Walls Resistance to the passage of heat

The traditional method of heating buildings was by burning wood or coal in open fireplaces in England and in freestanding stoves in much
of northern Europe. The ready availability of wood and coal was adequate to the then modest demands for heating of the comparatively small population of those times. The highly inefficient open fire had the advantage of being a cosy focus for social life and the disadvantage of generating draughts of cold air necessary for combustion. The more efficient freestanding stove, which lacked the obvious cheery blaze of the open fire, was more suited to burning wood, the fuel most readily available in many parts of Europe.

The considerable increase in population that followed the Industrial Revolution and the accelerating move from country to town and city increased demand for the dwindling supplies of wood for burning. Coal became the principal fuel for open fires and freestanding stoves.
During the eighteenth century town gas became the principal source for lighting and by the nineteenth century had largely replaced solid fuels as the heat source for cooking. From about the middle of the twentieth century oil was used as the heat source for heating. Following the steep increase in the price of oil, town gas and later on natural gas was adopted as the fuel most used for heating.

Before the advent of oil and then gas as fuels for heating, it was possible to heat individual rooms by means of solid fuel burning open fires or stoves and people accepted the need for comparatively thick clothing for warmth indoors in winter.

With the adoption of oil and gas as fuels for heating it was possible to dispense with the considerable labour of keeping open fires and stoves alight and the considerable area required to store an adequate supply of solid fuels. With the adoption of oil and gas as fuel for heating it was practical to heat whole buildings and there was no longer the inconvenience of cold corridors, toilets and bathrooms and the draughts of cold air associated with open fireplaces. The population increasingly worked in heated buildings, many in sedentary occupations, so that tolerance of cold diminished and the expectation of thermal comfort increased.

For a description of the history of the development of heating appliances over the centuries and the increased use of thermal insulation, see Volume 2, Fires and Stoves.

Of recent years the expectation of improved thermal comfort in buildings, the need to conserve natural resources and the increasing cost of fuels have led to the necessity for improved insulation against transfer of heat. To maintain reasonable and economical conditions of thermal comfort in buildings, walls should provide adequate insulation against excessive loss or gain of heat, have adequate thermal storage capacity and the internal face of walls should be at a reasonable temperature.
For insulation against loss of heat, lightweight materials with low conductivity are more effective than dense materials with high conductivity, whereas dense materials have better thermal storage capacity than lightweight materials.

Where a building is continuously heated it is of advantage to use the thermal storage capacity of a dense material on the inside face of the wall with the insulating properties of a lightweight material behind it. Here the combination of a brick or dense block inner leaf, a cavity filled with some lightweight insulating material and an outer leaf of brick against penetration of rain is of advantage.

Where buildings are intermittently heated it is important that inside faces of walls warm rapidly, otherwise if the inside face were to remain cold, the radiation of heat from the body to the cold wall face would make people feel cold. The rate of heating of smooth wall surfaces is improved by the use of low density, lightweight materials on or immediately behind the inside face of walls.

The interior of buildings is heated by the transfer of heat from heaters and radiators to air (conduction), the circulation of heated air (convection) and the radiation of energy from heaters and radiators to surrounding colder surfaces (radiation). This internal heat is transferred through colder enclosing walls, roofs and floors by conduction, convection and radiation to colder outside air.


The rate at which heat is conducted through a material depends
mainly on the density of the material. Dense metals conduct heat more rapidly than less dense gases. Metals have high conductivity and gases low conductivity. Conductivity is the amount of heat per unit area, conducted in unit time through a material of unit thickness, per degree of temperature difference. Conductivity is expressed in watts per metre of thickness of material per degree kelvin (W/mK) and usually denoted by the Greek letter X, (lambda).


he density of air that is heated falls, the heated air rises and is replaced by cooler air. This in turn is heated and rises so that there is a continuing movement of air as heated air loses heat to surrounding cooler air and cooler surfaces of ceilings, walls and floors. Because the rate of transfer of heat to cooler surfaces varies from rapid transfer through thin sheet glass in windows to an appreciably slower rate of transfer through insulated walls, and because of the variability of the rate of exchange of cold outside air with warm inside air by ventilation, it is not possible to quantify heat transfer by convection. Usual practice is to make an assumption of likely total air changes per hour or volume (litres) per second and then calculate the heat required to raise the temperature of the incoming cooler air introduced by ventilation.


Radiant energy from a body, radiating equally in all directions, is
partly reflected and partly absorbed by another body and converted to heat. The rate of emission and absorption of radiant energy depends on the temperature and the nature of the surface of the radiating and receiving bodies. The heat transfer by low temperature radiation from heaters and radiators is small, whereas the very considerable radiant energy from the sun that may penetrate glass and that from high levels of artificial illumination is converted to appreciable heat inside buildings. An estimate of the solar heat gain and heat gain from artificial illumination may be assumed as part of the heat input to buildings.

Transmission of heat

Because of the complexity of the combined modes of heat transfer
through the fabric of buildings it is convenient to use a coefficient of heat transmission as a comparative measure of transfer through the external fabric of buildings. This air-to-air heat transmittance coefficient, the U value, takes account of the transfer of heat by conduction through the solid materials and gases, convection of air in cavities and across inside and outside surfaces, and radiation to and from surfaces. The U value, which is expressed as W/m2K, is the rate of heat transfer in watts through one square metre of a material or structure when the combined radiant and air temperatures on each side of the material or structure differ by 1 degree kelvin (1°C). A high rate of heat transfer is indicated by a high U value, such as that for single glazing of 5.3 (W/m2K), and a low rate of heat transfer by a low U value, such as that for PIR insulation of 0.022 W/m2K.

The U value may be used as a measure of the rate of transfer of heat through single materials or through a combination of materials such as those used in cavity wall construction.

Conservation of fuel and power
Standard assessment procedure (SAP) rating

The requirement in the Building Regulations for the conservation of fuel and power for dwellings alone is that the person carrying out building work of creating a new dwelling shall calculate the energy rating of the dwelling by a standard assessment procedure (SAP) and give notice of that rating to the local authority.

The SAP rating is based on an energy cost factor on a scale of 1 to 100,1 being a maximum and 100 a minimum energy use to maintain a comfortable internal temperature and use of energy in water heating. While there is no obligation to achieve a particular rating, a rating of 60 or less indicates that there is inadequate insulation or inefficient heating systems or both, and the dwelling does not comply with the regulations.
Details of the notification of the SAP rating for new dwellings are held by the local authority. A prospective purchaser of the dwelling may well be put off where the rating is 60 or less and the local authority has not issued a Certificate of Compliance with the Regulations, whereas the purchaser will be encouraged by a rating of say 85, which shows compliance with the Regulations.

The SAP rating is calculated by the completion of a four page worksheet by reference to 14 tables. The sequential completion of up to 99 entries by reference to the 14 tables is so tedious and difficult to follow as to confound all but those initiated in their use, and is hardly calculated to inform householders in a way that is simple and easy to understand as claimed by the authors of Approval Document L.

Calculation methods

Three methods of calculating the figures necessary for the SAP for
dwellings are proposed in Approved Document L. They are:
(1) an elemental method
(2) a target U value method
(3) an energy rating method.
In the elemental method standard U values for the exposed elements of the fabric of buildings are shown under two headings: (a) for dwellings with SAP ratings of 60 or less and (b) for those with SAP ratings over 60. The standard U values are 0.2 and 0.25W/m2K for roofs, 0.45 W/m2K for exposed walls, 0.35 and 0.45W/m2K for exposed floors and ground floors, 0.6 W/m2K for semi-exposed walls and floors and 3.0 and 3.3 W/m2K for windows, doors and rooflights, the two values being for headings (a) and (b), respectively. The basic allowance for the area of windows, doors and rooflights together is 22.5% of the total floor area. The area of windows, doors and rooflights, larger than those indicated by the percentage value, may be used providing there is a compensating improvement in the average U value by the use of glazing with a lower U value.

As it is unlikely that the SAP rating of the majority of new dwellings, complying with standard U values, will fall below 60, the over 60 rating values are the relevant ones.

The target U value method for dwellings is used to meet the requirement for conservation of fuel and power by relating a calculated average U value to a target U value, which it should not exceed. The average U value is the ratio of:

total rate of heat loss per degree total external surface area
The target U value is:
total floor area x 0.57 + 0.36 for dwellings with SAP ratings total area of exposed elements 60 or less, and
total floor area x 0.64 + 0.40 for dwellings with SAP ratings total area of exposed elements of more than 60
The total area of exposed floors, windows, doors, walls and roof and the standard U values in the elemental method are used to calculate the heat loss per degree. Where the calculated average U value exceeds the target U value it is necessary to improve the thermal resistance of walls, windows or roof either separately or together so that the average U value does not exceed the target U value. As an option, account may be taken of solar heat gains other than those allowed for in the equation on which the method is based. This method is based on the assumption of a boiler with an efficiency of at least 72%. Where a boiler with an efficiency of 85% is used the target U value may be increased by 10%. The use of the elemental or target U value methods of showing compliance does not give exemption from the requirement to give notice of an SAP rating.

The energy rating method is a calculation based on SAP which allows the use of any valid conservation measures. The calculation takes account of ventilation rate, fabric losses, water heating requirements and internal and solar heat gains.

The requirement for conservation of fuel and power will be met if the SAP energy rating for the dwelling, or each dwelling in a block of flats or converted building, is related to the floor area of the dwelling and ranges from 80 for dwellings with a floor area of 80 m2 or less to 85 for dwellings with a floor area of more than 120 m2.

As there is a requirement to complete the SAP worksheet to determine an SAP rating, which has to be notified to the local authority, whichever method of showing compliance is used the most practical and economic method of approach is to use the standard U values for SAP ratings over 60 set out in the elemental method in the initial stages of design, and then to complete the SAP worksheet at a later stage and make adjustments to the envelope insulation, windows and boiler efficiency as is thought sensible to achieve a high SAP rating.
For a description of the requirements for conservation of fuel and power for all buildings other than dwellings see Volume 4.


The sensation of comfort is highly subjective and depends on the age,
activity and to a large extent on the expectations of the subject. The young ‘feel’ cold less than the old and someone engaged in heavy manual work has less need of heating than another engaged in sedentary work. It is possible to provide conditions of thermal comfort that suit the general expectations of those living or working in a building. None the less, some may ‘feel’ cold and others ‘feel’ hot. For comfort and good health in buildings it is necessary to provide means of ventilation through air changes through windows or ventilators, that can be controlled, depending on wind speed and direction and outside air temperature, to avoid the sensation of’stuffiness’ or cold associated with too infrequent or too frequent air changes respectively. As with heating, the sensation of stuffiness is highly subjective.

Air changes

For general guidance a number of air changes per hour is recom-
mended, depending on the activity common to rooms or spaces. One air change each hour for dwellings and three for kitchens and sanitary accommodation is recommended. The more frequent air changes for kitchen and sanitary accommodation is recommended to minimise condensation of moisture-laden, warm air on cold internal surfaces in those rooms.


Condensation is the effect of moisture from air collecting on a surface
colder than the air, for example in a bathroom or kitchen where water from warm moisture-laden air condenses on to the cold surfaces of walls and glass. To minimise condensation, ventilation of the room to exchange moisture-laden air with drier outside air and good insulation of the inner face of the wall are required.

A consequence of the need for internal air change in buildings is that the heat source must be capable of warming the incoming air to maintain conditions of thermal comfort, and the more frequent the air change the greater the heat input needed. The major source of heat loss through walls is by window glass which is highly conductive to heat transfer. This heat loss can be reduced to some small extent by the use of double glazing. Most of the suppliers of double glazed windows provide one of the very effective air seals around all of the opening parts of their windows. These air seals are very effective in excluding the draughts of cold air that otherwise would penetrate the necessary gaps around opening windows and so serve to a large extent to reduce the heat loss associated with opening windows to an extent that they may reduce air changes to an uncomfortable level.

There is a fine balance between the need for air change and the expectations of thermal comfort that receives too little consideration in the design of windows.


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