INTRODUCTION

Airtightness is all about minimising air infiltration in buildings. Air infiltration is the uncontrolled flow of air through gaps and cracks in the fabric of buildings. It is driven by pressure and temperature differences between the inside and outside of the building and is highly variable in response to changes in the weather. Infiltration is not a reliable substitute for properly designed ventilation of the interior. It may significantly increase heat losses to the outside and can depress comfort levels by allowing unwanted draughts and cold spots.

For commercial and public buildings, good design should separate the mechanisms that provide a good supply of fresh air to occupants from the adverse and unpredictable effects of air infiltration. This demands good ventilation design, coupled with a clear, workable specification for an effective and maintainable airtightness layer. In short: build tight, ventilate right. For many years BRE has been promoting this approach to considering ventilation as well as airtightness. Now with the current trend towards making buildings even more airtight than in the past, effective ventilation becomes essential.

Buildings where thermal comfort is not a primary design factor, eg those used for storage where close control of temperature and humidity and the exclusion of pollutants are necessary, will also benefit from careful airtightness design in the finished building.

This guide is aimed at those involved in the design, specification, building and commissioning of new or refurbished buildings; it is in three main sections:

1: Background principles: an introduction to the mechanisms causing air infiltration to occur and the benefits which result from increasing the airtightness of a building.

2: Design principles: designing and specifying for airtightness in buildings.

3: Achieving airtightness: undertaking airtight construction on site and testing that it meets the required standard specified.

The design of an airtight building depends on an understanding of the mechanisms which cause infiltration and identifying where they are likely to occur, either in the external building envelope or between conditioned (heated or cooled spaces) and non-conditioned spaces.