Winter Heating and Indoor Air: Managing the Trade-Offs
No thermostat setting gives you warm, humid, fresh and clean air at once. Heating collapses relative humidity. Sealing the house to keep that heat in also keeps carbon dioxide, VOCs and combustion by-products in. Ventilating fixes that but costs heat and, during a winter inversion, imports outdoor particulates. Combustion heating warms the room and emits nitrogen dioxide into it. The goal is not to win every trade but to keep each variable in a workable band: 40 to 50 per cent relative humidity, carbon dioxide under about 1,000 parts per million, and no unvented flame in a room you sleep in.
Most winter air advice hands you one lever and tells you to pull it hard. That is how a dryness problem fixed in January becomes a mould problem in March.
The four trades
Four tensions pull against one another: heat against humidity, airtightness against air exchange, ventilation against heat loss and outdoor pollution, and the heat source against what it emits. Work through them in that order and the contradictions resolve into a short list of actions.
Trade one: heat versus humidity
Relative humidity is a ratio — how much water vapour the air holds against the maximum it could hold at that temperature. Warm air holds far more.
Take outdoor air at minus 5 Celsius and 80 per cent relative humidity — damp-feeling air, outdoors. It carries about 3.4 hectopascals of vapour pressure. Warm it to 20 Celsius indoors and you have added no water at all, but the air's capacity has risen to roughly 23 hectopascals. Relative humidity lands in the low teens. In a real house it goes lower, because rooms run warmer than 20 Celsius, cold spells go well below minus 5, and ventilation is continuous. Ten per cent or under during a hard freeze is ordinary. That is drier than most desert air.
So humidify. But this is where people overshoot, because the ceiling is set by your coldest surface, not the room average. Relative humidity rises as air cools: air at 45 per cent and 21 Celsius in the middle of the room hits saturation on a single-glazed window at 5 Celsius. That is condensation, and persistent condensation is how you get mould on window reveals, in the corners of external walls and behind wardrobes. Dust mites, whose droppings are a major indoor allergen, multiply readily above about 50 per cent and struggle below it.
The working rule: 40 to 50 per cent in a reasonably insulated, double-glazed house; 35 to 40 in an older one with cold single glazing or solid walls, and lower again in a deep cold snap. If the windows stream each morning, you are above your building's ceiling whatever the hygrometer says.
Trade two: airtightness versus air exchange
Draught-proofing, insulation and new windows all slow the rate at which your air is replaced. A leaky old house might turn over its whole volume once or twice an hour unaided. A well-sealed modern one can drop below 0.2 air changes an hour, which is not enough for the people inside it.
What accumulates: carbon dioxide from breathing; VOCs from furnishings, paint and cleaning products; water vapour from showers, cooking and drying laundry; and combustion by-products if anything burns fuel.
Carbon dioxide at household levels is not itself harmful. Its value is as a proxy — easy to measure, produced almost entirely by occupants, and it tracks everything else that builds up when air sits still. Keep it under about 1,000 ppm. Outdoor air is around 420; a closed bedroom with two adults routinely passes 2,000 by morning.
The sleep angle matters most for headache-prone people: seven or eight unbroken hours in one sealed room is the longest exposure of your day. The symptom — a heavy, dull head on waking that clears an hour after getting up — is easy to blame on the pillow or the barometer.
Trade three: ventilation versus heat and outdoor air
Purge, do not trickle. A window ajar all day bleeds heat continuously and does relatively little for air exchange. Two windows open wide for five to ten minutes creates cross-flow that replaces the air almost completely, and because the walls, floor and furniture hold most of the stored heat, the room reheats quickly. It also limits the humidity damage, because you are not running a continuous dry-air feed through the house.
Ventilate at the source. Extractor fans over the hob and in the bathroom remove moisture and pollutants before they disperse — but only if they vent outside and you run them during cooking and for ten to fifteen minutes after.
Heat recovery changes the maths. Mechanical ventilation with heat recovery passes outgoing warm air and incoming cold air through an exchanger, recovering roughly 70 to 90 per cent of the heat. That is how this trade is meant to be resolved in a sealed modern house — a building project, not a February purchase, but worth knowing before you next do work.
Inversions are the exception — the winter air quality situation where outdoor air is the worse option. A warm layer aloft traps cold air in a valley for days, with nothing to disperse traffic exhaust, wood smoke and industry. Outdoor fine particulates then build well above anything in your house. Denver and Calgary sit in classic inversion terrain, Anchorage can hold stagnant cold air for a week, and interior cities like Minneapolis get stagnation on top of the deepest dryness.
The answer on those days is not to stop ventilating — you cannot filter out carbon dioxide or moisture. Keep the purges short, avoid opening up during the evening wood-smoke peak, and run a HEPA cleaner for the particulates. Filtration and ventilation solve different problems; neither substitutes for the other.
Trade four: the heat source itself
Sealed, flued and electric systems put nothing into the room; unvented combustion puts everything into it. And systems that move air move dust with it, while radiators and underfloor heating move almost none.
| Heating type | Main indoor-air downside | The fix |
|---|---|---|
| Ducted forced-air furnace | Circulates settled dust and allergens through the house | Pleated filter, MERV 11 to 13, changed on schedule |
| Radiators or hydronic underfloor | Very little — no air moved, so no dust circulated | Keep furniture off radiators so convection is not blocked |
| Heat pump, ducted | Same dust circulation as any forced-air system | Same filter discipline; gentler air movement helps |
| Ductless mini-split | Mould and bacteria on the indoor coil and blower wheel | Wash filters monthly, deep-clean every year or two |
| Wood-burning stove | Particulates leak into the room at every refuelling | Dry seasoned wood, quick refuelling, HEPA cleaner in the room |
| Open fireplace | Poor combustion, real particulates, and it pulls heated air up the chimney | Use it for pleasure, not heat; ventilate while it burns |
| Gas hob used for heat | Nitrogen dioxide and ultrafine particles straight into the kitchen | Never heat a room with a hob; run the extractor when cooking |
| Unvented gas or kerosene heater | All combustion products released indoors, carbon monoxide included | Brief use in a ventilated space only; never overnight or in a bedroom |
| Electric resistance heater | None for air chemistry; hot elements scorch settled dust | Keep them clean |
Nitrogen dioxide is the one most people underestimate. It is a respiratory irritant associated with asthma symptoms, and a gas hob without extraction can push kitchen levels above roadside concentrations within minutes. A recirculating hood that filters grease and returns the air to the room does nothing about it.
A brief note on carbon monoxide. The fix is cheap and complete: fit an audible carbon monoxide alarm in any room with a fuel-burning appliance and outside sleeping areas, and have boilers, stoves and flues serviced annually by a qualified person. Carbon monoxide is odourless, and its early symptoms — headache, nausea, dizziness, tiredness — look exactly like the winter complaints in this article. The tell is that everyone in the house feels it, pets included, and everyone improves on leaving. If that pattern appears, leave and get the appliance checked.
What helps symptoms, and what is folklore
For headache, sinus pain and morning grogginess, the things with a plausible mechanism behind them:
- Humidity in the 40 to 50 band, measured rather than guessed.
- Saline nasal spray or gel — treats the nose rather than the room.
- Bedroom ventilation: door ajar, trickle vent open, or a purge before bed.
- Extractor fans used properly — the cheapest NO2 and moisture reduction available.
- HEPA filtration during wood-smoke or inversion episodes, and in any room with a stove.
- A slightly cooler house: capacity you never add is humidity you never destroy.
The folklore, and what it actually does:
- A bowl of water on the radiator. Evaporates a trivial amount. Drying laundry indoors adds far more.
- Houseplants as purifiers. The famous chamber studies do not scale; you would need a jungle per room to match a cracked window.
- Salt lamps. Decorative. No measurable effect.
- Ionisers and ozone generators. Ionisers move particles onto surfaces rather than removing them. Ozone generators are worse than useless indoors — ozone irritates the lungs and reacts with household VOCs to produce formaldehyde and ultrafine particles.
- Scented sprays, plug-ins and diffusers. They add VOCs and mask odour rather than removing its source.
Telling indoor air apart from the weather
The two have different signatures. Pressure-driven symptoms follow you: they arrive within hours of a front, happen at work and at home alike, and resolve in a day or two whatever room you are in. Indoor-air symptoms are tied to a place — worse on waking, better on a day spent elsewhere, and grinding on for as long as the heating season lasts.
Two weeks of notes usually settles it, provided you log the hour symptoms began, the hygrometer and carbon dioxide readings, and what the barometer was doing at that moment rather than that day. Keeping the current barometric pressure on the same chart as your entries turns a vague "winter is bad" into two patterns with two fixes, and a barometric pressure forecast gives the lead time to tell which is arriving. If you already use a migraine tracker app, add humidity and carbon dioxide as fields — they are the two indoor numbers most likely to explain a morning headache the weather does not.
FAQ
Does heating dry air out, or is it the cold outside doing it? Both, in sequence. Cold air carries little vapour; heating it indoors multiplies its capacity without adding any water, so relative humidity collapses. Target 40 to 50 per cent, but your real ceiling is your coldest surface — if the windows stream each morning you are too high for your building, whatever the hygrometer says.
Is carbon dioxide in my house dangerous? At household levels, no. It matters as a proxy: it is the easiest thing to measure that tracks how well a space is ventilated, and everything else builds up alongside it. Under 1,000 ppm means adequate air exchange; two or three thousand in a bedroom by morning means the opposite.
Should I ventilate when outdoor air quality is poor? Yes, but briefly — short cross-ventilation purges rather than an open window all day, timed away from the evening wood-smoke peak, then a HEPA cleaner for particulates. Filtration handles particles; only air exchange handles carbon dioxide and moisture.
Is my gas hob really an indoor-air problem? It is an unflued combustion appliance in your kitchen, so yes. That does not mean replacing it. It means running an extractor that vents outside every time you cook, and never using the hob or oven as a room heater.
Do air purifiers help headaches? They help with the particulate part — wood smoke, inversion haze, allergens stirred up by forced air. They do nothing for dryness, carbon dioxide, nitrogen dioxide or gaseous VOCs without a substantial activated carbon stage.
My newly insulated house feels stuffy and damp. What happened? You cut the air exchange rate without replacing it with anything deliberate, so moisture and carbon dioxide that used to leak away now stay put. The fix is controlled ventilation: extractors used properly, trickle vents open, daily purges, and heat recovery if you do further work.
The short version
Winter indoor air is a set of trades, not a setting. Heating drops relative humidity into the low teens, so humidify — but only to 40 to 50 per cent, and less if the windows run wet. Sealing the house traps what you and your furnishings emit, so measure carbon dioxide and hold it under about 1,000 ppm. Ventilate in short wide-open bursts, and on inversion days keep the purges brief and filter indoors. Match the fix to the heat source: filters for forced air, cleaning for mini-splits, extraction for anything that burns. Fit a carbon monoxide alarm and never sleep with an unvented flame. Then log humidity and carbon dioxide next to the barometer, because the two patterns look alike and only the timing tells them apart.