Last Updated on October 24, 2026 by Umar Farooq
Air changes per hour in a house (ACH) is how many times the indoor air is replaced by outdoor air in one hour: 1 ACH means a volume of air equal to the whole house passes in and out every hour. It is the number behind most ventilation advice, and also the one people misquote most.
The short version: there is a floor to stay above, a typical figure that depends on the age of the building, and a trap where blower-door numbers get mistaken for everyday ones.
What one air change per hour looks like in a real house
The idea is simple: air out equals air in. Whatever a fan, a chimney or a gap pushes out has to be replaced by outdoor air coming in elsewhere.
The sum is the airflow divided by the house volume. Take a 100 m² home with 2.4 m ceilings, so 240 m³ (about 8,500 ft³). One air change an hour is 240 m³ of air every hour. That is my own arithmetic, not a published figure, but it shows the scale: slow and constant, not dramatic.
One caution. ACH is a whole-house average, and it assumes the air mixes evenly. It does not. A bathroom with the door shut can sit at a very different rate from the landing outside it, which is why an average of 0.5 ACH says nothing about a single steamy room.
The three figures people quote, and where they come from
Search results throw up everything from 0.35 to 15, because they mix standards, equipment ratings and room types. These three I can trace to a page I opened:
| Figure | What it is | Source |
|---|---|---|
| 0.35 ACH | Minimum whole-house ventilation, with a floor of 15 cubic feet per minute per person | EPA, citing ASHRAE 62.2 |
| About 1 ACH | What a modern building needs | Better Housing Better Health |
| Up to 4 ACH | What traditional buildings may experience | Better Housing Better Health |
For 0.35 ACH in my 240 m³ example, the sum gives about 84 m³ an hour, which is roughly 23 litres per second (about 49 cubic feet per minute). Again, my arithmetic.
Two figures you will meet that are not these. A “3 to 5 ACH” rating describes what a ventilation system can do, not what a house does on its own, and the Wikipedia article on air changes per hour notes such ratings often ignore house size. The very large blower-door numbers are a different thing again.
Blower-door ACH and everyday ACH are different numbers
A blower-door test measures leakage at a 50-pascal pressure difference. According to the same Wikipedia article, that roughly mimics a sustained 20 mph wind, so natural air change in calm conditions may be 10 to 25 times lower. A report that says your house is “6 ACH at 50 pascals” does not mean it changes its air six times an hour. It is a tightness score, not a ventilation rate.
Air changes per hour in a house and damp: more is not automatically drier
Ventilation lowers humidity only when the air coming in holds less moisture than the air going out. In winter that is usually true, so a steady air change does help with the water your household produces by breathing, cooking and showering. In a muggy summer it can bring moisture in. Opening windows works on the same rule.
There is also a ceiling to what extra air achieves. The Wikipedia article reports a 2010 New Zealand study in which seven of eight houses already had a natural rate of 0.75 ACH or more, and says that where the natural rate exceeds 0.75, forced ventilation is unlikely to help control condensation. If your house already leaks that much, a bigger fan is not your answer. Finding where the moisture is made is.
For the practical version, with fans, windows and vents sorted by when each helps, see how to ventilate a house to reduce moisture.
Why a tight new house needs ventilation on purpose
Better Housing Better Health says a modern building needs about one air change an hour and that a super-insulated one will likely need mechanical ventilation, which can be paired with heat recovery. The EPA makes a related point: tight enclosures may need supplemental supply air, particularly where there are fuel-burning appliances.
My reading is that modern houses did not get worse at ventilating. They stopped doing it by accident. Old buildings changed their air through gaps; a well-sealed one changes it only through the openings someone designed. If those are blocked, the moisture from a family of four has nowhere to go, which is a big part of why trickle vents and extractor fans should stay open and working.
Why a very leaky house is cold but not necessarily dry
Draughts are uncontrolled ventilation, as Better Housing Better Health puts it, and the cost shows up as heat loss and discomfort. A leaky house is easy to chill, but chilled is not the same as dry.
Cold surfaces are exactly where moisture condenses, and gaps do nothing to remove steam from a particular room. Wikipedia lists low airtightness among the causes of condensation and moisture risk. I would not claim a leaky house is always damper either, because it depends on how much water the household produces. Leakiness tells you about heat loss and very little about humidity. Sealing it is worthwhile, but does draught proofing cause damp explains why you then need to keep some deliberate exits.
Estimate your air change rate without a blower door
You cannot measure ACH with a hygrometer, and I will not pretend otherwise. You can reason about it, though.
- Age and construction. A pre-war house with open chimneys and sash windows sits towards the top of the range. A sealed new-build with double glazing and a whole-house fan sits towards the bottom.
- Open exits. Count trickle vents, extractor fans that vent outside, and unblocked air bricks. If you have none and the house is airtight, assume it is low.
- Symptoms. Windows that stay wet all morning and a stale smell on waking suggest too little air leaving where moisture is made.
- A reading trend. Track humidity for a week. If it falls sharply when you ventilate and climbs again within hours, you have a moisture source rather than a leak problem.
If you need a real number, for a retrofit or a heat-recovery system, a qualified assessor’s blower-door test is the proper route. For ordinary damp, a hygrometer and the habits in how to lower humidity in your house will tell you more.
Aim for controlled air, not for as much air as possible
The three figures give you a sensible frame: 0.35 ACH as a floor, about 1 for a modern building, up to 4 for a traditional one. The best answer on air changes per hour in a house is a rate you chose, with extraction at the moisture source and a steady background supply, rather than the leftover from gaps you hope are in the right places.
FAQ
How many air changes per hour should a house have?
The EPA, citing ASHRAE 62.2, gives 0.35 air changes per hour as the minimum, with at least 15 cubic feet per minute per person. Better Housing Better Health says a modern building needs about one air change an hour.
How do you calculate air changes per hour?
Divide the airflow by the volume of the house. In metric, ACH is airflow in cubic metres per hour divided by the house volume in cubic metres. A 240 m³ house with 120 m³ of air passing each hour is at 0.5 ACH.
Is a higher air change rate always better?
No. More air costs heat, and it only lowers humidity when the outdoor air is drier than indoors. Past roughly 0.75 ACH, the Wikipedia article says extra forced ventilation is unlikely to help with condensation.
What is ACH50 and is it the same thing?
ACH50 is the blower-door result at 50 pascals. It is a measure of leakiness, not of how often your air actually changes, and natural rates are typically many times lower.
Do old houses have more air changes per hour than new ones?
Generally yes. Better Housing Better Health says traditional buildings may experience up to 4 air changes an hour, compared with about 1 for a modern one, but individual houses vary a lot.
