Ventilation is the element of PAS2035 most frequently identified as inadequate at audit — and the one most likely to cause genuine harm if it is overlooked. Making a home more airtight without addressing ventilation does not just create a compliance problem. It creates conditions that lead to condensation, mould growth and poor indoor air quality, with real consequences for the health of residents.

Understanding why ventilation is consistently missed — and what adequate ventilation assessment and design actually looks like — is essential for every Retrofit Coordinator.

Why Ventilation Matters in Retrofit

Older UK housing stock was built with significant air leakage through gaps in the fabric — around window frames, under doors, through loft hatches and around service penetrations. This uncontrolled infiltration was inefficient for heating purposes, but it did provide a degree of background ventilation that reduced moisture buildup and diluted indoor pollutants.

Retrofit measures — particularly loft insulation, solid wall insulation, draught proofing and window replacement — reduce this uncontrolled air leakage. In a property where no compensating ventilation strategy is put in place, this creates a tighter building envelope without adequate ventilation. Moisture from cooking, bathing and breathing has nowhere to go. Condensation forms on cold surfaces. Mould grows.

This is not a hypothetical risk. It is well documented in both UK and European research, and it is the reason PAS2035 places such strong emphasis on ventilation assessment as a precondition for fabric improvement measures.

The PAS2035 Requirement

PAS2035 requires that any measures that affect the airtightness of the property must be accompanied by a ventilation assessment. This assessment must determine whether the existing ventilation provision is adequate given the proposed improvements, and must specify what ventilation measures are required to maintain acceptable indoor air quality and moisture levels.

The ventilation assessment is a required output at the design stage — it must be produced before installation begins, not added as an afterthought once measures are in place. Where the assessment identifies a ventilation deficiency, the design must include appropriate ventilation measures as part of the overall package.

What Adequate Ventilation Assessment Looks Like

A compliant ventilation assessment goes beyond simply ticking boxes on a checklist. It requires the assessor and designer to understand the existing ventilation provision in the property — including background ventilators (trickle vents), mechanical extract fans, passive vents, open fireplaces and any existing MVHR or MEV systems — and to assess whether these will remain adequate following the proposed fabric improvements.

The RdSAP 10 update, which came into effect in June 2025, significantly expanded the ventilation data that must be captured during the pre-retrofit assessment. Assessors must now count and classify all ventilation openings — trickle vents in windows, extract fans, passive stack vents, blocked chimneys and flues. This data forms the basis of the ventilation assessment and cannot be adequately modelled without it.

Why It Is Commonly Missed

Several factors contribute to ventilation being inadequately addressed in retrofit projects:

• Assessment data gaps: Until RdSAP 10, ventilation data was poorly captured in standard assessments. Projects using older assessment data may have insufficient information to complete a proper ventilation assessment
• Skills gap: Ventilation assessment requires specific knowledge that not all Retrofit Coordinators and designers have developed — it is not covered in depth in all RC training pathways
• Pressure to reduce costs: Ventilation measures add cost to a project. In programmes where there is pressure to maximise the number of properties treated within a fixed budget, ventilation requirements can be minimised or overlooked
• Complexity in practice: Older properties with mixed construction and multiple ventilation openings can be genuinely complex to assess. This complexity is sometimes treated as a reason to simplify the assessment rather than engage with it thoroughly

The Consequences of Getting It Wrong

A project that installs fabric measures without an adequate ventilation strategy creates conditions that are worse than the pre-retrofit state in terms of moisture and air quality — even if the energy performance has improved. Residents experience condensation and mould. Health impacts follow. The housing association faces complaints, potential Awaab's Law obligations and remediation costs.

At TrustMark lodgement, a missing or inadequate ventilation assessment will prevent a compliant lodgement. This creates a delay in funding claim and a requirement for remediation before the lodgement can proceed — often including additional surveys and potentially the installation of ventilation measures that should have been included from the outset.

Getting Ventilation Right

The starting point is thorough data collection at assessment stage — using RdSAP 10's expanded ventilation data requirements as the minimum baseline. The Retrofit Coordinator should review ventilation data from the assessment before the design stage begins and identify any properties where the data appears insufficient.

The ventilation strategy should be designed alongside the fabric measures, not added afterwards. Where mechanical ventilation is required — such as MVHR or MEV — this should be treated as a primary component of the measure package, not an optional extra. The cost of ventilation measures is substantially lower than the cost of remediation following mould damage, and significantly lower than the cost of a failed TrustMark lodgement.