PAS 2035 and ventilation in retrofit

What is PAS 2035?

PAS 2035 is the publicly available specification that sets the framework for whole-house domestic retrofit in the UK. It governs how energy efficiency measures such as insulation are assessed, designed and installed, including the ventilation that must go with them. It is mandatory for government-funded retrofit schemes. The current edition, PAS 2035:2023, replaced the 2019 version on 30 March 2025.

PAS 2035 sits alongside PAS 2030, which covers the installation work itself. Together they set out a whole-house approach, so measures are considered as a system rather than in isolation. A retrofit coordinator oversees the process, from assessment through design to sign-off, and ventilation is one of the things that must be considered throughout.

Sources: Retrofit Academy

What does PAS 2035 require for ventilation?

PAS 2035 requires a ventilation assessment as part of the retrofit, inspecting and reviewing the existing ventilation. Its ventilation requirements are aligned with Approved Document F. Functional ventilation testing, checking that systems actually work rather than assuming they do, is a core requirement. Where mould or condensation is present, an upgraded system such as MEV or MVHR must be specified.

The 2023 edition simplified and tightened the ventilation rules and brought them into line with Part F. The key change in practice is the emphasis on testing: it is no longer enough to install ventilation, it has to be shown to perform. That protects the homeowner from the classic retrofit failure of a sealed-up house that becomes damp.

Sources: Retrofit Academy

Why does a retrofit need a ventilation assessment?

Adding insulation and airtightness to an existing home reduces the uncontrolled airflow it used to rely on. Without a matching look at ventilation, that can trap moisture and cause condensation and mould. PAS 2035 makes the ventilation assessment part of the process precisely so that improving efficiency does not damage air quality or the building fabric.

Older homes ventilate partly by accident, through gaps, chimneys and draughts. A retrofit removes much of that on purpose, to save energy. The assessment makes sure the lost airflow is replaced with controlled ventilation, so the home stays healthy. It is the retrofit version of build tight, ventilate right.

Who does PAS 2035 apply to?

PAS 2035 applies to whole-house retrofit projects, and compliance is mandatory for government-funded energy efficiency schemes such as ECO and the Warm Homes programmes. A retrofit coordinator oversees the process. For a privately funded retrofit outside a funded scheme it is best practice rather than a strict legal requirement, but the ventilation logic applies either way.

If you are having insulation or other measures installed under a grant scheme, the work will be done to PAS 2035, and a retrofit coordinator will be involved whether you see them or not. If you are funding a deep retrofit yourself, following the same approach is the safest way to avoid creating a damp problem.

How does PAS 2035 relate to Part F?

PAS 2035 aligns its ventilation requirements with Approved Document F, the Building Regulations standard for ventilation. In effect a PAS 2035 retrofit has to deliver ventilation to the Part F standard for the work done, and to test that it performs. So the two work together: Part F sets the rates, and PAS 2035 makes sure a retrofit assesses and achieves them.

This alignment matters because it removes any gap between the two frameworks. A retrofit coordinator working to PAS 2035 is effectively making sure the home meets Part F ventilation levels after the work. See our Approved Document F guide for the underlying rates and requirements.

Sources: GOV.UK