Thermal comfort

The basicsWhat your body actually feels

Your body produces heat all the time and has to lose it at the right rate. If it loses heat too fast you feel cold, and if it loses heat too slowly you feel hot. Six things set that rate.

Four of them belong to the house:

• Air temperature, which is what the thermostat reads.
• Surface temperature: the walls, windows, floor and ceiling around you. You exchange heat with them by radiation.
• Air movement, whether it's a breeze, a fan or a draught.
• Humidity, which controls how easily sweat evaporates.

Two of them belong to you:

• Clothing.
• Activity, whether you're sitting at a desk, cooking or asleep.

A heater or air conditioner changes only the first of the six. Good design works on the other three that the house controls.

SurfacesWhy a warm room can feel cold

You lose heat to cold surfaces by radiation. It works like the warmth you feel from a fire across a room, but in reverse. On a winter night, the largest window is usually the coldest surface in the room.

How cold depends on the glass. In laboratory modelling of a severe winter night, with −18°C outside and 21°C inside, the inner surface of each type of glass reached:

A person sitting a metre from a large clear double-glazed window that night has the room's surfaces averaging under 12°C around them, even though the air and every other wall are at 21°C. The cold glass also chills the air next to it. That air sinks, spills across the floor and arrives at your ankles as a draught, at a speed you can feel on bare skin.

Canberra nights are milder than that test, but the order doesn't change: the better the glass, the warmer its inner surface and the closer you can comfortably sit to it. Uninsulated walls, slabs without edge insulation and poorly insulated ceilings do the same thing on a smaller scale.

What fixes it:

• Match the glass to the size of the window and to how close people will sit to it.
• Fit close-fitting curtains or cellular blinds with a pelmet or top seal, so cold air can't spill out behind them.
• Insulate walls, ceilings, floors and slab edges, so those surfaces stay near room temperature.
• Plan furniture with the windows in mind. A sofa or bed placed hard against a large window will always feel cold in winter.

Air movementBreezes and draughts

Moving air carries heat off your skin and speeds up the evaporation of sweat. In summer that's exactly what you want. A ceiling fan blowing down on you makes a room feel roughly 2 to 4°C cooler without changing the air temperature at all. A fan blowing upward gives about half that. Many modern ceiling fans use under 10 watts at the speeds people actually choose, against hundreds of watts for even a small air conditioner. A fan cools people rather than the room, so it's only worth running when someone is underneath it.

In winter the same physics works against you. Air leaking in around doors, skirtings, downlights and window frames is cold and arrives at floor level. Sealing those gaps is one of the cheapest comfort improvements there is. A sealed house still needs fresh air, but the air should come in through openings you control, not through cracks you don't.

What to plan for:

• Ceiling fans in living areas and bedrooms, with the wiring and ceiling framing allowed for at design stage.
• Windows on opposite or adjacent walls so that summer breezes can pass through a room.
• A draught-sealed building, including the entry, downlights and any wall or roof penetrations.

HumidityWhy the same temperature feels different

Humidity decides how well sweat evaporates. A comfortable range for most people is above about 20% relative humidity all year, and below about 60% in summer. At 30°C, dry air feels tolerable and humid air feels oppressive.

Canberra summer afternoons average 37% humidity, so evaporation works well. Fans, cross-breezes and cool night air are very effective here. On the NSW coast, humid summer nights stay warm. Air movement matters more there, and an air conditioner earns some of its keep by drying the air as much as by cooling it.

In winter, humidity is a building problem more than a comfort one. A family of four puts 10 to 14 litres of water into the air every day by breathing, cooking, showering and drying clothes. If that moisture has no way out, it condenses on the coldest surfaces. The answer is extraction at the source: rangehoods and bathroom and laundry fans ducted outside, combined with surfaces kept warm by good glazing and insulation. See Condensation and building breathability for more detail.

HealthA cold home isn't just uncomfortable

The international health benchmark is that a home in a temperate or cold climate should reach at least 18°C in the cold season. Older people, young children and people with heart or lung conditions may need it warmer. The evidence behind that figure is consistent. Colder homes are linked to higher blood pressure and more respiratory illness.

Cold is also the bigger killer. Across 13 countries, Australia among them, about 7.3% of all deaths are linked to non-optimal cold, compared with 0.4% for heat. Most of those deaths come from ordinary cold weather, not extreme cold snaps. In Australia's coldest climate zone, which includes Canberra, about 3.6% of deaths between 2006 and 2017 were linked to cold, and heat was the smaller risk.

Canberra's housing stock is part of the story. Measured through winter 2023, ACT rental homes had a median indoor temperature of 15.3°C, the lowest in the country. Homes spent 84% of the time below 18°C, and one reading fell to 4.5°C.

The remedy is not complicated. Insulation in new homes, insulation retrofitted to old ones, and enough ventilation to keep them dry do most of the work. A well-designed new Canberra home should hold 18°C or better through a winter evening with modest heating, and keep most of that warmth until morning.

PeopleComfort depends on who, where and when

People aren't fixed thermostats. If you can open a window, put on a jumper or switch on a fan, you'll accept a wider range of temperatures, and your sense of comfortable shifts with the season. A home designed to let people adjust things will feel comfortable more of the time than one that relies only on heating and cooling.

Different rooms also need different things:

• Living areas are used from morning to late evening. They need winter sun, warm surfaces and summer shade.
• Bedrooms are cooler at night under bedding. A bedroom's real enemy is heat stored up during the day: a west-facing room that has soaked up afternoon sun stays warm until well after midnight.
• Home offices are where people sit still for hours. That makes them sensitive to cold surfaces and draughts, and the rating assumes bedrooms are empty during the day.
• Bathrooms feel coldest when you're wet. A warm, draught-free bathroom matters most on a Canberra winter morning.

The household matters as much as the rooms. A baby, an older parent or someone with a heart or lung condition needs a warmer house. Tell me early, and the design can plan for it.