There are four ways in which our buildings lose heat:
1. Convection - the bulk movement of heat
2. Conduction - due to molecular vibrations
3. Evaporation - due to energy being consumed as water evaporates
4. Radiation - the transfer of heat through electromagnetic waves.
These mechanisms are in turn controlled by four factors:
As measured by a digital or mercury thermometer, and sometimes referred to as ‘dry bulb’ temperature. If this is below 16°C we tend to have to wear excessive clothing or be involved with rigorous activity; above 28°C and we require excessive air movement to reduce sweating even at very low activity rates.
Air temperature is controlled by convection and evaporation, and so warming it is done by introducing a heat source and moisture. Cooling is done by materials absorbing the heat, or since the heat is essentially held in the water molecules, taking moisture away (as with air conditioning).
Air temperature is also controlled to a lesser extent by conduction, and so having cool surfaces will help to cool and warm surfaces will help to warm, although much of the work will be being done by evaporation.
We need a certain amount of air movement in order to improve air quality and to help us perspire, and air movement (windspeed) of less than 0.1m/sec leads to a feeling of stuffiness. An airspeed of over 1.0m/sec indoors however and we start feeling the discomfort and the disturbance – papers blow about in the draught. Outside, air movement of 2.0m/sec is good, particularly if it is humid, but anything over 5.0m/sec is too high for comfort.
Air movement helps to cool surfaces by enabling and accelerating evaporation and perspiration. This can be useful for both us and our buildings, if we are too hot and humid, otherwise it can make us feel cold even if air temperature is high.
The wind plays an important and yet badly studied part of our buildings’ thermal dynamic, and preventing heat loss by shielding buildings from the South West prevailing winds can be just as important as providing insulation elsewhere.
In essence, relative humidity is given as a percentage of water vapour in air at any given temperature, compared to saturated air at the same temperature. The warmer the air, the more water vapour (and heat) that it is able to hold. If you cool the air, the water will fall out.
We are sensitive to humidity because of our evaporation/perspiration mechanism – below 20% and we feel discomfort due to dryness of eyes, lips, and throat, whereas above 90% and we feel damp and clammy.
Air conditioning essentially works by condensing the water vapour out of hot, humid air, which causes the air to lose heat. This process is also what is happening when the humid air in a room condenses on a cold window or other cold surface, thereby creating a feeling of coolth, air movement, and discomfort.
Long-wave (infrared) thermal radiation is given off by all objects that are above -273°C. Substantial radiation is lost by any object, but this is balanced by that gained from objects around. In order for us to lose heat, we must be surrounded by surfaces that are cooler than us – generally this is preferable since we need to lose a substantial amount of our metabolic heat this way (hence generally having our heating set in the region of 20°C).
Radiators ironically largely heat through convection and so perhaps should be better named, whereas underfloor heating is much more radiative, directly heating interior surfaces rather than the air.
Naked flames also heat using radiation (as well as through convection), which is why you can huddle round a fire outside in the winter and still feel hot. Naked flame also has the added advantage of heating through psychology – our association with fire and safety, warmth, and food is ancient, and simply looking at a fire can help us feel warmer and content.