Leaky Buildings Don’t Help As Much As You Think
Spoiler: Natural leakage doesn’t cut it. If you think your building is “breathing” enough to manage moisture loads, you’re in for a damp surprise. Let’s unpack the myth and look at what really works for keeping your building dry, healthy, and mould-free.
Why We Still Hear “Leaky Is Better”
Somewhere along the way, the idea stuck that air leakage was good. A bit of “natural ventilation” never hurt anyone, right?
Well, wrong.
This mindset comes from older buildings that leaked like sieves. And yes — in the days before double glazing, synthetic building wraps, and air control layers — fresh air found its way in. But it also meant cold, drafty homes and sky-high energy bills.
Today, we try to build more airtight for comfort, durability, and performance. But the assumption that a bit of leakage will still solve your moisture problems lingers. And that’s where things start going sideways.
How Much Air Do You Actually Need to Move?
Let’s do the math. Humans produce moisture — around 4 litres per person per day, just by cooking, showering, breathing, and living.
To remove that through passive air leakage alone, you'd need to replace about 464 m³ of air per person, per day.
That’s the entire volume of a 182 m² house with 2.5 m ceilings. Every. Single. Day.
Now, most new homes sit around 5 ACH (air changes per hour at 50 Pa). That sounds like a lot, but in real-life pressure conditions, it’s not shifting nearly enough air to keep up with internal moisture generation.
Unless your home is blowing at 13+ ACH50, you’re not getting close.
The Moisture Load Isn’t Going Anywhere
Your building doesn’t magically lose moisture just because it leaks a little. Here’s what happens instead:
Moisture Accumulates in the Wrong Places
Warm, moist air finds cold surfaces. Think:
Inside wall cavities
Behind ceiling linings
Around poorly sealed junctions
At thermal bridges
That’s where it condenses. It doesn’t escape — it hides. Quietly soaking into materials over time until mould, damage, or rot shows up.
Ventilation Can’t Be Left to Chance
Moisture management needs active systems: fans, extract points, ducting, and controls. Otherwise, you’re just hoping air goes where it should. That’s not a strategy — that’s a gamble.
Common Excuses We Hear (And Why They Don’t Work)
Let’s clear up a few misconceptions.
“I open the windows every day.”
Do you, though? And even if you do, do you open them long enough — and during the right conditions — to create meaningful air exchange? What about winter? Or night? Or days when it’s raining sideways?
“We don’t have condensation issues.”
No visible condensation ≠ no moisture problems. Hidden mould and rot don’t announce themselves until it’s too late. Our blower door testing regularly uncovers homes with hidden damage, even when things looked “fine” on the surface.
“My house is old and drafty, so I don’t need ventilation.”
That’s the myth in action. Even drafty homes trap moisture — and you still need proper extraction in wet areas like bathrooms and kitchens. Passive leakage is inconsistent and uncontrollable. That’s the opposite of what you want for a healthy, dry building.
What the Code (and Science) Actually Say
H3: NZ Building Code Clause G4
The code requires that ventilation be adequate to remove moisture. But it doesn’t say “leave gaps in your walls and hope for the best.” It refers to extract fans and mechanical systems where needed.
ASHRAE, Passive House, and Everyone Else
Modern standards all agree: rely on controlled ventilation, not infiltration. Passive House even designs to airtightness levels 10x better than typical new homes — and compensates with balanced ventilation systems that actually work.
Case Study: What We See in the Field
At BEO Buildingscience, we’ve tested hundreds of homes across New Zealand and Australia. Here's what we’ve learned:
Leaky ≠ Safe
Even homes at 8–10 ACH still suffer condensation and mould issues if the ventilation strategy is poor or nonexistent.
Fan Underperformance is the Norm
Even when fans are installed, they’re usually:
Underrated (too small)
Underused (turned off too early)
Not maintained (clogged or broken)
Which brings us to this: if you don’t size and run your fans properly, you’re not ventilating.
So What Does Actually Work?
Start with the Numbers
4L moisture per person per day
25 L/s fan = needs 5 hours of runtime per person
50 L/s fan = needs 2.5 hours per person
If your fan isn’t running that long, it’s not removing that moisture.
Add a Run-On Timer or Sensor
Hook your fan up to a humidity sensor or run-on timer so it doesn’t cut out when the mirror clears.
Consider Continuous or Balanced Systems
For new builds or major renos, look at:
Continuous systems with balanced supply and extract (simple and effective)
Balanced heat recovery systems (MVHR/ERV) if airtightness is part of the goal
Why This Matters (A Lot)
Moisture is the silent destroyer of buildings. It shows up in:
Fogged windows and musty smells
Mould in wardrobes and ceiling corners
Damaged insulation
Degraded linings and fixings
Hidden rot in timber framing
Respiratory health issues
Getting your ventilation right isn’t a luxury — it’s a foundation for a durable, comfortable home.
The Bottom Line
If you’re still relying on a “leaky” building to solve moisture issues, you’re gambling with your health and your building’s lifespan.
Here’s what to take away:
Most homes are not leaky enough to manage moisture loads
4L of moisture per person requires 464 m³ of air replaced daily — more than your whole house volume
25–50 L/s fans need hours, not minutes, of runtime to keep up
Passive leakage is inconsistent and ineffective
Mechanical ventilation — even simple fans — needs to be sized and run properly
