External Components

Michael Mcevoy

• Publisher: Routledge
• Year: 2014
• eBook ISBN: 9781317893981
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Analyses, in conjunction with Internal Components, the performance requirements of building components and the effectiveness of typical solutions. External components integrates logically with the theoretical aspects explored in other titles in the Mitchell's building series. It encourages evaluation of alternative methods for putting components together.

1 Introduction

1.1 The ecology of components

There are considerable repercussions for the global environment in the process of design and specification of building components. Energy is consumed at every stage of their fabrication and throughout their working lives. In the first instance there is the energy cost of obtaining the necessary raw materials, then manufacture, incorporation into a building and finally in terms of the efficiency of their performance in use. The relative ecological importance of these factors can be gauged from the size of the construction industry in Western economies (in Europe, it is the second largest industry after agriculture).

This environmental impact manifests itself in various ways. For example, in the short to medium term, the depletion of the world’s store of oil can be expected to profoundly affect patterns of energy consumption. This will be felt in the way buildings are occupied and constructed and in the pricing of the wide range of oil-based products used in construction. Of particular and immediate concern is the extent to which tropical rain forests are being felled to supply hardwoods to the building sites of the northern hemisphere.

Alternatives to the use of non-renewable tropical hardwoods are described in Friends of the Earth’s The Good Wood Manual. Because materials that are ‘closer to nature’ are more environmentally benign, most of their preferred alternatives are timbers from managed forests in Europe and North America. Some synthetic substitutes, as well as the treatments used to preserve low-durability softwoods, are associated with other problems such as pollution, high energy consumption in manufacture and health hazards. Building construction is still a major user of hardwoods (see chapters 4 and 7). Friends of the Earth advise that unless a source of supply is known to be managed and sustainable, tropical hardwoods should not be specified. The guide discusses alternatives for the detailing of timber components, such as doors and windows, to achieve durability without the use of tropical hardwoods.

Since the 1970s, concern about resource depletion has resulted in more stringent measures to reduce energy consumption being incorporated into the Building Regulations. Attention has recently been focused on environmental pollution and the greenhouse effect. The emission into the atmosphere of carbon dioxide, an inevitable product of the combustion of fossil fuels, is a major contributory factor. Ironically, the foamed plastics insulation materials being used in building to conserve energy, require the use in their manufacture of CFC gases that have a highly deleterious effect on the ozone layer and are also an agent responsible for global warming. Alternative insulants are polystyrene (which can be obtained CFC free), cork or mineral fibre (see section 8.2.3). Manufacturers are now producing foamed plastics that employ blowing agents other than CFCs but some of the alternative gases are also contributors to the greenhouse effect. When deciding which insulation to specify, full technical descriptions should be sought.

The internal environment within buildings is altered by the chemical content of the materials used in their construction. Little is known about the health effects of exposure to the long-term release of gases from organic compounds such as wood-based boards and plastics, although this is a current cause of concern. Some chemicals and fibres used in building materials produce an allergic reaction in some people but only universally dangerous materials such as asbestos, formaldehyde, radon and lead are subject to regulation. Manufacturers are required to display on their products the presence of hazardous materials in order to conform with Health and Safety at Work and Control of Substances Hazardous to Health Regulations.

In response to the need to make construction more environmentally responsible, the Building Research Establishment (BRE) have introduced a method of rating building performance — BRE environmental assessment method (BREEAM). At present this is applicable only to office blocks but versions for other building types are being developed. The method evaluates aspects of design for which there is good evidence of the environmental problems they can cause, and credits are given under three headings:

1. Global issues: global warming (carbon dioxide emission per year), ozone depletion (absence of CFCs in insulation materials and refrigerating systems, halons in fire-fighting equipment), rain forest destruction (timber obtained from sustainable sources) and resource depletion (use of recyclable materials).

2. Neighbourhood issues: Legionnaires’ disease (from cooling towers), local wind effects, reuse of existing sites.

3. Indoor issues: Legionnaires’ disease (from water supplies), lighting, indoor air quality, hazardous materials (absence of lead in paint, asbestos products, urea formaldehyde insulation).

These topics obviously have considerable implications for the design of the external envelope of buildings. The BRE have also designed a domestic energy model (BREDEM) which is the basis of energy classification schemes such as the national home energy rating (NHER), developed by the National Energy Foundation.

Building components and materials not only vary in terms of their energy performance in use but also have different levels of embodied energy (the amount consumed in extracting raw materials, fabricating components and incorporating them into buildings). Several studies have been made of the energy used to produce building materials, demonstrating a ratio of 1 : 23 between the lowest and highest levels of embodied energy. Aluminium, for example, uses particularly high levels of energy in its extraction whilst employing only 10 per cent of the excavated material. On the other hand, to a large extent, the countries producing aluminium do so using hydroelectricity rather than energy derived from fossil fuel. Conversely, although timber’s embodied energy level is much lower than that of metals, an increased use of quick-growing softwood for structural purposes would require its treatment with toxic chemicals. Growing more trees would however, as a result of increased photosynthesis, positively reduce the concentration of atmospheric carbon dioxide. The discussion concerning embodied energy and building components is a complex and contradictory one which is as yet unresolved. A recent reference is The Green Construction Handbook by Ove Arup and Partners, published by JT Design and Build.

A comprehensive standard has been introduced for environmental management systems, BS 7750:1990. This describes how companies can quantify the environmental impact of their activites to arrive at a total environmental performance rating and so reduce waste, pollution and the consumption of raw materials and fuel. The design of products should also minimize the environmental results of their manufacture, use and eventual disposal. An overall environmental strategy is established and independently assessed; the company is then entitled to use the British Standard kitemark (see MBS: Internal Components).

1.2 Durability

A related issue is the assessment of the total lifetime cost of building materials to arrive at a specification suitable for a building’s use and anticipated lifespan. The factors included in this calculation are initial purchase and construction, fuel consumption and running costs, maintenance costs and the cost of eventual removal and disposal.

Life-cycle costing is recommended in BS 7543:1992 Guide to durability of buildings and building elements, products and components to determine at what stage it is no longer economically viable to retain and repair rather than replace components or whole buildings. BS 7543 was necessitated by the expanding range of materials used in building and the increasingly sophisticated methods available to understand the agents that cause deterioration. Durability as defined within the standard as ‘the ability of a building and its parts to perform its required functions over a period of time and under the influence of agents’.

The various types of agent are:

• Weathering agents such as changes in ambient temperature, water, atmospheric gases, air contaminants such as pollution or sea spray, freeze/thaw cycles and the wind.

• Biological agents including micro-organisms, insects and plants.

• Stress agents whether causing sustained stress such as structural creep or intermittent stresses such as earthquakes.

• Che...

Table of contents

• Cover
• Halftitle Page
• Title Page
• Copyright Page
• Table of Contents
• Acknowledgements
• Preface
• 1 Introduction
• 2 Prefabricated building components
• 3 External glazing
• 4 Windows
• 5 Rooflights and patent glazing
• 6 Structural glazing
• 7 External doors
• 8 Roofings
• SI units
• CI/SfB
• Index