120
110…………………………….Loud disco
100…………………………….Noisy factory
90……………………………..Heavy lorry
80……………………………..High street corner
70……………………………..Vacuum cleaner
60……………………………..Normal conversation
50
40…………………………….Suburban living room
30
20…………………………….Quiet countryside
10
0…………………………….Threshold of hearing
The level of sound that we are able to tolerate in our immediate environment is also a variable; while we may expect to work in a noisy atmosphere, perhaps over 160 decibels, which may be so loud as to necessitate the wearing of some kind of ear-protection, none of us would be happy to tolerate that level of noise pollution while we are at home. We design the interior spaces that we inhabit to be as free from unwanted outside noise as is economically and aesthetically possible.
Light
As a diurnal species, we humans prefer to go about our business in the light of day. This is the third factor that needs to be considered during the planning phase of constructing a building for human habitation.
While people generally prefer to make use of natural daylight whenever possible, sometimes it becomes necessary to utilize an artificial source of illumination; for instance on dull or cloudy days or during the hours of darkness.
The nature of vision. The human eye is an amazing organ. Light entering the eye via the cornea, a membrane which covers the outside of the eye, passes through the circular opening (pupil) in the coloured part of the eye (the iris). Behind the iris is a lens, the shape of which is changed by the ciliary muscles, so as to focus an image of the object being viewed onto the retina of the eye. The retina is at the back of the eye and is packed with two different types of light-sensitive cells, rods and cones. There are around 130 million rod cells, which we use for black and white vision in poor lighting situations. The cone cells are used for colour vision when lighting is at a more ideal level. These type of cell number around 7 million.
The human eye.
When light energy falls onto the cells in the retina, it causes chemical signals to be transmitted to the brain via the optic nerve. These signals are then perceived as images. This is how human beings “see”.
Human vision has evolved to work at it’s most efficient in natural daylight. In these lighting conditions the cone cells in the retina are stimulated and we are able to see in colour. Also, there is a concentration of cone cells in the fovia at the centre of the retina, which enable us to see in great detail.
Measurement of light. Light is a form of energy and can be measured by the standard unit of energy, the Watt. However, we find it more convenient in terms of building design to use a measure of the flow of light energy as a standard.
Luminous Flux is the rate of flow of light energy. It is measured in units called lumen (lm).
When luminous flux falls onto a surface, it illuminates that surface. The lighting effect is called illuminance.
Illuminance is the density of luminous flux reaching a surface.
UNIT: lux (lx) where 1 lux = 1 lumen/metre²
Summary of comfort levels – Illuminance.
50,000 lux
SUNLIGHT
10,000 lux
SHOP DISPLAY
COMFORT POINT
100 lux
HALLWAY
0 lux
NO LIGHTING
The level of ambient light with which we are comfortable is again subject to the type of activity in which we are engaged at any given time. Whilst at work in an office, a luminance level of around 500 lux would be desirable, whereas we might be happier with a level of around 100 lux in a bedroom.
Desirable Illuminance Levels:
Entrance halls 150 lux
Stairs 150 lux
Passageways 100 lux
Machinist work area 300 lux
Very fine detailed work 1000 lux
Clerical work 500 lux
Typing 750 lux
Supermarket 500 lux
Classroom 300 lux
Laboratory 500 lux
Bar 150 lux
Restaurant 100 lux
Kitchen 400 lux
Living room 50 lux
Halls and landings 150 lux
Bedroom 50 lux
Gymnasium 500 lux
Swimming pool 300 lux
The Nature Of Heat
Heat is a form of energy, which we measure in Joules (J).
This is not to be confused with temperature. The temperature of an object or a space is a measure of how much heat it contains, relative to its size.
We can measure temperature on various scales, although for the purposes of this exercise I will refer only to 2 scales: Celsius (C) and Kelvin (K).
Celsius scale
Freezing point of water = 0° C Boiling point of water = 100° C
Kelvin scale
0° K is the point at which all heat energy has been extracted from a body. This point is known as absolute zero and occurs at –273° C.
Heat transfer. Heat energy will always travel from a region of high temperature to one of lower temperature. If several bodies of varying temperature are placed close together, then heat will be exchanged between them until they are all of equal temperature.
This equalising of temperature can occur by three basic processes of heat transfer:
- Conduction
- Convection
- Radiation
Conduction can occur in solids, liquids and gases, although the speed at which it occurs will vary. If a material is heated in one area only, the molecules in that area will acquire energy and become hot. These molecules will then transfer some of their energy to neighbouring molecules, which will in turn become hot themselves. In this way the whole of the material will eventually become hot. This process is most effective in solid materials; particularly metals. This is why we say that metal is a good conductor of heat, as opposed to most gases and liquids, which are known as insulators.
Convection can occur in fluids (liquids and gases) but never in solids. It is the transfer of heat energy through a material by the bodily movement of heated particles.
Natural convection occurs when a sample of fluid, such as air, is heated and so expands. The expanded air is less dense than the surrounding air and so it rises, to be displaced by cooler air. This cooler air is then heated in turn by the heat source, rises to be displaced itself and in this way a convection current is created.
By this process a single source of heat, such as an electric fire, can heat a room throughout.
Radiation is the transfer of heat by electromagnetic waves. Heat is transferred from the Sun to the Earth through the vacuum of space, where conduction and convection are not possible. The process of radiation is responsible for this type of heat transfer.
Thermal Insulation
In order to maintain a constant temperature within a building it is necessary to restrict the rate at which heat energy is exchanged with the surroundings. Keeping heat inside a building for as long as possible conserves energy and reduces heating costs.
Thermal insulation is the major factor in reducing the loss of heat from buildings. Adequate insulation should be a feature of all good initial design but insulation can also be added to existing buildings. The relatively small cost of extra insulating materials is quickly paid for by the reduction in the size of the heating plant required and by the annual savings in the amount of fuel needed.
Insulating materials.
A thermal insulator is a material which opposes the transfer of heat between areas at different temperatures. This, in essence, is the main aim of incorporating insulating materials in building design. By this method we hope to maintain a comfortable indoor temperature without the poor economy of reliance on expensive heating or cooling systems.
The best insulating materials have their atoms spaced widely apart. Gases, which have the most widely spaced atoms, are the best insulators against conduction.
Air, which is a mixture of gases, is the basis of insulators such as expanded plastics and cavities.
For air to be an effective insulator it must be stationary, otherwise it will allow the transfer of heat by conduction.
Materials that do not readily absorb or emit radiant heat restrict heat transfer by radiation. Aluminium foil, for example is an example of this type of insulator.
Some different types of insulation product commonly used in construction:
-
Rigid preformed materials. Example: aerated concrete blocks
-
Flexible materials. Example: fibreglass quilts
-
Loose fill materials. Example: expanded polystyrene granules
-
Materials formed on site. Example: formed polyurethane
-
Reflective materials. Example: aluminium foil
U Values
The U value of a section or component of a building is a measure of the rate of heat transfer of that particular component (a wall or a window for example), by all mechanisms (conduction, convection and radiation) taken together as a whole.
We can use the U value to calculate the rate at which heat energy from inside a building, for example, will travel through a particular component of that building to the outside of the building, taking into account all the quantities of the materials used in the construction of that component. This measurement is called the overall thermal transmittance coefficient.
The coefficient, or U value, is measured as the rate of heat flow in watts through 1 m² of a structure when there is a temperature difference of 1° C across the structure.
For ease of calculating U values, the construction industry makes assumptions concerning the rate at which various components will allow heat to be transferred through them. These are known as standard U values and are a constant measurement used throughout the industry.
The standard U value of a wall built with 215mm bricks and a 15mm skim of plaster for example, is 2.3 W/m² K. This means that where there is a temperature difference of 1ºC between one side of the wall and the other, the rate of heat transfer in Watts will be 2.3.
U values can be of great use when we are attempting to maintain an indoor temperature in a cost-effective way.
For example, the standard U value of a single glazed window is 5.7 W/m² K but the standard U value of a double glazed window is only 2.8 W/m² K.
This means that almost twice the amount of heat energy is transferred through the single glazed window as through the double glazed window; therefore it will cost almost twice as much to replace the heat lost through the single glazed window that it would to replace the heat lost through the double glazed window.
Cut-away diagram of a double glazing window.
By calculating the U values for the many various components of the entire building, we can identify which areas and surfaces are acting as ‘heat-sinks’ by draining heat energy away to the outside of the building and act accordingly, perhaps by adding extra insulating materials to these areas.
Humidity. This is the measurement of moisture in the air around us. A relative humidity range of between 40 to 70 per cent is required for comfortable conditions; too low and we can experience dryness of the throat and skin. Too high and human beings may begin to sweat, especially if high humidity is experienced with high temperatures. High humidity together with low temperatures will cause the air to feel chilly.
Summary of comfort measurements – relative humidity.
100%
SATURATED AIR
70%
HUMID DAY
COMFORT POINT
40%
DRY DAY
0%
DRY AIR
Ventilation. A number of statutory regulations specify minimum rates of air supply in occupied spaces. The normal process of breathing gives significant quantities of latent heat and water vapour to air. Body-odours, bacteria, and the products of cooking, smoking and washing also contaminate household air.
In any occupied space ventilation is necessary to provide oxygen and to remove contaminated air. Fresh air contains about 21 % oxygen and 0.04% carbon dioxide while expired air contains about 16% oxygen and 4% CO². The body requires a constant supply of oxygen but the air would become unacceptable well before there was a danger to human life. As well as being a comfort consideration the rate of ventilation has a great effect on the heat loss from buildings and on condensation in buildings.
Heat gains. A building gains heat energy as well as loses heat energy, and both processes usually occur at the same time. In a location with a temperature climate such as the British Isles, the overall gains are less than the overall losses, but the heat gains may still provide useful energy savings.
Factors which affect heat gains in a typical home:
- Solar heat gains from the sun
- Electrical appliances in the home
- Water heating and cooking
- Light fittings
- The occupants of the building themselves
- The position of windows around the building
Typical heat gains in a building.
These types of heat gain may be beneficial when it comes to maintaining a comfortable temperature in the home. Solar heating in particular is a great factor to be considered when calculating heat gains in a building. In cold climates it is desirable to design buildings with windows that are south facing. However, it is possible to gain too much solar heat energy and we may want to consider this in our designs. Heavy blinds may be required to shut out the suns rays when a building becomes uncomfortably hot.
Heat balance. The thermal comfort of human beings requires that the inside temperature of a building is kept constant at a specified level. In order to maintain constant temperature the building will generally require heating or cooling, and both of these processes involve the consumption of energy.
By being aware of the nature of heat and how good building design can effect losses and gains of it, we can ensure that we maintain a comfortable temperature for habitation in a cost-effective way.