Burp? Nah, Build Tight, Ventilate Right
Every house should be built with a way to bring in fresh air on purpose. Until recently almost all weren’t.
I’ve always been a person who notices when a room has gone stale and gets up to open a window or a door for a few minutes. I was doing it long before “burping the house” was something people were talking about. Meeting rooms especially get nasty. In 2020 I bought a CO2 sensor from Aranet, and it turned a vague feeling into a number. That was the useful part. Once you can see it, you can connect it to specific things: tiredness that doesn’t compute especially. You might be surprised what happens in your car when the vents are closed.
Since moving to Big Sky I’ve lived in two places, and both are heated with electric baseboard. No furnace, no ducts, no air handler, nothing moving air around the house at all. Both are also built tight, which is what you want in a Montana winter. The combination is the problem. In the bedroom I can watch CO2 climb past 800 ppm within 20 minutes of closing the door. Outdoors it’s about 425.
The house has no lungs
A tight house with baseboard heat is a fish tank with no pump.
The building science world has had a slogan for this for decades: build tight, ventilate right. The first half caught on. Energy codes now push hard on airtightness, and Montana’s target for new homes is 4 air changes per hour at 50 pascals on a blower door test. Code does require mechanical ventilation in a house that tight, but it lets a bathroom exhaust fan on a timer count. That fan pulls air out of the bathroom. It doesn’t deliver fresh air to the bedroom with the door closed, and in a really tight house it can’t pull much in from anywhere.
What every dwelling really should have (in every climate!) is a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV), designed in from the start. Both are balanced: one fan pulls stale air out, another brings fresh air in, and the two streams pass each other through a core that moves the heat from the outgoing air into the incoming air without mixing them. An ERV moves some moisture across too. You get fresh air and keep most of the heat you paid for. Well designed systems are also aware of ventilation fans in kitchen and baths and will supply make-up air when they are on.
CO2 isn’t the full story, but it is a good proxy
CO2 is the number a cheap sensor can measure. It isn’t necessarily the thing hurting you.
The research on CO2 itself is genuinely mixed. A 2012 Lawrence Berkeley study found decision-making scores dropped at 1,000 ppm and dropped a lot at 2,500 ppm (The space station runs at 3000 ppm because of equipment limitations, not great Bob!). Harvard’s 2016 COGfx study put people in simulated offices and found cognitive scores 61% higher in low-VOC “green” conditions and 101% higher with extra ventilation on top. But some independent attempts to reproduce the pure CO2 effect, injecting CO2 into otherwise clean air, have found little or nothing. I’d put it this way: nobody has settled whether 1,500 ppm of CO2 alone makes you dumber.
Sleep is less ambiguous. A Danish study put sixteen students in dorm rooms for a week with a CO2-controlled fan and a week without, about 835 ppm versus about 2,400. With the fan on, they slept measurably better, felt less sleepy the next day, and concentrated better.
The point is that CO2 rises with everything else people and houses put into the air: moisture, body odors, VOCs off furniture and cleaning products, cooking particles. If CO2 is high, the house isn’t being ventilated, and everything else is building up with it. The meter is a proxy for all of it.
What else is in there
The big sources, roughly in the order I’d worry about them:
Combustion indoors. Gas stoves suuuuuck, and I’ve written about why they’re not better for cooking either. They put nitrogen dioxide into the house every time you cook. Stanford measured more than a hundred homes and found the NO2 reaches bedrooms within an hour and stays elevated for hours; people in homes under 800 square feet get about four times the long-term exposure of people in homes over 3,000. A 2022 analysis attributed 12.7% of US childhood asthma to gas stoves. Kashtan’s group also found benzene from the flame itself, not the food. And they leak methane even when off. Wood stoves, fireplaces, and unvented heaters belong on this list too.
Radon. It comes up out of the ground, you can’t smell it, and it’s the leading cause of lung cancer among non-smokers. Montana is bad for it: DEQ says 48% of homes tested statewide are over the EPA action level, and an MSU summary puts Gallatin County at 37%. A test kit is cheap. Do it.
VOCs and formaldehyde from new furniture, flooring, paint, glue, and cleaning products. Worst in new or freshly renovated spaces of course. Or in a bedroom with a new mattress…
Particulates from cooking (any stove, any fuel) and from wildfire smoke in summer.
Moisture, which turns into mold in a tight house that never exchanges air.
Houseplants don’t fix any of this. A Drexel review of thirty years of studies worked out you’d need somewhere between 10 and 1,000 plants per square meter of floor to match the air exchange of a building or a couple of open windows.
I feel like altitude makes it worse
Maybe I’m imagining it, but I believe this matters more up here. Researching it is what led me to write this article.
At Big Sky’s elevation the air pressure is about 77-80% of sea level, so every breath already carries a fifth to a quarter less oxygen. The CO2 building up in a building doesn’t steal a meaningful amount of oxygen on its own; the math there is tiny. But it sure feels like that stale air and thin air stack gives you less margin. Unfortunately I haven’t found a study that tests it directly.
What I can support:
Your CO2 meter is probably lying low. The common NDIR sensors count CO2 molecules in a little chamber, and at altitude there are fewer molecules of everything in that chamber. Uncompensated, a sensor at Big Sky reads roughly 20% low, so a bedroom at 1,500 shows up as 1,200. Some meters correct for this with a built-in pressure sensor (the Aranet4 does, it’s a nice bit of kit with temp and humidity too) and some need the altitude set by hand.
Gas burns dirtier here. There’s less oxygen per cubic foot for the flame, so appliances have to be derated for altitude, and incomplete combustion makes more carbon monoxide. Research on unvented appliances found significantly more CO near 9,000 feet than near 5,000. And CO is WAY worse for you than CO2.
The windows stay shut longer. For a long stretch of the year nobody is airing out a house at subzero temperatures. Tight construction plus a long closed season means whatever goes into the air stays there.
What to do about it
Measure first. Get a CO2 meter that compensates for pressure and put it where you sleep. Watch it for a week. Outdoor air is about 425 ppm; a lot of guidance treats 1,000 as the line where you should be doing something.
Burp the house, sigh. Open windows on opposite sides for five or ten minutes. In winter, short and wide open beats a crack left open all day; you swap the air before the walls and furniture cool down.
Sleep with the door open, or crack a window. The bedroom is where the numbers get bad, because it’s a small closed room with one or two people breathing in it for eight hours. I have a fan blowing air at the door.
Add a single-room ventilator. This is the easy retrofit for a house with no ducts. The Lunos e2 is a pair of through-wall units that take turns, one exhausting while the other supplies, swapping every 70 seconds, with a ceramic core that stores heat from the outgoing air and hands it back to the incoming air. Panasonic’s WhisperComfort Spot ERV goes in a ceiling or wall and runs both streams through a core continuously. Either one gives a bedroom its own lungs for the cost of a hole in the wall.
Build one. I built a small HRV myself. I made a frame of corrugated plastic, and internal baffles of heavy duty aluminum foil and foil tape. It just needs to be air tight inside and out. People have built decent ones from corrugated plastic sign board stacked into a crossflow core with a couple of small fans; the better builds report around 60% heat recovery.
Design it in. If you’re building or gutting a house, put in a ducted whole-house ERV. An HRV helps dry out a house that has condensation problems which is unlikely in our climate, an ERV holds onto some of the humidity a Montana winter strips away. So I’d go ERV myself, Montana summers are only going to get hotter, and the air is dry most of the year.
Remove the sources. Induction instead of gas. A range hood that actually vents outside, used consistently. A radon test. A HEPA purifier for smoke season, which sadly also will probably only be getting worse, keeping in mind that a purifier cleans particles and does nothing for CO2.
All new construction, especially in an affluent place like Big Sky, should be targeting “passivhaus” levels of energy efficiency now. They cost slightly more to build but are way more comfortable to live in, and vastly cheaper to operate over time. And getting the fresh air is a natural part of that thinking.
Hopefully you can breath a little easier with some of this info…