What an Indoor Air Quality Monitor Measures in a Tight House

Which of the numbers on the screen are trustworthy, which are an index in disguise, and which ones you can act on tonight.

What an Indoor Air Quality Monitor Measures in a Tight House

These devices became inexpensive at roughly the same moment New York houses
started getting seriously air sealed, and the two facts are related. A leaky
house is ventilated by accident. A tight one is ventilated on purpose, or not
at all — and a monitor is how you find out which of those you are living
in.

What it will not do is interpret itself. A screen of coloured numbers
invites either alarm or complacency, and neither is much use. The value is in
knowing what each parameter is actually reporting, how much to trust it, and
which readings translate into something you can do this evening.

First: This Is Not a Safety Alarm

The most important thing to establish before buying one. A residential
carbon monoxide alarm is a life-safety device, built to a product standard,
required in New York residences, and designed to wake people up. An air quality
monitor is an instrument. Some monitors include a carbon monoxide sensor, and
that sensor is generally not a substitute for the alarm your code official
expects to see.

Keep the alarms, wherever they are required, and treat the monitor as
something you add on top. If your kitchen has a gas range, the
combustion side of that question is a different subject
from measurement, and it is the one with the emergency procedure attached.

The Parameters, and What Each One Is Really Telling You

Carbon Dioxide Is the Ventilation Number

If you only watch one line, watch this one. People exhale carbon dioxide
continuously, so its concentration indoors rises when occupancy exceeds air
change and falls when fresh air arrives. That makes it a direct read on whether
the house is being ventilated at the rate the people in it need — which is
exactly the thing air sealing changes.

The pattern is more informative than any single value. A bedroom that climbs
steadily overnight with the door closed and drops when the door opens is
telling you something specific and fixable. A living room that rises whenever
guests arrive is behaving normally. For guideline values, use the published
guidance from a public health or ventilation standards body rather than a
device maker’s colour scheme, which is a marketing decision as much as a
technical one.

One trap: some inexpensive units do not measure carbon dioxide at all. They
infer it from a volatile compound sensor and report it under a name like
estimated CO2. That reading responds to cleaning spray and cooking, not to
breathing, and it will mislead you about ventilation specifically. If the
ventilation question is why you are buying, confirm the unit has a real
optical sensor.

Particles, and the Fact That Cooking Dominates

In most homes without smokers, the largest particle events of the week are
cooking — searing, frying, toasting, anything that browns. Watching a monitor
during dinner is the fastest way to understand why kitchen ventilation is
treated so seriously, and it usually converts people to running the hood
without any further argument.

Outdoor air matters too. Wildfire smoke reaches New York, and on those days
the correct response inverts: the outdoor air is the source, so you filter and
close up rather than ventilate. A monitor is what tells you which regime you
are in on a given day, and that is genuinely hard to judge by eye or nose.

Humidity Is the One That Damages Buildings

Humidity acts on the house itself, not only on comfort. Too low in a New
York winter and you get static, dried joinery and cracking. Too high and you
get condensation on cold surfaces, and sustained damp is what mould needs.

Because it is a building problem, the responses are building responses.
Persistent damp in a basement or crawl space usually has a moisture source that
needs finding before anything else, which is why
a dehumidifier belongs at the end of that sequence
rather than the start, and why
a basement vapour barrier helps in some positions and
traps moisture in others. Bathrooms are the classic recurring spike, and
a fan that runs on humidity rather than on memory
solves the specific failure of somebody switching it off too early.

Radon Deserves Its Own Test

Parts of New York have geology associated with elevated radon, and it is not
something a general-purpose monitor settles. A continuous consumer radon sensor
is a useful screening and trend tool, but the measurement that matters —
appropriate duration, controlled conditions, an accredited laboratory — is a
distinct procedure. If a screening reading is elevated, follow the New York
State Department of Health and EPA guidance for testing and mitigation rather
than acting on the display.

This intersects with efficiency work directly: sealing a basement floor and
changing the pressure regime of a house can change how soil gas moves into it,
so a house being tightened is a reasonable house to test.

Placement, and Reading It Honestly

• Put it where people are. A bedroom or a living room at
  roughly seated or sleeping height tells you about the air people breathe. A
  hallway shelf tells you about a hallway.
• Keep it away from the obvious. Directly beside the stove,
  in a draught from a window, or above a radiator gives you readings about that
  appliance rather than the room.
• Give it time. Most sensors need a warm-up period, and some
  need days of operation before their baseline settles.
• Read trends, not moments. The useful question is what
  changed and what you did just before it changed. A single number without a
  history is close to meaningless.
• Expect device disagreement. Two monitors in one room will
  not match exactly. Consistency over time on one device is what you are
  after.

What to Actually Do With a Reading

Three responses cover nearly everything, in this order. Remove the source
where you can: run the hood while cooking, store solvents outside living space,
fix the leak causing the damp. Ventilate deliberately: open a window, run the
bathroom or kitchen fan for longer than feels necessary, or — in a genuinely
tight house — install mechanical ventilation designed to bring in outdoor air
without throwing away the heat. Filter what remains, which is the right
response for outdoor particles and the wrong one for carbon dioxide, since a
filter does nothing to it.

Where This Fits in Efficiency Work

None of this argues against tightening a house. A leaky building wastes
energy and is uncomfortable, and its accidental ventilation is unreliable in
exactly the weather when you want it least. The point is that ventilation
becomes something you design once the leaks are gone —
which is why sealing comes before the heating
equipment, and why a blower door test is the
measurement that tells you how far the house has moved.

A monitor is the cheap instrument that closes that loop after the work. If
you have had wall cavities filled or
foam applied, running a monitor for a few weeks
afterwards tells you whether the house now needs ventilation it used to get by
accident. That is a much better basis for a decision than either anxiety or
the assumption that a tighter house is automatically a healthier one.

Frequently Asked Questions

Can an air quality monitor replace my carbon monoxide alarm?

No. A residential carbon monoxide alarm is a life-safety device built to a product standard and required in New York residences, designed to be loud enough to wake people. A monitor is an instrument for observing conditions. Some include a carbon monoxide sensor, and that is a useful addition rather than a replacement. Keep the alarms your code official expects and treat the monitor as extra.

Which reading is the most useful?

For a house that has been air sealed, carbon dioxide, because it is a direct proxy for whether ventilation is keeping up with the people in the building. Humidity is a close second and arguably more actionable, because it acts on the structure and drives condensation and mould risk rather than just comfort.

Why does my monitor show high CO2 when nobody is smoking or cooking?

Because carbon dioxide comes from breathing, not from those activities. It rises whenever people occupy a space faster than fresh air replaces the air they have used, which is why a closed bedroom climbs overnight. It is a ventilation signal. If your unit estimates carbon dioxide from a volatile compound sensor rather than measuring it optically, though, cleaning products and cooking will move that number and it is not telling you about ventilation at all.

Are the VOC readings meaningful?

Treat them as the softest number on the screen. Most consumer devices report an index derived from a general-purpose sensor rather than a concentration of any specific compound, and they respond strongly to cleaning products, cooking and new furnishings. They are useful for spotting that something changed and poor for saying what.

Can I use one to test for radon?

For screening and trends, yes. For the measurement that decisions are made on, no. Radon testing of record involves appropriate duration, controlled conditions and an accredited laboratory. Parts of New York have geology associated with elevated radon, so follow the state Department of Health and EPA guidance on testing and mitigation rather than acting on a continuous display.

Where should I put it?

Where people actually are — a bedroom or living room at roughly seated or sleeping height. Avoid placing it beside the stove, in a window draught or above a radiator, because it will then report on that appliance rather than the room. Give it time to settle; some sensors need days before the baseline is stable.

Does air sealing my house make the air worse?

It removes accidental ventilation, which is unreliable anyway and weakest in mild weather. What it requires is that ventilation becomes deliberate — running exhaust fans properly, or in a genuinely tight house installing mechanical ventilation that brings outdoor air in without discarding the heat. A monitor after the work is a good way to find out which of those you need.

Should I filter or ventilate?

It depends on where the problem is coming from, which is exactly what a monitor helps you decide. Filtration addresses particles, so it is the right response when outdoor air is the source, as on a wildfire smoke day. Ventilation addresses carbon dioxide and humidity, which no filter touches. Source control — the hood while cooking, the leak fixed — usually beats both.