How to Measure Your Home's Ventilation and Fix a Stuffy Room

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Every house feels fine, right up until three people have been in a small bedroom for two hours. Then somebody opens a window, the room changes, and everyone relaxes. That instinct is correct, and it is also the entire science of this post.

The measurement takes about twenty minutes, costs the price of a CO2 sensor, and produces a number you can act on: how many times the air in a room is replaced each hour. Here is how to do it.

Why CO2 is the right thing to measure

Carbon dioxide is a tracer, and tracers are how you measure invisible flows.

People breathe out CO2 at a roughly steady rate. Put it in a sealed room, leave, and watch the level climb. Then open the house up to fresh air and watch it fall. The rate of that fall is a direct read on how much fresh air the room is getting relative to its volume. You are measuring airflow, not guessing at it.

CO2 is also a reasonable stand-in for something harder to measure. The air we exhale carries respiratory aerosol, which is where a good share of airborne respiratory viruses travel. Those particles and the CO2 come from the same source, in roughly the same proportions, at roughly the same moment. So a room’s CO2 is a reasonable proxy for how much exhaled aerosol has been diluted, and a CO2 sensor is cheap in a way that a particle counter is not.

Two honest caveats. First, CO2 says nothing about cooking fumes, cleaning products, or VOCs from furniture and paint — those need PM2.5 or VOC sensors. Second, CO2 is a proxy for people-generated air, so a very airtight room with nobody in it will read beautifully while being full of stove pollution.

But for the question “is this room actually exchanging air with the outside,” CO2 is the right instrument, and it is the cheapest one that gives a number.

Air quality sensors: CO2, PM2.5, VOC and NOx explainedPlays from YouTube when you press play

Step one: measure your outdoor baseline

Before anything else, find out what CO2 looks like outside your window right now. This step is the one people skip, and skipping it will corrupt everything downstream.

The number everyone quotes is 420 ppm, roughly the global average. Your actual local outdoor value is usually higher, and in a city it can be substantially higher. One Bristol garden measured 578 ppm on a still summer evening — about 40 percent above the figure everyone repeats. Local traffic, a road, a bus depot, or a neighbouring house’s exhaust can all push it up.

Put the sensor outside, or in an open window with the room behind it closed, and let it stabilise. Record that number. It is your baseline, and it is the point your indoor air will decay back toward.

Write it down. You will need it.

Step two: raise the indoor CO2

Close the room up. Doors shut, windows shut, turn off any ventilation you would normally rely on. Then get several people into it and keep them there for an hour or two. Two or three adults in a small bedroom will push a well-sealed room from baseline to somewhere in the 1500 to 3000 ppm range.

The reason for several people is simply that you need enough signal above the noise. A single person in a large room moves the number by a hundred ppm, which is hard to time accurately.

Now everyone leaves. The room stays closed. This is the important part, because from this moment on nothing is generating CO2, and every point of CO2 in the air is only being removed by ventilation.

Science at home: measuring ventilation with CO2 sensorsPlays from YouTube when you press play

Step three: log the decay

Start recording. Note the indoor CO2 at the moment the last person left, call that the start value. Then note the readings as they fall.

The curve is a clean exponential decay, and the shape is what makes the calculation easy. Find the reading that is halfway between your start value and your outdoor baseline. Note how many minutes that took. That is the half-decay time.

flowchart TD
    A["Read outdoor baseline CO2"] --> B["Close room and occupy it with several people"]
    B --> C["Everyone leaves, room stays shut"]
    C --> D["Record the indoor start value"]
    D --> E["Log readings as the value falls"]
    E --> F["Find the reading halfway to baseline"]
    F --> G["Time the half decay in minutes"]
    G --> H["Divide 0.693 by minutes"]
    H --> I["Multiply by 60 to get air changes per hour"]
    I --> J["Open more windows and repeat"]

Step four: get the number

Here is the formula in words, because you will remember it that way:

Air changes per hour equals 0.693 divided by the half-decay time in minutes, multiplied by 60.

The 0.693 is the natural logarithm of two, and it falls out of the exponential decay. Everything else is arithmetic on a stopwatch.

Worked example. Start at 2000 ppm, outdoor baseline 578 ppm. Halfway is 1289 ppm. Suppose that reading arrives 80 minutes after you left the room. Air changes per hour is 0.693 over 80, multiplied by 60, which is 0.0087 times 60, which is about 0.52 ACH.

That 0.52 is the number almost every house gets with everything shut.

Step five: repeat with windows open

Now redo it with one window open, and then with two windows open on opposite sides of the house.

This is where the result stops being depressing and starts being actionable:

type: bar
title: Air changes per hour in a typical home
x: All windows shut, One window open, Two windows open
Air changes per hour: 0.52, 1.43, 4.8

Read that left to right. Everything shut: 0.52 air changes per hour. One window open: 1.43. Two windows open on opposite sides of the house: 4.8.

The jump from one window to two is not linear and it is not a small improvement. Opening a window on one side of a house gives you a flow path only if there is somewhere for the air to go. Open two on opposite sides and you create a through-draft, and the flow rate rises by more than three times.

The expert guidance for good ventilation is around 4 air changes per hour. That is the number to aim for, and the two-window result clears it comfortably while a single window does not.

Do the experiment room by room

This is the part that turns a number into a renovation.

Ventilation is not a property of a house. It is a property of a particular room, on a particular side of the house, on a particular day, with a particular wind direction. A flat that gets 4.8 ACH in the living room may get 0.4 in the back bedroom, permanently, because that window faces a neighbour’s wall.

So run the measurement separately in every room that matters: the bedroom you sleep in, the room the kids do homework in, the room where you sit for hours. Twenty minutes each. Write the ACH on the wall.

You will almost always find one room is dramatically worse than the rest, and it will be the one you use most. That room is your ventilation project, and now you know the size of it.

What else matters

CO2 measures dilution. It does not measure comfort, and the three factors that most affect how a room actually feels are worth attention on their own terms.

Cross-flow. Air needs somewhere to go. A single high window in a closed room moves almost nothing, because there is no return path. Two openings at different heights and on different walls moves a great deal. This is why bathroom extractor fans work so much better when the door is left slightly ajar — the door gap is the return path. The same principle governs which pair of windows to open.

Wind direction. Ventilation is enormously easier with the wind. Open the upwind side first, and check which way your smoke goes. A breeze across a room can do the work of two open windows, and a still, cold evening can make a 0.5 ACH house feel reasonably aired while the CO2 is still climbing.

Season and temperature. Thermal stack effect pulls warm, CO2-rich air up and out of a room, which helps without any window at all — and then reverses overnight as the house cools, drawing air back in. This is why a bedroom with a radiator under the window can behave differently overnight than it did in the afternoon.

A short version worth keeping

Put a CO2 sensor in the room you spend most of your time in. Watch the number rather than the feeling. Open two windows on opposite sides, not one. Aim for something near 4 air changes per hour, measured room by room. And if a room reads above about 1200 ppm while someone is working or sleeping in it, that room has a ventilation problem worth solving — and now you have the number to justify solving it.

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