It is easy enough to measure the air temperature of a room with a thermometer but it is far more difficult to say at what temperature a person feels comfortable in a room. Whether internal good feeling predisposes us to comfort and emotional upset causes us to be more critical of our environment is something that cannot be gauged. Watching a good late-night thriller on the television you may not notice the drop in room temperature that has occurred since the central heating automatically switched itself off at 10 pm. The film is absorbing, the chair ensures that your body is relaxed, the elderberry wine is of a particularly promising vintage and you finished the autumn digging in the morning.
All the scientists are able to do is measure the physical conditions within a room, such as air temperature, humidity, the surface temperature of walls, floor and ceiling, and the spread of draughts within the room and then, noting how the people are dressed and what they are doing, they ask them how comfortable they feel. By doing a survey in a large enough sample of buildings they can produce a range of conditions within which people engaged in certain activities can be said to be comfortable. This in turn leads to official tables of recommended temperatures to be achieved in various buildings: to find these, in the UK, refer to The Shops, Offices and Railway Premises Act and the Parker Morris recommended temperatures for Local Authority housing.
The environmental measurements that can be taken within a room relate to how people lose heat from their bodies. Unless you are ill your body temperature should be around 37°C. This relatively high temperature means that each person continually loses heat to the surroundings. Heat is generated within the body as food is digested. The more work we do, the more heat is generated and the greater is our need for food. Sawing wood for two hours on a frosty Sunday afternoon will produce a better appetite for tea-time crumpets than sitting reading in front of the fire. However, because most of our calorie intake is used simply to keep the body functioning we still need to eat even in hot climates. At rest you give off a little over 100W and this can rise to around 250W for work such as scrubbing the floor.
Providing the conditions that suit a given level of activity remain constant, you remain comfortable. When conditions change, most people’s bodies, except those of the very young, the ill and the very old, compensate automatically by altering the rate at which heat is lost from the skin’s surface. If these compensatory mechanisms are not adequate you might increase your insulation by putting on a jumper when the sun goes in, or you might increase the rate at which air movement removes heat from your skin surface by opening a window on a hot day. The only time you are unable to practise this type of automatic compensation is when you are asleep. Going to sleep with three blankets on and the window open on a frosty night may cause you to wake hot and clammy the next morning when the weather has changed to a warm muggy drizzle.
The 100W rate of heat loss at rest is determined by two main factors: one is the temperature difference between your skin and the surrounding mass of air (which governs the heat lost by convection); the other is the difference in temperature between your skin and the surface temperatures of the walls, floor, furniture of the room (which governs the heat lost by radiation). The amount of water in the air of the room, the humidity, has very little effect on the heat loss balance and, therefore, on comfort. It is only when you are already hot and sweating that humidity has any effect. This is why we tend to feel uncomfortable on hot, humid days.
Quality of heating
The fact that we lose heat by both radiation and convection also has a bearing on the different types of heating that can be used within a room in order to maintain comfortable conditions.
If you are sitting in a room where the air temperature is very low, but you are near a source of radiant heat (such as a stove which was lit early in the morning), you may still feel comfortable. You are gaining more radiant heat than you are losing and, even though the heat lost to the cold air may be considerable, the total heat loss rate is still in balance.
The amount of heat lost from your body will also remain constant if you sit near an electric fan heater. Here you will be hotter than the warm air which might be at 20°C and you will continue to radiate heat to the air and the surrounding surfaces of the room (which are at less than air temperature); but there will be very little heat lost by convection to the warmed air.
A more normal central heating system with water-filled radiators both warms the air by setting up convection currents and radiates out heat to anyone standing near the radiators, as the surface temperature of a radiator is higher than that of skin. Interestingly, many people will say that they feel more comfortable when seated next to a source of radiant heat (a coal fire or a stove) than in a room with central heating.
Insulating to control room temperature
By insulating your house you alter how comfortable you feel within it. Apart from reducing the heat loss which may mean that the building can be heated with, say, a single, centrally placed stove, the insulation will affect the temperatures of the surfaces surrounding you within a room. When the room is heated these surfaces will be at the same temperature as the air within the room and providing the windows are covered with insulated shutters there will be no very cold spots to which you will be losing heat through radiation. Less heat will be needed to keep you feeling comfortable. At the same time, if you have stopped up all the draughts, there will be no cold air to increase your heat loss by convection. Draughts, which usually only blow about the feet, induce a feeling of discomfort as they cool only a part of the body.
What you may lose in setting up a house that consumes little energy is the control of separate room temperatures that some forms of central heating now give. It would not be possible to pre-set temperatures in an insulated house heated by a single source of radiant heat such as a woodstove. The room containing the stove would be the warmest but the stove would also warm the room above it, and, to some extent, those around it, allowing for the fact that warmed air would rise up the stairwell. The only way you could control the heating of a room would be by opening and shutting doors. Such a system might be suitable for a house that was continuously occupied. At the other extreme where the insulation is placed on the inside walls, each room can be very quickly warmed up by using a gas fire in each; such a system might suit a household out at school and work each day. If you do some hard physical work in this type of insulated room you may end up feeling rather hot; but work leading to a heat loss rate of 175W (moderate housework such as changing a bed) is generally done only for a short time in most rooms, except perhaps the kitchen.
What the recommended temperatures do not allow for are different patterns of use, such as bedrooms which children might use as playrooms during the day or as study rooms in the evening. Using systems such as central heating often with pre-set temperatures also tends to pre-set the ways in which the house is used. In an insulated house which is continuously occupied, it is much easier to keep all the rooms at a reasonably comfortable temperature which may free the way in which they are used. If a room is warm you will use it whereas you may be unwilling to suffer half an hour
It is far easier to alter the amount of clothing you wear than it is to adjust the temperature of a room. Today the levels of insulation of our clothing are less than was usual at the turn of the century; we have become accustomed to having our buildings heated and to wearing less. We also vary the amount of clothes we wear according to the seasons. If you are wearing a shirt and jeans in the summer you might need a temperature of 23°C to feel comfortable when sitting indoors; but if you wear a shirt and jeans, and a sweater, woollen socks and vest in winter, you will need only a temperature of 20°C indoors. The fact that clothing is adaptable solves the problem of varying the temperature of a room to suit the activity. Add an overcoat to your normal winter wear and you could sit happily in a room heated to 17°C, but for the fact that you do not expect to have to wear an overcoat indoors, so that you are unlikely to say that you feel ‘comfortable’ in one.
For the reasons discussed, any scale of temperatures recommended to give comfort within a building must be fairly arbitrary. However, if you insulate your house and put in some kind of heating system, whether a single stove, central heating or even a low temperature solar-heated water system, you should find some place within the building where you feel comfortable wearing normal clothing. As a general rule, you can leave bedrooms in the insulated house to be warmed by the heat rising through the floor and up the stairs. It is possible (according to the Building Research Establishment) for most people, except for babies, the old and the ill, to sleep happily when the temperature goes below 0°C. But if you are to be able to force yourself out of bed and to dress comfortably in the morning perhaps 15°C would be a better target temperature. The sitting room should contain the source of heat in order to maintain the temperature at a comfortable 20°C; but you can get away with heating the kitchen to only around 17°C.
Casual gains, such as the sun coming through windows, will alter the amount of heat within the building on different days. But if you are willing to substitute the never varying temperatures of a central heating system (with the consistently high bill) for the energy-saving methods of insulation and reduced heating, you may find that you accept more variable temperatures. On cold cloudy days it may be necessary to shut the bedroom doors and wear an extra jumper; but the fact that in so doing you are saving energy may help you to feel both physically and mentally more comfortable.