I live in a 70’s house that has had double glazing installed but little else in the way of thermal improvements since then. Imagine the following situation. Yesterday was a mild winter’s day, temperatures around 10°C, and I was comfortably warm working at home. Today the weather’s changed, it’s been cold all night and now it’s hovering around zero and cloudy outside. Wearing the same clothes, and with the thermostat set at the same temperature, I feel chilly. After spending the morning cradling numerous cups of tea and still not warming up I finally turn the thermostat up a degree. Why, if the air temperature inside my house is the same on both days, do I feel colder today?
We are used to thinking about how warm we feel indoors purely in terms of air temperature – it is, after all, the only thing displayed on the thermostat. Yet whether we feel too cold, too hot, or just right, our thermal comfort, is dependent on much more than just the temperature of the air around us.
Models of thermal comfort are pretty well developed but are poorly understood by most people outside academia. I’ll focus in this post on feeling too cold, but broadly the same principles apply in reverse for overheating in the summer.
Whether we feel warm or cold depends on how fast our bodies are losing heat to the world around them. This depends on the air temperature in part; if the air next to our bodies, or on the outside of our clothing, is cold then we lose heat to it more quickly than if it is warm, but this is only part of the story. Let’s look at the other other factors affecting how fast we lose heat to our surroundings.
The speed at which the air is moving around you, as well as its temperature, also affects your heat loss. People who spend time in the mountains know this effect, wind-chill, very well. For a given air temperature, it’s much harder to stay warm on a windy day. While it never gets properly windy indoors, the same rules apply; air moving fast around you means the layer of air next to your skin/clothes that you have just warmed up is replaced with colder air more frequently, increasing your rate of heat loss.
Air moving quickly in a building can obviously be from draughts of outside air, which bring the added risk that the air moving past you is colder than the rest of the room, but even in a well-sealed building cold weather can generate fast-moving air indoors. Cold surfaces such as single or double glazed windows, especially large ones, cool the air adjacent to them and cause powerful down-draughts that set the air moving too quickly for comfort around the rooms they are in.
As well as losing heat through conduction to the air next to us, we also lose heat through radiation, regardless of the air temperature, to the solid surfaces surrounding us. This radiative heat loss depends on the temperature of those surfaces and how close we are to them. The warmer they are the less heat we lose in this way, the colder they are the more heat we lose. This radiative effect is very noticeable in extreme situations – a campfire on a cold night warms you mostly through radiation, which is why you feel colder instantly when someone stands between you and the fire. The same effect is true in buildings, when the internal surfaces of external walls, windows, floors and ceilings are colder, we lose more heat to them through radiation*.
The humidity of the air is the final piece of the thermal-comfort jigsaw. The drier the air is, the more easily moisture evaporates from our skin, taking heat with it. So, for a given air temperature, a room at 60% humidity will feel warmer than a room at 40% humidity. Cold air holds less moisture than warm air, so ventilating a room (either deliberately or through leaks) with cold air tends to bring less moisture indoors than ventilating with warm air, resulting in a lower relative humidity.
Going back to the original question of why I might feel colder indoors when it is cold outside, even with the thermostat set at the same temperature, all of these factors play a role:
- The air in a room may be moving faster on the cold day, due to windows and other poorly-insulated surfaces being colder when the external temperature is lower, and thus generating more powerful down-draughts.
- External draughts due to leaky building fabric or trickle vents will be colder, even if the average internal air temperature is the same on both days.
- The surfaces surrounding me in the room will be colder on the cold day, so heat loss via radiation will be increased.
- The humidity of the air in the room may be lower on the colder day, making the same air temperature feel colder.
I’ll take a look in the next post about how very low-energy buildings affect how comfortable we feel in cold weather.
*Edit November 30 2019. A friend who’s cleverer than me pointed out that my description here is not quite right. You don’t lose more heat through radiation to a cold surface than a warm one, you just gain less heat from it. Any object that is warmer than absolute zero (-273.15°C) radiates heat, so you feel colder stood next to a cold wall than stood next to a warm wall because it is radiating less heat at you, not because you are radiating more heat to it.