What is BS 5925:1991 Code of practice for ventilation principles and designing for natural ventilation?

BS 5925:1991: Code of practice for ventilation principles and designing for natural ventilation is an older, foundational British Standard that outlines the principles and design considerations for achieving natural ventilation in buildings. It focuses on non-mechanical systems, relying primarily on natural forces such as wind pressure (wind-driven ventilation) and temperature differences (buoyancy or stack effect) to move air through a dwelling.

While the minimum performance requirements for ventilation in the UK are now primarily mandated by the Building Regulations 2010, specifically through Approved Document F (Ventilation), the core principles detailed in BS 5925 remain highly relevant in designing and assessing naturally ventilated residential properties, particularly in:

  • Residential Retrofit: When upgrading existing homes, designers often rely on controlled natural ventilation measures (like trickle vents and improved window operability) which are informed by the airflow principles in this standard.

  • New Extensions: In extensions or renovations, the standard’s guidance on using internal and external pressure differences to achieve cross-ventilation is key to ensuring adequate air quality without mechanical intervention.

Practical Examples

  1. Airflow and Pressure: The standard informs the optimal placement and sizing of air inlets (e.g., trickle vents in window frames) and air outlets (e.g., kitchen/bathroom extract fans acting as an outlet, or even a chimney flue) to harness wind pressure differences across a home. For instance, designing an extension with openable windows on opposite walls to ensure cross-ventilation, as promoted by the standard’s principles.

  2. Condensation Risk: BS 5925 includes methods and calculations for determining minimum ventilation rates necessary to mitigate the risk of surface condensation caused by high internal moisture levels. A retrofit project might reference these principles when calculating the necessary background ventilation rate (e.g., Equivalent Area of trickle vents) to prevent mould growth in a newly insulated room.

Supersession and Context:

It’s important to note that the quantitative minimum ventilation performance for new and existing dwellings in England and Wales is now legally governed by Approved Document F (ADF). The current version of ADF provides specific, prescriptive requirements and Approved Methods that supersede the numerical airflow rates that were recommended in the 1991 standard. However, the principles behind natural ventilation design still draw heavily from this foundational document.