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PAS 2035 Moisture Risk: What It Means for Your Home
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PAS 2035 Moisture Risk: What It Means for Your Home

13 min read

PAS 2035 Moisture Risk: What It Means for Your Home

Solid masonry wall beside internal insulation

PAS 2035 moisture risk is the chance that retrofit work, such as insulation or airtightness upgrades, changes how heat, moisture and air move through your home and leads to condensation, mould or timber decay. The standard expects existing damp, leaks and ventilation problems to be found and dealt with before measures go in, and expects the design to manage moisture across the whole dwelling. If you are planning insulation, or have already had it fitted and now see mould, a survey that documents what is happening is the sensible first step.

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TL;DR

  • PAS 2035:2023 defines moisture risk as a whole-dwelling issue: heat, moisture and air flows must stay in balance so the building fabric and occupants are not harmed.
  • Insulation and airtightness can raise the risk of condensation, mould and rot if existing damp, leaks or poor ventilation are not resolved first.
  • Before retrofit, the assessment should record defects, leaks, rising or penetrating damp, condensation, mould and the state of ventilation and timbers.
  • Wrong diagnosis leads to wrong remediation, so an evidence-based survey comes before any treatment.
  • If mould or damp has appeared after insulation, a documented inspection shows the cause and the scope of work.

Table of Contents

What PAS 2035 means by moisture risk

PAS 2035 is the British Standards Institution specification for retrofitting dwellings for improved energy efficiency. In the 2023 edition, moisture risk is described as the risk to comfort, health and building fabric that arises from how heat, moisture and air flows interact and affect dynamic moisture equilibrium. It is tied specifically to internal surface condensation and mould growth, and to moisture accumulating in the structure and causing rot and decay.

Two points matter in practice.

  1. It is assessed for the whole dwelling. A new layer of insulation on one wall changes conditions in the rooms, the roof space and the floor, so the standard does not treat each measure in isolation.
  2. The aim is balance, not dryness at any cost. PAS 2035 links occupant health and safety to controlling flows of heat, moisture and air so that healthy internal conditions are kept and the fabric does not decay.

This is why a house that was sound before an upgrade can start to show mould afterwards. The building has not become faulty by itself. The conditions it was coping with have changed.

Which edition applies

PAS 2035:2023 supersedes PAS 2035:2019+A1:2022, which stayed current until it was withdrawn on 30 March 2025. BSI Knowledge lists the 2023 edition as current. The wording on moisture in buildings was brought into line with BS 5250, the code of practice for managing moisture in buildings.

TrustMark has published transition arrangements between the 2019 and 2023 editions. One change is that the old risk pathways have been removed. If a retrofit assessment was written under the older edition, ask the installer or retrofit coordinator which edition it follows.

How insulation and airtightness raise moisture risk

The mechanism is simple. Warm, moist indoor air condenses when it meets a cold surface. Adding insulation to the inside of a wall makes the original masonry colder in winter, and sealing gaps reduces the natural air change that carried moisture out.

The research behind this is consistent:

  • The government-funded DEEP study found that warm, moist air condenses on cold surfaces and that damp and mould are a risk where this persists, especially with high relative humidity, under-heating and under-ventilation.
  • In the same study’s case studies, internal wall insulation increased moisture risk compared with uninsulated cases, and the risk rose even with a thin layer, because the wall becomes colder and relative humidity at critical points goes up.
  • UCL research notes that airtightness without adequate ventilation can lead to long periods of high humidity, and surfaces become more prone to moisture problems even without visible condensation.
  • The same UCL work states that floor, roof or internal wall insulation can increase the risk of wood rot by reducing the ability of embedded timber, such as joist ends, to dry out when it is detailed or installed inappropriately.
  • Historic England describes retrofit going wrong where additional insulation produced an indoor environment that could not regulate humidity, leaving the building reliant on trickle vents.

If you have noticed mould appearing in the weeks after new insulation, our guide to checks in the first 24 to 48 hours after insulation explains what to record. Mould along corners and reveals is often a thermal bridging problem that a survey can pin down.

What the PAS 2035 assessment should look at

The standard asks for the dwelling to be appraised before any measure is chosen. According to the 2023 specification, the assessment covers:

  • the dwelling’s heritage, structure, construction and condition;
  • installed services, including ventilation, heating, hot water and lighting;
  • existing construction or structural defects, leaks, and any condensation or mould growth;
  • existing ventilation, including recording fans and background ventilators and reporting air-flow testing against minimum requirements;
  • the construction in enough detail to establish main materials, thermal transmittances and moisture properties, which may need test equipment or invasive assessment.

The design stage then has to confirm the condition of the existing building. That includes leaks and rainwater ingress, faulty rainwater goods, rising or penetrating damp, damp, mould and condensation, and limited ventilation in a loft or underfloor void.

TrustMark’s design guidance adds that the design should identify areas that would affect the installation or be made worse by it. It names areas of dampness from leaks, penetrating or rising damp, and damp or rotting timbers.

Interstitial condensation and solid walls

Condensation can also form inside a wall build-up. PAS 2035:2023 includes assessing interstitial condensation, but notes that for solid walls with internal wall insulation in exposed locations, a calculation to BS EN ISO 13788 might not be enough as the basis of the moisture risk assessment. For a homeowner, this means a solid-wall house with a history of damp deserves a closer, evidence-based look than a desktop calculation.

Treat existing damp before you insulate

Insulation does not cure damp. It can hide it, or make it worse. The government’s internal wall insulation guide warns that insulating around joist ends that already have high moisture levels can accelerate decay or cover up an existing problem such as weather ingress or a leak.

Floor joist end beside internal wall insulation

That guide gives a practical timber rule: where joists in external walls have a moisture content above 20%, internal wall insulation should not be installed around them. The source of moisture should be found and removed, and the timber allowed to dry first.

The sequence is therefore:

  1. Find the moisture source. This might be a failed gutter, defective pointing, a leaking pipe, bridged damp-proof course, or condensation from poor ventilation.
  2. Repair the source. Rainwater goods, masonry and plumbing come before any internal finish.
  3. Allow drying and re-check. Take moisture readings on timbers and walls, not just a visual impression.
  4. Treat timber where needed. Decay found at joist ends, skirtings or floor timbers is dealt with before it is covered.
  5. Then specify the insulation and ventilation. Choose measures that suit the wall, with the ventilation strategy decided at the same time.

The RICS journal makes a related point. Professor Mike Parrett FRICS says dampness investigations should be holistic and evidence-based, and that a wrong diagnosis leads to wrong remediation. He also warns that inappropriate or unnecessary damp-proofing can fail to resolve the moisture source and may worsen moisture retention in vulnerable fabric.

Managing moisture in the design

Once the existing condition is known, PAS 2035:2023 requires moisture to be managed in the design in line with BS 5250. That means evaluating the whole dwelling and specifying suitable vapour permeable or vapour-balanced solutions, so moisture does not become trapped in the construction.

BS 5250:2021 recommends three ways to assess moisture risk: prescriptive guidance, modelling, and applying the moisture-safety principles in its Annex A. It also stresses looking at real-world effects, meaning the building as designed, as built and in service, and joining up condensation, rain penetration and rising damp rather than treating each alone.

Where insulation is installed in stages, PAS 2035:2023 requires the design to include measures that limit risks from cold surfaces, including condensation and mould. This includes the use of heating and continuous ventilation, or other appropriate measures. If you are insulating the loft this year and the walls in two years, the plan should say what keeps the untreated walls safe in between.

Ventilation is the part homeowners most often overlook. If your home needs help moving moist air out, a positive input ventilation assessment may be part of the answer, and our page on condensation and black mould covers how we approach it.

Documentation: what you should be able to see

PAS 2035 puts a Retrofit Coordinator in charge from inception through to post-completion evaluation. TrustMark’s update says site visits are mandatory on PAS 2035:2023 projects where external or internal wall insulation, room-in-roof, under-floor, party wall or flat roof insulation is installed.

Documentation: what you should be able to see

The same update requires digital evidence, photographs or video, of installation progress and quality. This includes images of the measure itself, the ventilation in place at the time of the visit, design interface details where several measures meet, and any areas of concern or non-conformance.

If you are a homeowner or landlord, ask for this record at handover. It is also useful later: if mould appears, photographs of what was behind the finish and what ventilation existed on the day give a surveyor a starting point.

Our view on PAS 2035 moisture risk

We start from the same principle the standard does: moisture is a cause to be found, not a symptom to be covered. Most calls we take about mould after insulation come down to one of three things: a moisture source that was never dealt with, ventilation that was not matched to the new airtightness, or timber that was already damp when it was boxed in.

That is why we survey first. We look at what you can see, take readings, document the findings with original photographs, and set out a defined scope of work before anyone proposes treatment. You then know what is wrong, what we would do about it and what our written guarantee covers.

We are also clear about our remit. Where a problem needs structural input or another specialist, we say so and refer you on rather than stretch beyond what we do. Our aim is the same as the standard’s: a home where heat, moisture and air stay in balance, and where the fix lasts.

Dan, Director / Surveyor

Quay

When to get a survey

A survey is worth booking in these situations:

  • Before insulation or a ventilation change. Find existing damp, leaks, decay and condensation first, so the retrofit design starts from facts.
  • After insulation, when mould or condensation has appeared. Record where it is, when it started and what changed.
  • When damp appears on solid walls. Our solid-wall moisture articles explain the common patterns.
  • When damp follows rain. That often points to penetrating damp rather than condensation.
  • When timber looks soft, cracked or smells musty. A timber survey establishes whether it is woodworm, wet rot or dry rot.

Landlords have an extra reason to act early. Our note on ventilation assessment for landlords under Awaab’s Law covers what we look at in rented homes.

What to tell us before the visit

It helps to gather a few things. Tell us what you can see and where, when it started, what work was done recently and whether the heating and ventilation have changed. Take dated photographs of affected walls, ceilings and windows. If insulation was installed, find the paperwork and any handover photographs.

Book a survey with Quay Point

We are survey-led. We diagnose from the visible symptoms and measured findings, give you written findings and a defined scope of work, and treat only what the inspection supports. Our work covers damp, mould, timber, leaks and re-plastering across Dorset, Hampshire and Wiltshire, with written guarantees on our workmanship.

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If you want the cause identified before you spend money on insulation or repairs, book a damp and timber survey, or see our full range of services. You can also call 01202 287134 and tell us what you can see.

FAQ

PAS 2035 is a specification, not a law that applies to every home. It is the framework used for publicly funded retrofit schemes and by installers who work to it. Check the terms of any funding or contract to see whether it applies to your project.

Can insulation cause damp or mould?

It can, if it is installed over existing moisture problems or without suitable ventilation. Research from DEEP found that internal wall insulation increased moisture risk in its case studies, because it makes the wall colder. A pre-retrofit survey and a ventilation plan are how this risk is managed.

What should I do if mould appears after insulation?

Photograph the affected areas, note when it appeared and keep the installation paperwork. Do not paint over it or cover it. Book an inspection so the cause, whether condensation, a leak or trapped moisture, is established before any treatment is chosen.

Should damp be treated before retrofit work begins?

Yes. PAS 2035:2023 expects the design to identify existing dampness from leaks, penetrating or rising damp, and damp or rotting timbers. Government guidance says the moisture source should be removed and timber allowed to dry before insulation goes around it.

Who should investigate damp before a retrofit?

RICS guidance says investigations should be evidence-based and carried out by trained practitioners who are qualified to carry out pathological building surveys. Where a defect needs structural or other specialist input, it should be referred on.

Sources

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