PAS 2035: Introduction to ventilation requirements

Introduction to ventilation requirements in PAS 2035

PAS2035 and PAS2030;2019 have introduced mandatory ventilation assessments and upgrades as part of the new standards that apply to all publicly funded energy efficiency retrofit measure.

Ventilation requirements have the potential to cause the most failures if they are not fully understood.

Retrofit Assessors and Retrofit Coordinators have key roles in creating a ventilation strategy for a property.

The ventilation standards

PAS2035 Annex C.2.1 - For every dwelling proposed for improvement, the Retrofit Designer shall use the information provided by the Retrofit Assessor to assess the adequacy of the existing ventilation according to the following criteria.

C.2.2 - Existing ventilation shall be assessed as inadequate for the improved dwelling if one or more of the following are apparent:

  • there is evidence of condensation and/or mould growth in the dwelling;
  • there is no ventilation system, or the ventilation system is incomplete or not functional;
  • there are not undercuts of at least 7 600 mm2 beneath all internal doors, above the floor finish, to allow air to move through the dwelling;
  • there is no provision for purge ventilation of each habitable room (e.g. by opening windows) as required by Approved Document F [N11].

How to avoid ventilation failures

It is the correct combination of IEV and background ventilators that make a complete ventilation system.

To avoid failures, the following clause must be fulfilled:

PAS2035 Annex C.2.3 - An acceptable, complete ventilation system shall be:

An Intermittent Extract Ventilation (IEV) system consisting of correctly sized extract fans in all wet rooms and correctly sized background ventilators (to admit “fresh” external air) in all living spaces and bedrooms.

What are IEVs?

IEVs have to have a certain output to match the room type they are in, and are rated in an extraction rate of litres per second (l/s).

Intermittent Extract Ventilation systems are extractor fans that go in kitchens, bathrooms, utility rooms and toilets to expel the wet air from the room.


Must have extraction fans that extract 30l/s for cooker hoods and 60l/s for all other fans. All kitchen cooker hood fans must exhaust externally.

Utility rooms

Must have a 30 l/s fan.

Bathroom/ Shower rooms

Must have a 15l/s fan.


Must have a 6 l/s fan.

What is background ventilation?

Background ventilators are vents that can be opened and closed to let fresh air in from outside the property.

Trickle vents in new windows are the most common type of background ventilators.

Background ventilators can also be installed as wall vents that go through the walls of the property.

Working out the level of ventilation required

The level of background ventilation required can be calculated on a whole property or room by room basis.

The ventilation requirements are recorded by the retrofit assessment software by the Retrofit Assessor during the assessment, but installers need to know these ventilation rates when planning ventilation upgrades and preparing a ventilation strategy.

Each Habitable room and utility room requires 6,500mm2 of background ventilation - this number relates to the air that flows through the size of the vent openings.

  • A window trickle vent would provide 4,000mm2
  • A 6x9 wall vent would provide 10,200 mm2

It is worth noting that ventilation should be discussed with customers up front as it can cause controversy with not all householders embracing the requirements.

Click here to view an example of a ventilation strategy.

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