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Part F: What it means in practice

The significant revisions to Part F are designed to align it with those changes made to Part L. The aim of the joint revisions is to ensure a balance of both airtightness and ventilation in new homes. This means there are now minimum energy efficiency levels for all ventilation systems. But what are the implications of these regulations?

A key issue is that the use of trickle ventilation looks set to become more difficult, with additional guidance published for dwellings with air permeability tighter than 5.0 m3/(h.m2). What this means is that approximately 50 per cent more background ventilation will be required for dwellings with intermittent or passive stack ventilation systems.


This may have the effect that designers and builders will be motivated to move towards using continuous ventilation systems, which perform better in SAP 2009 and are also simpler to standardise. Continuous ventilation systems also have the potentially significant advantage of not requiring trickle vents.


The changes to Part F mean that the breadth and level of testing will increase due to the mandatory requirement for fixed mechanical ventilation systems to undergo testing and commissioning where possible. Furthermore, air flow should be measured for both intermittent and continuous mechanical ventilation installations - this includes cooker hoods for kitchens and extractor fans for bathrooms. And that’s not all, as this data must then be sent to the building control body within five days of completion. Under the changes, it will also be mandatory to submit sufficient information to the building owner in relation to the systems installed in a new dwelling, as this will enable the owner to be better informed of correct usage.


While the most significant implications of Part F are those to dwellings with air permeability of less than 5.0 m3/(h.m2), in practice the additional ventilation measures will actually apply to buildings with an air permeability of less than 3.0 m3/(h.m2). This is because the corresponding changes to Part L mean a confidence factor’ of 2.0 is added to untested units. The alternative would be to test every dwelling - an expensive and impractical option. By choosing a continuous mechanical extract system, the changes to Part F eliminate the need for background ventilation in dwellings designed with air permeability rates above 5.0 m3/(h.m2.


There are also benefits in SAP 2009 for continuous systems, which may lead designers to adopt whole house mechanical extract ventilation systems, possibly including heat recovery as well.

Ultimately, the changes to Part F, in tandem with those to Part L, will help create homes that are more airtight, but no less ventilated than those of today. For
consumers, that’s a good thing.


For the housebuilding industry it means increased testing and increased data-sharing - not a bad thing, but something that may require additional time and extra resources. And that is something that can begin today.