Design and Detailing for Airtightness

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1.1 Aims of this Guide
1.2 Target audience
1.3 How to use this Guide

As thermal insulation levels have risen in the last few years the proportion of energy lost to draughts has increased to the extent that now in some cases around half of all heat losses are due to air leakage across the building fabric [1]. Given that approximately half of all energy used in the UK is in buildings [2], it is not hard to see that draughts account for a staggering amount of energy - and therefore cost - wastage.

The situation is such that further increasing thermal insulation levels would be largely unproductive unless airtightness is conscientiously addressed. Air leakage has been shown [3] to reduce the effectiveness of thermal insulation by up to 70% and so it is clear that if energy efficiency is to be improved in buildings, the next efforts will have to focus on airtightness.

Many people make the mistake of thinking that an airtight building is necessarily a ‘stuffy’ building. This is not the case. All buildings have to be ventilated for health and comfort and airtight buildings are no different. An adequate ventilation system (which may well include openable windows as well as fans etc.) has to be planned for every building. The difference will be that a great deal of unplanned air leakage needs to be stemmed (see right).

As described in Chapter 6, the additional costs of creating an adequately airtight building can be negligible, but even where costs are increased, these can be more than offset by a reduction in the capital cost of heating and ventilation equipment, not to mention the long term savings in energy.

Given that the vast majority of building stock is existing, a great deal of attention will need to be given, in the foreseeable future, to remedial works to existing buildings. This guide specifically includes examples of good and best practice remedial work in terms of airtightness and shows that such works can offer substantial benefits without undue disruption or cost.

The Scottish Ecological Design Association (SEDA) has commissioned this Guide to help address the above problems and provide practical guidance on how to save energy and costs and protect building fabric. On the basis that prevention is cheaper and easier than cure, one purpose of this guide is to enable Designers to design inherently more robust and durable solutions which avoid costly and time consuming remedial works on site.

The general guidance here is firmly focused on the idea of practical design and detailing, and should be read in conjunction with other guidance on sustainable design, energy efficiency and airtightness where necessary to provide an overall design framework.The details provided have been fully costed, tested and subjected to a Defects Liability insurance assessment. They are offered as viable alternatives to standard details, and illustrate the possibilities that exist. It simply remains for you, the reader, to apply them appropriately in the context of your next project…

1.1 Aims of this Guide

• To highlight benefits of airtightness which include both energy and cost efficiency, improved comfort and reduced risks of damage to building fabric
  
• To improve awareness of the need for airtightness in construction
  
• To promote detailing and specification solutions which create airtight and efficient buildings thus reducing the need for remedial works - ‘prevention rather than cure’
  
• To show that new build and remedial airtightness are achievable without undue cost penalties to construction works
  
• ... and in this way to help to ‘mainstream’ the good and best practice outlined in the document.

1.2 Target audience

This Guide will help all those who wish to improve the airtightness and energy efficiency of buildings through their construction, e.g:

• clients –building owners and users,
  
• principal and specialist contractors,
  
• interior designers
  
• architects and technicians
  
• structural engineers
  
• building service engineers
  
• building surveyors
  
• quantity surveyors/ cost consultants
  
• maintenance and facilities managers
  
• project managers
  
• planning officers and building control officers
  
• funding bodies and their professional advisors
  
• government and non-governmental agencies,

1.3 How to use this Guide

This Guide is divided into six sections. The first two sections provide an overview of the issues surrounding airtightness. Sections Three, Four and Five describe the requirements for the design process, the procurement and the testing involved in designing for airtight buildings.

Section Six provides a number of representative details which have been optimised in terms of airtightness. These are compared with standard details for a variety of construction types, and costed. This section will be primarily of interest to the design team. It should be read in conjunction with sections Three, Four and Five in particular, as all details must be placed in a suitable context.

At the end of this Guide there is an annotated list for further reading, as well as a list of useful contacts and websites.

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Footnotes:
1. BRE, Airtighness in Commercial and Public Buildings 2002, p.3

2. See, for example p2 of the final report of the Sustainable Buildings Task Group, available at http://www.dti.gov.uk/sustainability/

3. For a detailed analysis of the problem, generally termed ‘convective bypass and blow-through’ refer Lowe R, Impacts of construction defects on heat loss and CO2 emissions from dwellings

4. EU Energie ‘SHINE” projects. UK involvement by Hyde Housing Association (0208 297 7500, Contact Sally Buckley), Ecological Development (0207 837 6308) Dyke Coomes Associates (0207 702 7558)

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