Not every retrofit project carries the same level of risk, and PAS 2035 doesn’t assess them all the same way. Installing loft insulation in a house built after 2012 and retrofitting a high-rise block within a conservation area need very different levels of scrutiny – which is exactly what the standard’s three Risk Pathways exist to reflect. Get the pathway wrong and you either under-assess a genuinely complex building or over-engineer a straightforward one. Here’s what separates Pathway A, B and C, and which one brings a mandatory air tightness test with it.
March 2025 update: PAS 2035:2023 became the mandatory standard, replacing the Risk Pathway A/B/C grading described below with continuous Retrofit Coordinator oversight across every project stage. See our full breakdown of what changed under PAS 2035:2023.
In this blog, you’ll learn:
- The three PAS 2035 Risk Pathways, and what separates them
- Which pathway requires a mandatory air tightness test
- What additional diagnostic testing high-risk pathways typically need
- How we help coordinators find genuinely qualified Pathway C assessors
What are the PAS 2035 Risk Pathways?
Under PAS 2035:2019 risk pathways were determined by a number of factors, including a building’s age, size and the complexity of the work planned. The Retrofit Coordinator determined the risk pathway at the outset. The general principle: the more complex the project – multiple units, complicated installations – the more in-depth the onsite assessment needed to be.
PAS 2035 Risk Pathway A
The lowest-risk pathway, covering simple measures for standard properties – installing loft insulation in a property built after 2012 was a typical example. A standard whole-dwelling retrofit assessment could be carried out for Pathway A projects by a competent Retrofit Assessor.
PAS 2035 Risk Pathway B
Required for high-risk, multiple, or complex measures installed across several properties. The Retrofit Coordinator needed more experienced Retrofit Assessors for Pathway B, since additional information was required to complete the final assessment: fuel use metrics had to be included in each SAP calculation, and a thorough ventilation assessment was needed to identify danger areas – generally where condensation is likely to collect and black mould could develop.
PAS 2035 Risk Pathway C
The highest-risk pathway, for large-scale projects with multiple high-risk measures planned. Risk Pathway C covered all high-rise buildings, complex structures, and any home within a conservation area – a high-rise block of apartments was a typical example.
Were Air Tests Required for Pathway C Projects?
Yes, under PAS 2035:2019. Assessment requirements at this stage went beyond Pathway B’s advanced SAP calculations and ventilation tests – an air tightness test was also mandatory. Genuinely capable Pathway C assessors were rare: it was uncommon to find an individual or company that could offer comprehensive retrofit assessments to Pathway C standard, so additional time and resource was often needed at the assessment stage.
When Is Air Testing Mandatory Under PAS 2035:2023?
The trigger has moved from the old Pathway C category to the presence of fabric measures. Under Section 8.2.35, a retrofit designer or coordinator must develop an airtightness strategy for any domestic retrofit project involving building fabric measures – insulation or replacement windows, for example. If that strategy sets an explicit airtightness target, an actual air permeability test becomes mandatory to prove compliance. In practice, that means the question is no longer “is this a high-risk, complex building” – it’s “does this project touch the building fabric, and has a target been set for it.”
What Additional Factors Apply to High-Risk Pathways?
More complex projects in atypical buildings will often need a thermal imaging survey. A borescope cavity inspection may also be necessary to gather enough detail to properly inform the retrofit design process.
Commissioning a Retrofit Assessor who can undertake all of this in a single visit saves real cost and time on complex projects – but finding that combination of skills is difficult. There’s a nationwide shortage of assessors who combine working PAS 2035 knowledge with genuine experience assessing high-risk pathway projects.
What Sets Our Retrofit Assessors Apart
Finding a Retrofit Assessor who’s also a qualified tester can be genuinely difficult – there’s a nationwide shortage of assessors who combine working PAS 2035 knowledge with hands-on experience in air tightness testing, Part F ventilation testing, thermography and borescope inspection. We’ve built our team specifically to close that gap: every assessor is PAS 2035-qualified, and many are cross-trained across this additional testing, so a single site visit can cover the assessment and the testing Pathway C requires, rather than needing two separate specialists. That combination has grown into ongoing partnerships with organisations like the Centre for Sustainable Energy, who commission us specifically for Risk Pathway C testing.
Delivering PAS 2035 Risk Pathway C air tightness testing, carried out by engineers trained for sensitive, in-home visits rather than empty construction sites:
What’s Next?
Getting the risk profile right at the outset shapes everything that follows – the assessment scope, the testing required, and ultimately whether the retrofit design has the evidence behind it to be signed off with confidence. Whether you’re coordinating a straightforward single-dwelling project or a portfolio of high-risk, complex buildings, that’s the foundation we help put in place.
We carry out PAS 2035 retrofit assessments and the additional testing high-risk, complex projects require, for portfolios and programmes nationwide.
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