PAS 2035: Introduction to ventilation requirements
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
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
there is no provision for purge ventilation of each
habitable room (e.g. by opening windows) as required by Approved Document F
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
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
Must have a 30 l/s fan.
Must have a 15l/s fan.
Must have a 6 l/s fan.
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 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
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
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