Six in the morning, and the rig did nothing wrong overnight. It is warm, the bed was comfortable, nobody got up, and yet your head is dull and every thought arrives through a wet towel. You open the door for the dog, stand on the step in the cold a minute, and come back in feeling roughly human.
That last part is the diagnosis. Nothing had gone wrong; the air in there had simply been through your lungs several times over by morning. Carbon dioxide is what you breathe out, it accumulates in a small sealed space, and long before it does anything worse it makes people slow and headachy. It also shares two letters with something genuinely dangerous, so before anything else:
Carbon dioxide is not carbon monoxide. CO2 is the gas you exhale. It collects in a sealed camper, it makes you foggy, and a vent fixes it. CO, carbon monoxide, is a poison produced by combustion, it has no smell, and it kills sleeping people. Two similar abbreviations, no overlap at all in how seriously you treat them. Nothing here substitutes for a certified CO alarm, and the alarms are their own subject.
Why a bedroom on wheels is the worst case
Outdoor air sits somewhere around 400 to 420 parts per million of CO2, and that is the floor everything else works up from. A well ventilated room indoors runs a few hundred above it with nobody noticing anything. North of about 1,000 ppm most people report the same short list: drowsiness, poor concentration, a headache that clears within minutes of stepping outside.
Now hold your sleeping quarters up against that. Very small, sealed against weather on purpose, two of you and a dog breathing in it for eight hours. A house bedroom has several times the air volume and far more accidental leakage. A camper with good seals in January, every window shut and the roof vent closed, is about as near a sealed box as anybody sleeps in. You are not being poisoned. You are re-breathing, which is a different problem with a cheaper answer.
The same breath is putting water in your walls
Here is what makes this worth a sensor rather than a shrug. Breath carries water as well as CO2, two adults asleep put out a surprising amount of it, and in a cold rig that water goes straight to the coldest surface it can find: the inside of the windows, the wall behind the mattress, the back corners of the overhead cabinets where nobody looks until something smells wrong.
So the CO2 number is a leading indicator for something more expensive than a foggy morning. A rig running high on CO2 overnight is depositing water into its own structure at the same time and for the same reason, which is how a camper that has never been rained on ends up with soft spots and mould. Put CO2 and humidity on one graph and there is little left to interpret: both climbing through the night, both falling within minutes of a cracked vent, means the rig is not exchanging enough air.
Three ways to put a number on it
The cheap monitor that is guessing
There is a whole shelf of inexpensive air quality monitors with a CO2 figure on the display, and plenty of them are not measuring CO2 at all. They carry a VOC sensor and infer a number from it, sold as “eCO2” or “equivalent CO2”, and it responds to anything volatile in the air: cooking, a cleaning spray, hand sanitiser, a new plastic thing off-gassing.
It will happily show 2,000 ppm because you opened a marker pen.
The sensor that genuinely measures carbon dioxide is NDIR, non-dispersive infrared, which measures how much infrared the air absorbs at a wavelength CO2 absorbs. If the listing does not say NDIR, assume it is estimating; anybody who paid for the better sensor says so in the first line. What the cheap version costs you is the measurement being true. An eCO2 figure tells you something in the air changed, which is not nothing, but it will not reliably tell you the bedroom needs a vent open, and it fires at the wrong moments.
The Govee H5106 is worth naming, not because it is a bad monitor but because people ask. It is a real instrument and the hub can see it broadcasting away, but the telemetry is encrypted with your Govee account key, so what is on the air is not readable by us. It stays a candidate for that reason and will probably stay one, because that is a decision at their end rather than a job sitting on our list.
The Aranet4, which is the better instrument
The one van dwellers actually trust, and the one that turns up in every serious thread on the subject. Proper NDIR. An e-ink display you can read across a dark room without waking anybody, and battery life measured in years rather than weeks. As a standalone instrument it is excellent, and if you buy nothing else you will not regret it.
What it will not do today is turn up in your dashboard with everything else. It is a candidate rather than supported for two reasons we would rather publish than paper over. It only broadcasts once Smart Home integrations are switched on in the Aranet app, so a unit out of the box is silent. And the broadcast layout has moved between firmware versions, so a decode written from a document can be quietly wrong on somebody else’s unit. We want a live capture in hand first.
Anything that speaks BTHome
The third option is not a product. BTHome v2 is an open broadcast standard, and anything that speaks it, RoamVitals reads: the Shelly BLU range, pvvx-flashed sensors, most of what people build themselves. Thirteen measurements come out of that format in our decoder and CO2 is one of them, alongside PM2.5, PM10, TVOC, temperature and humidity, all itemised per device in the compatibility database.
That is a different proposition from a monitor with a screen. A BTHome CO2 sensor over the bed is a number you can put a threshold on, look at as a graph over breakfast, and line up against humidity from the same device rather than against your memory of the night. The trade is that nobody holds your hand: you choose the sensor off its specification, and you decide whether the thing inside it is any good.
Two caveats we publish rather than bury. Encrypted BTHome devices need their bind key saved against the sensor in the hub before anything decodes at all. And because BTHome is a format rather than a product, we cannot promise any particular device using it is well built. What we can promise is that if it broadcasts BTHome v2, we read it. Buy the sensor on its own merits, and check it is NDIR.
So which one
- You want to know, and you will look at a display. An Aranet4. Better instrument, and it needs nothing else to be useful.
- You want it in the dashboard with the tanks and the temperatures. A BTHome CO2 sensor, which is the one that works with us today.
- You are chasing damp. BTHome again, reporting CO2 and humidity from the same spot, because the correlation is the diagnosis.
- You want a rough sense and nothing more. Open a vent. Free, and it is the fix in almost every case anyway.
Cracking the vent, and why people resist it
The fix is nearly always ventilation, and in an RV that means a roof vent open an inch, not a window. Warm air leaves through the roof and pulls fresh air in low, so a vent moves air through the whole rig while a cracked window mostly moves it past the window.
People resist because it costs heat, in the weather where heat is the point. You are already losing heat to the moisture going into the walls, and that loss is permanent in a way the propane is not. A vent open overnight in January costs a little gas. Wet insulation costs a floor.
If you alert on it, set the threshold near where people start to feel it rather than at some dramatic number, and let it arrive while somebody is still awake to open something. “Crack a vent before bed” is a useful message at nine o’clock. At four in the morning it is just something you read over coffee.
None of which makes a small sealed room any bigger. What it settles is the argument you would otherwise have every night about whether the heat is worth more than the air, and a week of data settles it for good. After that the vent gets cracked before bed without anybody thinking about it, and six in the morning stops being the worst part of the day.
