National foreword

This Published Document is the UK implementation of CEN/TS 18351:2026.

The UK participation in its preparation was entrusted to Technical Committee RHE/2, Ventilation for buildings, heating and hot water services.

A list of organizations represented on this committee can be obtained on request to its committee manager.

Contractual and legal considerations

This publication has been prepared in good faith, however no representation, warranty, assurance or undertaking (express or implied) is or will be made, and no responsibility or liability is or will be accepted by BSI in relation to the adequacy, accuracy, completeness or reasonableness of this publication. All and any such responsibility and liability is expressly disclaimed to the full extent permitted by the law.

This publication is provided as is, and is to be used at the recipient’s own risk.

The recipient is advised to consider seeking professional guidance with respect to its use of this publication.

This publication is not intended to constitute a contract. Users are responsible for its correct application.

This publication is not to be regarded as a British Standard.

Compliance with a Published Document cannot confer immunity from legal obligations.

This Published Document was published under the authority of the Standards Policy and Strategy Committee on 30 November 2026.

This Technical Specification (CEN/TS) was approved by CEN on 31 August 2026 for provisional application.

The period of validity of this CEN/TS is limited initially to three years. After two years the members of CEN will be requested to submit their comments, particularly on the question whether the CEN/TS can be converted into a European Standard.

CEN members are required to announce the existence of this CEN/TS in the same way as for an EN and to make the CEN/TS available promptly at national level in an appropriate form. It is permissible to keep conflicting national standards in force (in parallel to the CEN/TS) until the final decision about the possible conversion of the CEN/TS into an EN is reached.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czechia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and United Kingdom.

European foreword

This document (CEN/TS 18351:2026) has been prepared by Technical Committee CEN/TC 156 “Ventilation for buildings”, the secretariat of which is held by BSI.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN shall not be held responsible for identifying any or all such patent rights.

Any feedback and questions on this document should be directed to the users’ national standards body. A complete listing of these bodies can be found on the CEN website.

According to the CEN/CENELEC Internal Regulations, the national standards organisations of the following countries are bound to announce this Technical Specification: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the United Kingdom.

Introduction

This is the first edition of the document, making a significant step forward in addressing a topic that, until now, has not been properly standardized in any CEN publications. Natural ventilation and hybrid ventilation have been discussed in various research and design contexts, but there has been a lack of unified guidance specific to their practical application in building design. This document fills that gap by providing a dedicated and structured framework for the design of natural ventilation systems and hybrid ventilation systems. It reflects the growing recognition of the need for clear, actionable guidance in this emerging area of ventilation design.

This document provides an introduction and a comprehensive guidance on the ventilation principles and the design aspects that may be considered in the design of natural ventilation systems and hybrid ventilation systems in non-residential buildings to achieve acceptable indoor air quality (IAQ).

Natural ventilation systems can either be designed as a stand-alone system or integrated with a mechanical ventilation system (or vice versa) offering the strengths of the two systems resulting in a hybrid ventilation system.

Traditionally, ventilation needs have been met by natural ventilation in which the flow process is driven by wind and differences in temperature. Making natural ventilation the oldest form of ventilation. Natural ventilation has developed from being largely uncontrolled systems to a demand-controlled ventilation system with cooling and ventilation capabilities. Hybrid ventilation was introduced much later at the end of the 20th century and was described as systems using both natural ventilation and mechanical ventilation. This shift also calls for a corresponding transition for those involved in the ventilation system design. Hence, this document is to support the design of these ventilation systems.

The primary role of ventilation is to achieve acceptable IAQ for the occupants by replacing the indoor air, but can also be applied to achieve acceptable thermal comfort. This document focusing on IAQ, only. Besides ventilation, the IAQ can be controlled by source control, filtration and air cleaning. All important aspects and considerations when designing a ventilation system. The design of ventilation systems commonly considers other aspects like energy, acoustics, maintenance, and durability. These aspects are only partly covered in this document. The requirements to fulfil the acceptable IAQ can either be set formulated as performance-based or in a prescriptive manner, both methods are covered in this document.

IAQ is a fundamental component and plays an important role achieving overall indoor environmental quality (IEQ), which encompasses all aspects of the indoor environment that affect occupant health, comfort and well-being. IEQ includes not only air quality, but also thermal comfort, lighting, acoustics, and other environmental conditions.

Resilient ventilation system design is also an important part to ensure future robustness of the ventilation solution and how this can cope with various external impacts like future weather conditions.

This document has a link to the following CEN publications:

EN 16798‑1 which includes possible levels of indoor air quality requirements.
EN 16798‑3 applies to the design, energy performance of buildings and implementation of mechanical ventilation, air conditioning and room conditioning systems for non-residential buildings and can be seen as complementary to this document.
EN 16798‑7 which gives information on calculation of air flow rates in buildings.
CEN/TS 18335 applies to the design of ventilative cooling systems to achieve acceptable thermal comfort.
FprEN 15665 “Ventilation for buildings - Ventilation systems in residential buildings - Design” applies to the design of ventilation systems for residential buildings to achieve acceptable indoor air quality.

1 Scope

This document gives guidance on the design of natural ventilation systems and hybrid ventilation systems in non-residential buildings to achieve acceptable indoor air quality. It is applicable to spaces intended for human occupancy in non-residential buildings.

The guidance for design in this document is applicable to natural ventilation systems and hybrid ventilation systems, encompassing the natural ventilation part of the hybrid ventilation systems, only. The type of control and the design process of hybrid ventilation systems is covered by this document.

This document does not set the requirements for the indoor air quality, but utilizes these as input parameters when designing natural ventilation systems and hybrid ventilation systems.

This document does not apply to:

thermal comfort and overheating;
health risk from exposure to tobacco smoke;
air cleaning;
design of mechanical ventilation systems.

2 Normative references

3 Terms and definitions

4 Abbreviated terms

5 Design process and principles of natural ventilation systems and hybrid ventilation systems

5.1 General

5.2 Primary purpose of ventilation

5.3 Natural ventilation principles

5.4 Hybrid ventilation principles

6 Design requirements

6.1 General

6.2 Primary ventilation requirements

6.3 Additional design aspects

7 Natural ventilation system design procedure

7.1 General

7.2 Conceptual design approach

7.3 Basic and detailed design approach

8 Hybrid ventilation system design procedure

8.1 General

8.2 Basic and supporting system

8.3 Conceptual design approach

8.4 Basic and detailed design approach

Bibliography