Your Blower Door Test Came Back at 4.2. Is That Good?
ACH50 is the number that tells you how airtight your home really is. Here's what it means, how it's calculated, and what you should be targeting.
You’ve just received your blower door test report. There’s a number on it — 3.8, or 6.1, or 0.72. Maybe your builder is pleased with it. Maybe they’re not sure what to say.
The number is your ACH50 — Air Changes per Hour at 50 Pascals. It’s the single most important metric for understanding how airtight your home is, and whether your building envelope is performing the way it should.
This page explains what it means, how it’s calculated, and how your result compares to Australian benchmarks — including NCC 2022, MVHR requirements, and Passive House certification.
What is ACH50?
A blower door test works by mounting a calibrated fan in an external doorway and using it to pressurise (or depressurise) the building to exactly 50 Pascals above normal atmospheric pressure — roughly equivalent to a 32 km/h wind blowing on all sides simultaneously.
The fan measures exactly how much air is required to maintain that pressure. That airflow figure — in cubic metres per hour (m³/h) — is the raw test result. On its own it doesn’t tell you much, because a 600 m² warehouse and a 60 m² apartment could leak the same volume of air while having completely different airtightness levels.
ACH50 solves this by dividing the airflow by the building’s volume. The result is a normalised number — comparable across buildings of any size.
The formula
ACH50 = airflow at 50 Pa (m³/h) ÷ building volume (m³)
Example: 200 m² home, 2.7 m ceilings, test result 600 m³/h
Volume = 200 × 2.7 = 540 m³ → ACH50 = 600 ÷ 540 = 1.11
Building volume is usually floor area multiplied by average ceiling height. For multi-storey homes, add each level’s area × its ceiling height. Your test report may calculate this for you — but it’s worth confirming the volume figure used is accurate.
What do the numbers actually mean?
Context is everything. A result that sounds low can be excellent or alarming depending on what you’re trying to achieve.
| ACH50 | Benchmark | What it means |
|---|---|---|
| 15–20 | Average Australian home | Significant leakage. Draughts common. High energy use. |
| 5–8 | Good new construction | Typical for careful builders. Noticeable improvement over average. |
| ≤5 | NCC 2022 better practice | Above code. Good performance for most homes. |
| ≤3 | MVHR recommended | MVHR will operate as designed. Strong envelope performance. |
| ≤0.6 | Passive House | Certification threshold. Near-zero uncontrolled infiltration. |
Why ≤3 ACH50 for MVHR? MVHR systems are designed to control ventilation precisely — supplying fresh air and extracting stale air in measured amounts. In a leaky building, uncontrolled infiltration through gaps and cracks undermines this balance. At 3 ACH50 or below, the system can operate as designed. Above this, you’re effectively running a high-precision ventilation system in a building that’s also ventilating itself at random.
It’s not just about energy bills
Most people associate airtightness with heating and cooling costs — and the relationship is real. Every cubic metre of warm air that leaks out on a cold night is replaced by cold air coming in somewhere else, which your heating system then has to warm again. But the implications of a poor ACH50 go further than energy.
Moisture in the building fabric: Air carries moisture. When warm, humid indoor air moves through gaps in the building envelope into wall cavities, it can cool below its dew point and condense inside the structure. Over time this creates ideal conditions for mould — often in places you can’t see.
Uncontrolled ventilation paths: A leaky building isn’t randomly ventilated in all directions equally. Air follows paths of least resistance — often through roof cavities, around electrical boxes, or at wall junctions. These can draw pollutants, dust, and allergens from unconditioned spaces into living areas.
MVHR system effectiveness: If you’ve invested in MVHR, airtightness directly determines whether you get the outcome you paid for. A system sized for a 3 ACH50 home installed in a 10 ACH50 building will be outpaced by uncontrolled infiltration — fresh air supplied by the unit is diluted by outside air coming in through gaps.
Acoustic performance: Air gaps transmit sound. A more airtight envelope is also a quieter envelope — particularly relevant for homes near busy roads, flight paths, or urban noise sources.
Calculate your ACH50
Enter your blower door test result and building dimensions below. If you haven’t had a test yet, try the default values — they represent a typical 200 m² Adelaide home — and adjust the airflow to see how the rating changes.
ACH50 Calculator
Enter your blower door result to see your ACH50 rating and how it compares to Australian benchmarks.
Your ACH50 result
1.11
air changes per hour at 50 Pa
Excellent result. Your home is airtight enough for MVHR to work as designed and for the building to perform well above standard. Energy use, draught, and moisture control will all benefit significantly.
✓ NCC 2022 better practice
≤5 ACH50
✓ MVHR recommended
≤3 ACH50
✗ Passive House
≤0.6 ACH50
ACH50 = blower door result (m³/h) ÷ building volume (m³) · Volume = floor area × ceiling height (for single-storey homes)
Where does air leakage actually come from?
The frustrating thing about airtightness is that the biggest leaks are usually invisible. You can’t feel a 0.5 m² total leakage area spread across hundreds of small gaps — but the cumulative effect on your ACH50 is significant. The most common leakage paths in Australian new construction:
Ceiling/roof junction
The most common single source of leakage — where the ceiling plane meets the top plate or roof space.
Electrical and plumbing penetrations
Every cable, pipe, or conduit through the ceiling, walls, or floor is a potential leakage path if not carefully sealed.
Window and door perimeters
Frames that aren't properly taped or sealed to the wall structure can leak significantly, especially at corners.
Exhaust fan and range hood penetrations
Openings through the building envelope for exhaust fans and range hoods require careful detailing to avoid bypassing the airtight layer.
Downlight recesses
Recessed lights in ceilings are a notorious leakage path — the housing creates a direct opening into the ceiling cavity.
Slab perimeter (slab-on-ground)
The junction between slab, wall frame, and floor covering is often unsealed. Can be a significant source in single-storey slab homes.
This is why a blower door test mid-construction — before linings go on — is so valuable. With a smoke pencil and a depressurised building, leakage paths become visible and can be sealed while they’re still accessible. Trying to improve ACH50 after handover is significantly harder and more expensive.
ACH50 vs. natural infiltration — what’s the difference?
The 50 Pa test pressure is much higher than real-world conditions. To estimate natural infiltration — the amount of air that actually leaks under everyday wind and temperature conditions — a commonly used rule of thumb divides ACH50 by 20.
| ACH50 result | Estimated natural ACH | Context |
|---|---|---|
| 20 | ~1.0 ACH | High — substantial uncontrolled ventilation through leaks |
| 5 | ~0.25 ACH | Moderate — some background infiltration |
| 3 | ~0.15 ACH | Low — controlled ventilation important |
| 0.6 | ~0.03 ACH | Very low — ventilation must be fully designed |
Important: natural infiltration is not the same as ventilation. Uncontrolled infiltration comes in through random gaps, often at the wrong times, carrying dust, allergens, or outdoor pollutants. Designed ventilation — through MVHR or exhaust fans — brings fresh air in through filtered, controlled paths. As homes get more airtight, the distinction matters more.
Same house, different results
Without airtightness focus
🏗 Standard build, no airtight strategy
🔍 No mid-construction test
📊 Result: 9.4 ACH50
💨 MVHR installed but under-performing
❌ Draughts, moisture issues, MVHR can’t compensate
With airtightness strategy
🏗 Same design, airtight detailing specified
🔍 Mid-construction test + targeted sealing
📊 Result: 2.1 ACH50
💨 MVHR operating as designed
✅ Comfortable, draught-free, healthy indoor air
The difference between these two outcomes is rarely the design — it’s the process. Airtightness is achieved through careful detailing at every penetration, junction, and edge, combined with testing to verify that the work has been done and to find what was missed.
How Big Is the Hole in Your House?
Turn your ACH50 into a hole you can picture — the equivalent leakage area of your building envelope
Airtightness Explained
What airtightness means and how it’s achieved in construction
Blower Door Testing
HiPer Haus blower door testing service in Adelaide and SA
What Is MVHR?
How mechanical ventilation with heat recovery works and why airtightness matters for it
Window Condensation
How airtightness and humidity interact — and what your windows are telling you
Mould in New Homes
Why airtight homes develop mould without proper ventilation
Frequently asked questions
What is ACH50?
ACH50 stands for Air Changes per Hour at 50 Pascals. It's the standard metric used to express a building's airtightness after a blower door test. The number tells you how many times the building's entire air volume would leak out per hour if held at a constant pressure of 50 Pascals. Lower is better — a lower ACH50 means fewer uncontrolled air leaks through the building envelope.
How do I calculate ACH50 from a blower door test?
ACH50 = airflow at 50 Pascals (m³/h) ÷ building volume (m³). The blower door test report will give you the airflow figure directly. Building volume is typically floor area × average ceiling height. For a 200 m² single-storey home with 2.7 m ceilings, volume = 540 m³. If the test result is 600 m³/h, ACH50 = 600 ÷ 540 = 1.11.
What is a good ACH50 for a new Australian home?
The average Australian home sits around 15–20 ACH50. Good new construction typically achieves 5–8 ACH50. NCC 2022 better practice targets ≤5 ACH50. For homes with MVHR, ≤3 ACH50 is recommended for the system to work properly. Passive House certification requires ≤0.6 ACH50. Any result below 5 is respectable for new construction; below 3 is excellent.
Does my airtightness result matter if I don't have MVHR?
Yes. Airtightness affects energy efficiency, draught control, and moisture management regardless of your ventilation strategy. A leaky building loses heat through every gap and crack — increasing heating and cooling costs directly. Better airtightness also reduces the risk of moisture condensing inside wall cavities, which is a leading cause of mould in new homes. That said, airtightness and ventilation work together: a more airtight home needs a more deliberate ventilation strategy.
What is the difference between ACH50 and natural ACH?
ACH50 is measured at an artificially elevated pressure of 50 Pascals — much higher than normal conditions. Natural ACH is a rough estimate of how much air actually leaks under everyday conditions. A common rule of thumb divides ACH50 by 20 to estimate natural infiltration: a home at 5 ACH50 has roughly 0.25 natural air changes per hour. This is useful context but should not be confused with designed ventilation — natural infiltration is uncontrolled and cannot substitute for a ventilation strategy.
ACH50 is a number. What matters is what you do with it.
A blower door result tells you where your building envelope stands — but the value comes from acting on it. Testing mid-construction, finding the leaks, and sealing them is the single most cost-effective way to improve energy performance, moisture control, and indoor air quality in a new home.
Want to know your ACH50?
HiPer Haus carries out blower door testing across Adelaide and South Australia — mid-construction and at completion. We identify leakage paths, explain what the result means, and can advise on targeted sealing to improve your result.