Every remodel we finish makes a house tighter. Better windows, sealed rim joists, spray foam where there used to be a draft. That is the whole point, and it is where the energy savings come from. But a tight house holds on to whatever is already inside it, and that changes what you need to pay attention to.
Air quality in a home is not one thing. It is four or five different gases, each with its own source, its own detector, and its own consequence for getting it wrong. Here is how we think about it on a job, starting with the one almost nobody tests for.
Carbon dioxide: the one that tells you the room stopped breathing
Carbon dioxide will not hurt you at the levels found in a house. That is exactly why it gets ignored, and it is also why it is the most useful number you can put on a wall.
Outdoor air sits around 420 parts per million. An occupied room with decent ventilation stays under about 800. A closed bedroom with two adults and the door shut will climb past 1,500 by morning, and a small room with poor airflow can push past 2,500. At those levels people report waking up groggy, with a headache, feeling like they never really slept.
Nothing about that is toxic. The CO2 is a marker. If carbon dioxide is stacking up in a room, the air in that room is not being replaced, and everything else in the air is stacking up right alongside it. Humidity. Off-gassing from new cabinets and carpet. Cooking byproducts. Carbon dioxide is simply the easiest of those to measure, which makes it a proxy for the ventilation problem you cannot see.
A basic CO2 monitor runs forty to a hundred dollars and needs no installation. Put one in the main bedroom for a week. If it reads over 1,200 in the morning, the fix is usually mechanical ventilation, a cracked window, or leaving the bedroom door open. On a larger remodel this is the argument for an ERV or HRV, which brings in outside air without throwing away the heat you already paid for.
Carbon monoxide: the one that actually kills
Carbon dioxide is a comfort problem. Carbon monoxide is a different category entirely, and the reason is that it gives you nothing to react to. No smell, no color, no taste, no irritation. It binds to your blood roughly two hundred times more readily than oxygen does, so the symptoms are drowsiness and confusion, which are the two states least likely to make someone get up and leave the house.
In April 2004 the parents of the musician “Weird Al” Yankovic were found dead in their home outside San Diego. Nick was 86, Mary was 81. They had lit a wood fire in the fireplace and the flue was closed. When relatives finally opened the door, the house was full of smoke. He was in a chair in the front living room. She was on the bathroom floor, which suggests she got up, understood something was wrong, and did not make it any further than that.
Two people who had lived with that fireplace for years, doing an ordinary thing on an ordinary evening. A single working detector would have changed the outcome.
What produces carbon monoxide in a home is anything that burns fuel: gas furnace, gas water heater, gas range, wood stove, fireplace, an attached garage with a vehicle running in it, and a portable generator, which is the one that catches people during outages. A generator in a garage with the door open is still a generator in the garage.
Three things we tell every client. Put a detector on every floor and outside every sleeping area, because carbon monoxide mixes with air rather than settling, so it will reach the bedroom regardless of mounting height. Do not skip the basement if the furnace or water heater lives down there. And check the date on the back, because these sensors expire, typically somewhere between seven and ten years, and a dead sensor inside a live-looking unit is worse than no detector at all because it buys false confidence.
Radon: the one that is specifically a problem here
Radon is a radioactive gas that comes up out of the ground as uranium in the soil and rock breaks down. It is the second leading cause of lung cancer in the United States behind smoking. It is odorless, and unlike carbon monoxide it will not harm you tonight. It harms you over decades, which is exactly why so few people get around to testing for it.
The Upper Cumberland sits on limestone. Karst country, full of fractures, sinkholes and cave systems, and those fractures are pathways. Soil gas travels through cracked rock far more freely than through dense clay, and a stone or block foundation, a dirt-floor crawlspace, or an older basement with an open sump gives it somewhere to arrive.
Here we would rather be accurate than dramatic. The EPA places Putnam County in Zone 2, the moderate band, with a predicted average indoor screening level between 2 and 4 picocuries per liter. That is not the worst designation on the map and we are not going to pretend otherwise.
But a zone is a county-wide prediction, not a measurement of your house. Homeowner-submitted readings around Cookeville have averaged right around 4.0 pCi/L, which is precisely the level at which the EPA says to fix the building. Two houses on the same street can differ by a factor of five depending on foundation type, how the basement is sealed, and where the fractures happen to run.
So the zone says moderate and the local readings sit at the action line. That is not a contradiction. It is the reason a test is worth doing rather than assuming. A short-term kit is inexpensive, a long-term test over ninety days gives a truer annual picture, and the lowest lived-in level of the house is where you start. If the number comes back high, the remedy is a sub-slab depressurization system, which is a pipe and a fan that vent soil gas above the roofline. It is well-understood work and it costs less than most people expect.
There is a remodeling angle too. Sealing a house up without addressing the foundation can raise indoor radon, because you have removed the leakage that was diluting it. If we are tightening your envelope and you have a basement or crawlspace, test before and test again after.
The rest of the spectrum, and why commercial buildings are different
Three gases covers a house. Step into commercial and industrial buildings and the list gets considerably longer, because the hazard follows whatever the building actually does.
Wastewater plants, agricultural operations, pulp mills and oil and gas sites monitor for hydrogen sulfide. It is the rotten-egg gas, and it has a genuinely vicious property: at low concentrations you smell it clearly, and at high concentrations it deadens your sense of smell within minutes. The warning disappears exactly when the danger peaks, which is why those environments depend on fixed instruments rather than on somebody noticing. The specialists at PureAire have a solid breakdown of how hydrogen sulfide detection is handled in those settings if you want the detail.
Cold storage warehouses and ice rinks watch ammonia, still the refrigerant of choice at industrial scale. Labs, breweries, welding shops and anywhere holding bulk nitrogen, argon or carbon dioxide monitor for oxygen deficiency, because an inert gas leak does not poison anyone. It simply displaces the air in the room, and the first symptom is losing consciousness. Restaurants and taprooms with bulk CO2 for carbonation run into the same thing in a basement keg room.
None of that belongs in a house. We mention it because the underlying logic is identical at every scale: work out what the building contains and what it burns, then put a sensor on the specific thing that can go wrong. A detector is not a general-purpose safety device. It answers one question, and it only helps you if it is the right question for your building.
What we do about it on a remodel
When we are working on your house, this is the short version of what we look at:
- Combustion appliances get checked whenever we touch the envelope. Tightening a house changes how a furnace, water heater or fireplace draws. Sealing the shell without confirming those appliances still vent properly is how a safe house quietly becomes an unsafe one.
- CO detectors on every level, outside sleeping areas, with the expiry date checked. If yours predates your last remodel, it is probably finished.
- Radon testing before and after any basement or crawlspace work, particularly if we are sealing, insulating or finishing that space.
- Ventilation sized for the house we are handing back, not the leaky one we started with. That is where a CO2 monitor earns its keep, because it tells you whether the ventilation is genuinely working rather than merely switched on.
None of this is expensive relative to the cost of a remodel. It is a handful of detectors, one test kit, and asking the question at the right point in the schedule instead of a year later.
If you are planning work on a basement, an addition, or anything that tightens up the shell of an older Cookeville home, ask us about it while the walls are still open. It is a much easier conversation at that stage.