New Zealand homes test many times leakier, which is the plain physical reason they are expensive to heat and never quite comfortable (BRANZ). Our guide to why NZ homes are cold and damp traces the same problem from the occupant’s side.

Because New Zealand’s Code has historically regulated insulation rather than air leakage. Clause H1 sets thermal requirements, and those have tightened, but there is effectively no airtightness target a house must meet and no requirement to test. The practical consequence catches people out: a home can be fully compliant, newly built and signed off, and still leak air continuously through its junctions.

That gap matters because insulation and airtightness work together. Insulation slows heat moving through materials; airtightness stops heat being carried out bodily by moving air. Excellent insulation with a leaky envelope is like a good jacket left unzipped, and it is one reason people are surprised that a well-insulated new home still feels draughty.

Four things, and only the first is obvious. Heat, carried straight out of the building by air you have paid to warm. Moisture, because warm indoor air holds water and carries it into wall and roof cavities where it meets cold surfaces and condenses, quietly degrading the structure over years. Noise, since the gaps that pass air pass sound. And dust and pollen, which arrive with every draught.

The durability point deserves emphasis, because it is the one nobody sees. Condensation inside a wall does not announce itself; it works away for years and shows up as a problem long after the cause is buried.

Not if it is done properly, and the phrase to remember is build tight, ventilate right. Sealing the envelope does not remove the need for fresh air, it removes the pretence that draughts were supplying it. Once the building is tight you introduce air deliberately, filtered and heat-recovered, at a steady rate, which is what mechanical ventilation with heat recovery does continuously and quietly.

The alternative, relying on leakage, gives you ventilation exactly when you do not want it and none when you do: hardest on cold windy nights, almost nothing on a still day. That is why a leaky house manages to be both draughty and stale. The companion piece, mechanical ventilation with heat recovery, explains how the fresh air side works.

By detailing and care, not by buying a product. The airtight layer has to be continuous around the whole building, which means it must be designed as a line you can trace on a drawing without lifting your pen, and then built that way. Most leakage happens at junctions and penetrations rather than through the middle of walls: floor to wall, around windows and doors, at the ceiling, and wherever a pipe, cable or duct passes through.

That makes it a workmanship issue, and it is one of the clearest examples of why the invisible parts of a build decide its quality, which we argue in what sets a high-end build apart. It also has to be planned before construction, because every trade that comes afterwards can puncture it. A blower door test at the right stage confirms it worked while there is still time to fix anything.

Airtightness is the measure of how much air escapes your home uncontrolled, tested by blower door as ACH50, and it is the difference between a home that holds its warmth and one that leaks it. The Passive House target is 0.6 ACH50; typical older New Zealand homes are far leakier, and the Building Code sets no meaningful requirement, so it will only happen if you specify it. Sealing the envelope also protects the structure from moisture and makes the house quieter.

Airtight is not sealed-in: pair it with proper mechanical ventilation and the home is fresher than a leaky one, not stuffier. If you want a home built and tested to that standard in Nelson Tasman, arrange a consultation and we will talk you through how we detail and verify it.