Ventilation after insulation and how to avoid mould

Approved Document F contains a chart that tells you, in around five minutes and with no equipment, whether the energy efficiency work done to your home has left it under-ventilated by the standard building control uses. It sits in Section 3, it is called Diagram 3.1, and you use it by counting every energy efficiency measure your house has ever had. Two minor measures with no major work lands you in Category A, where the guidance says no further ventilation is needed. A third minor measure, still with no major work, tips the same house into Category B, where Approved Document F expects a ventilation system. Almost no consumer page explains this, and it decides whether the trickle vents in your new windows are a valid answer or a wasted fitting.

This guide walks you through that count for your own house. It uses the England and Wales Approved Document F Volume 1, published by the Ministry of Housing, Communities and Local Government (MHCLG), and gives the equivalent rules for Scotland and Northern Ireland where they differ. If you want the diagnosis side of damp in detail, the complete insulation and condensation diagnosis guide covers it properly. This one answers a narrower question: what ventilation standard your house is now measured against, given what has already been done to it.

First, a rule change that has not happened

MHCLG published the 2026 edition of Approved Document F Volume 1 on 24 March 2026, and it takes effect on 24 March 2027, or 24 September 2027 for higher-risk building work. Until then the 2021 edition, in force since 15 June 2022, governs work in England. The parts that matter to a homeowner are identical in both: Table 1.1 extract rates, Table 1.7 background ventilator areas and the Table 3.1 measure categories are word-for-word the same. If a salesperson tells you a 2026 ventilation rule change means you must buy something, they are describing a change that has not taken effect and, in the numbers they are quoting, has not happened at all.

Step one, count your measures against Table 3.1

Paragraph 3.8 of Approved Document F is the clause people miss. The count must include energy efficiency measures fitted at any time since the dwelling was originally constructed. The work you are planning now is one part of the total, and often the smaller part. A 1930s semi that got loft insulation in 1998, cavity fill in 2012 and new windows in 2021 carries all three into the count.

Source: MHCLG, Approved Document F Volume 1, Table 3.1. Where a measure is not listed, paragraph 3.8 says to use the closest match in the table.

Two things trip people up. Replacing more than 30% of window and door units is a major measure, and in a typical three-bedroom house a full replacement is close to 100%, so it counts as major on its own. And sealing over a chimney is major, which surprises owners who capped a redundant flue years ago and never thought of it as insulation work.

Step two, read your category off Diagram 3.1

Diagram 3.1 is a grid. Minor measures run along the top from 0 to 6, major measures down the side from 0 to 4. Here is the same grid written out, read directly from the chart in the 2026 edition.

Source: MHCLG, Approved Document F Volume 1, Diagram 3.1. The Welsh Government version uses the same grid with the same boundaries.

Illustrative calculation, not a real property. Assume a 1930s three-bedroom semi with a cavity wall, roughly 90m² of internal floor area across two storeys. The owners have, over 25 years: renewed the loft insulation (minor), had cavity wall insulation (minor), fitted a sealed loft hatch (minor), replaced all the windows and both external doors (major, because it exceeds 30%), and had the chimney sealed over when the gas fire came out (major). That is three minor and two major. On the grid, two major measures put Category B at 0 to 3 minor, so this house is Category B. Add one more minor measure, say sealing around service penetrations during a rewire, and four minor with two major crosses into Category C.

The point of the example is the accumulation. No single job in that list looks dramatic. Cavity wall insulation alone is a minor measure and, on its own, leaves the house in Category A. It is the running total across decades that changes what the guidance asks of the house.

What each category requires you to install

Paragraphs 3.10 to 3.12 set out the consequences.

• Category A. The measures are unlikely to have reduced ventilation below the requirement in F1(1), so no further ventilation provision is necessary. Paragraph 3.10 stops there. Note that if your package of measures included replacing windows, paragraph 3.3(b) sends you to paragraphs 3.14 to 3.16 as well, whatever category you land in. (The Welsh document makes the same point inside paragraph 3.10 itself, which is why you sometimes see it cited that way.)
• Category B. Ventilation has probably been reduced below F1(1), and further provision should be made by one of three means: natural ventilation following paragraphs 1.47 to 1.59, continuous mechanical extract ventilation following 1.60 to 1.66, or mechanical ventilation with heat recovery following 1.67 to 1.73.
• Category C. Ventilation has probably been reduced significantly below F1(1). The natural ventilation option survives only with expert advice on the design, sizing and positioning of ventilators. Otherwise it is continuous mechanical extract or MVHR.

For both Category B and Category C, if you choose MVHR, Approved Document F says existing background ventilators should be covered or sealed shut to avoid unintended air pathways. That is the only route on which sealing your trickle vents is the correct action rather than a mistake. The MVHR explainer covers how those systems work.

Who actually checks this, and when

Worth being straight about this, because it changes how urgently you should treat your own count. Nobody comes round to inspect a house because it quietly accumulated five measures over 25 years. There is no retrospective sweep and no register.

What triggers a check is new building work. The Building Regulations apply to building work as defined in regulation 3, and Approved Document F notes that ventilation equipment is a controlled service or fitting, so providing or extending it counts as building work in its own right. In practice the count gets used at four moments: when you notify a building control body about work; when a competent person scheme installer self-certifies a window replacement (the gov.uk Approved Document F FAQ confirms scheme members “self-certify that their work complies with the relevant requirements of the Building Regulations”); when a retrofit assessor surveys your home for a grant-funded scheme under PAS 2035; and when a buyer’s solicitor asks for the certificates during a sale.

It is also worth being precise about what Approved Document F is. It is not the law. The law is requirement F1(1) in Schedule 1 to the Building Regulations 2010. The approved document is guidance approved by the Secretary of State, and it says of itself that it sets out what “in ordinary circumstances, may be accepted as one way to comply”. The same passage adds that “there may be other ways to comply with the requirements”, to be agreed with the building control body early. Paragraph 3.4 repeats that for ventilation specifically, and paragraph 3.7 offers an air permeability test under Approved Document L as an alternative to the whole counting method. Contravening the regulations themselves is a different matter: the building control authority can prosecute in the magistrates’ court or Crown court.

So the count is not a summons. It is the number a building control officer, a retrofit designer or a surveyor will reach for, and knowing yours before they do is the entire advantage.

Why trickle vents may no longer be a valid route for your house

Table 1.6 of Approved Document F lists the three system types and, next to each, the dwellings the guidance covers. Natural ventilation, defined there as background ventilators plus intermittent extract fans, is marked “Less airtight dwellings”. Continuous mechanical extract and MVHR are both marked “All dwellings”, covering less airtight and highly airtight alike.

Approved Document F defines a highly airtight dwelling as one achieving either a design air permeability below 5m³/(h·m²) at 50Pa or an as-built air permeability below 3m³/(h·m²) at 50Pa, or both. A less airtight dwelling is simply one that is not highly airtight, so it has to sit at or above both thresholds rather than only one of them. Paragraph 1.46 adds that where a naturally ventilated dwelling turns out to be highly airtight on measurement, either expert advice must be sought or a continuous mechanical extract system should be installed.

So the honest answer to “can I just fit trickle vents” is: only if your house is still in the less airtight bracket. A Category C house that has had wall insulation over more than half its external area, a sealed suspended floor and a capped chimney is unlikely to be. Nobody can tell you which side of the line you sit on without a pressure test, which is why paragraph 3.7 offers that test as the alternative to the counting method.

If natural ventilation is still open to you, these are the minimum sizes. Equivalent area is measured in mm² and is not the same figure as free area, so check the manufacturer’s declared equivalent area rather than the hole size.

Source: MHCLG, Approved Document F Volume 1, Table 1.7. Paragraph 1.57 adds a count as well as an area: at least five ventilators across the habitable rooms and kitchen, or four in a one-bedroom property. Paragraph 1.58 requires at least three in an open-plan kitchen and living space. Paragraph 1.55 says a fan and a background ventilator in the same room should be at least 500mm apart. Table 1.7 does not apply at all if the dwelling has only one exposed façade, or has 70% or more of its openings on the same façade, or has a kitchen with no external wall through which a ventilator can be fitted. Those cases need expert advice.

Then there is the clause that catches window buyers. Paragraph 3.15 deals with replacement windows where the originals had no background ventilators, and it offers three ways to demonstrate compliance rather than one flat rule. The default route is ventilators in the new windows at 8,000mm² equivalent area in habitable rooms and the kitchen and 4,000mm² in a bathroom. If the dwelling will have continuous mechanical extract, that drops to 4,000mm² in each habitable room for windows outside wet rooms. The third route is any other provision demonstrated to a building control body. A note adds that where those minimum areas are not technically feasible, the ventilators should get as close to them as is feasible. Whichever route your installer intends to use, get it in writing before you order. Separately, the gov.uk Approved Document F FAQ states that a night-vent or night-latch, where a window locks slightly ajar, is not an appropriate background ventilation solution, because it is not sufficiently secure. Our trickle vent regulations guide goes into the window-replacement side in more detail.

Positive input ventilation is not a Part F compliance route

The words “positive input ventilation” and the abbreviation “PIV” do not appear anywhere in Approved Document F Volume 1. We searched the full text of the 2026 edition to confirm it. Table 1.6 recognises three system types and PIV is not one of them. Paragraphs 3.11 and 3.12 offer only those same three routes for Category B and Category C.

The design guidance used on grant-funded retrofit goes further and rules PIV out by name. That document was written by ecmk in consultation with TrustMark and other scheme providers including Elmhurst Energy, the Installation Assurance Authority and Quidos, and TrustMark hosts the PDF, so it is scheme-provider guidance on how to apply PAS 2035:2023 rather than the BSI standard text itself. Its section 15.1 lists what counts as a complete ventilation system for a dwelling with Category C measures: continuous mechanical extract from all wet rooms combined with correctly sized background ventilators, or a whole-dwelling supply and extract MVHR system, or “other ventilation systems, if they can be shown to meet an equal level of performance and ventilation effectiveness, and extract moisture at source. In this case, expert advice shall be sought.”

Printed directly beneath those three options is a Note: “The above excludes the use of Positive Input Ventilation systems (PIV) as these do not ‘extract at source’.” PIV is therefore not a back door into that third clause. It is named and excluded from it.

What remains is the general provision in Approved Document F paragraph 3.4, which allows ventilation solutions other than those described, as long as it can be demonstrated to a building control body that they comply. That is an argument you or your installer would have to make to building control about your specific house. It is not a box a product can tick off the shelf, and it is a different thing from the PAS 2035 route. Whether PIV dries out a damp house is a separate question from whether it makes you compliant, and sales material tends to blur the two.

Scotland is the exception, and it is worth knowing if you live there. The Scottish Government’s domestic Technical Handbook does discuss positive input systems at clause 3.14.11, and says they “have been successfully installed in existing dwellings with the objective of overcoming problems of surface condensation and mould growth”. The same clause warns that building tighter to reduce uncontrolled air movement “may have a detrimental effect on the operation of input air ventilation systems”, so they may not be appropriate in new dwellings. PIV is therefore acknowledged in Scottish guidance for existing homes and absent from English and Welsh guidance entirely.

How the four UK nations differ

Approved Document F applies to England. Wales has its own Approved Document F Volume 1, which took effect on 23 November 2022 and uses the same Diagram 3.1 grid with the same category boundaries. The Table 3.1 wording is not quite identical: the Welsh major measure at item (g) reads “Replacement of greater than 30% of the total existing windows or door units (when carried out with any other minor or major measure)”, a qualifier the England table does not carry. Wales also folds the window cross-reference into paragraph 3.10 and its Table 3.1 note, where England puts it in paragraph 3.3. Scotland and Northern Ireland use different documents with no measure-counting method at all.

Two practical consequences. Scotland asks for more trickle ventilation per habitable room than England and Wales do, and Section 3.14 warns that where a designer uses fabric infiltration rates below 5m³/(h·m²) at 50Pa in the SAP calculation, the trickle areas in its Table 3.5 may not suffice and an alternative system should be adopted. Northern Ireland’s Technical Booklet K asks for less on replacement windows, 5,000mm² for habitable rooms and 2,500mm² for a kitchen, utility or bathroom, where the originals had no ventilator or the size is unknown.

Is it a ventilation problem or a cold bridge

Appendix B of Approved Document F gives the numeric test, and it is more useful than most homeowners realise once you understand the units. Paragraph B10 applies to all dwellings, new and existing: the moisture criteria are likely to be met if, during the colder months, the moving average surface water activity on the internal surfaces of external walls stays below 0.75 over a month, 0.85 over a week and 0.95 over a day.

Surface water activity sounds obscure. The glossary in Appendix A settles it: in steady-state conditions the figure is numerically equal to the equilibrium relative humidity of the air at that surface, except that relative humidity is normally written as a percentage. A surface water activity of 0.75 is 75% relative humidity at the wall face. That is measurable with a surface hygrometer, and it is the number that separates a whole-house humidity problem from a localised one.

Be careful with the other table. Table B3, the 65% monthly, 75% weekly and 85% daily indoor relative humidity figures, applies to new dwellings only under paragraph B11. Plenty of blog posts quote it as a universal standard for existing homes. Approved Document F does not.

That leaves a gap worth naming, because it is the gap most people fall into. If you own an existing home and point a normal hygrometer at the middle of the room, Approved Document F gives you no number to compare the reading against. The criterion that does apply to your house is measured somewhere else entirely: at the surface of an external wall, not in the air in the middle of the room. Those two readings are not interchangeable. Room air at 55% can sit against a cold window reveal reading 85%, because the air touching that colder surface holds the same water vapour at a higher relative humidity. Take the reading at the wall, at the coldest spot you can find, and treat the room reading as a trend line for whether things are improving rather than as a pass mark.

Used together with where the mould actually is, that gives you a differential:

• Ventilation. Mould spread evenly across cool external-wall surfaces, behind furniture and inside wardrobes, with room humidity high across the whole house. Surface readings above 0.75 in more than one room point this way.
• Cold bridge. Mould in a band along a wall-to-ceiling junction, at window reveals, or at the line of a concrete lintel, while the rest of the room reads normally. That is a fabric geometry problem, and our guide to preventing cold bridges covers the fix.
• Penetrating damp. A discrete patch that is wet rather than mouldy, worse after driving rain, often below a defective gutter, sill or render crack.

The scale of each is worth knowing. According to the English Housing Survey 2024 to 2025, 1.4 million dwellings, 5% of the stock, had a damp problem, up from 4% in 2021 and 2022. Serious condensation affected 3% of all dwellings, penetrating damp 2% and rising damp 1%. Separately, 30% of households reported problems with condensation, damp or mould, rising to 46% in the private rented sector.

What to do about the mould itself, and when to treat it as urgent

Fixing the ventilation removes the cause. It does not remove what is already on the wall, and the timescales are different: a category count and a new extract system take weeks, while mould on a bedroom wall is a health question this week.

The joint OHID, DHSC, MHCLG and UKHSA guidance is written for rented housing providers, but its health section applies to any home. It identifies the people at greatest risk as children and young people, whose organs are still developing; older people; anyone with asthma, COPD or cardiovascular disease; people with weakened immune systems, including cancer patients and transplant recipients; people with mental health conditions; those who are pregnant or recently postpartum; and anyone bedbound, housebound or struggling to heat their home. If someone in one of those groups sleeps in the affected room, move them out of it while you sort the cause, and speak to a GP about any new or worsening respiratory symptoms. The guidance is explicit that action should not wait for medical evidence.

On the cleaning itself, the same guidance draws a line by area. Small patches on hard surfaces can be dealt with using an appropriate mould and mildew cleaner. Larger areas “should only be addressed by qualified professionals”. Whoever does it should wear a mask, gloves and goggles, and windows should be left open during and after the work. Do not dry-brush or sand mould, which puts spores into the air you are breathing.

Treat all of that as symptom management. If the surface readings at the wall stay above 0.75 through the winter, the mould comes back, and the count you did at the top of this guide is what tells you which system stops it.

What each compliant route costs installed in 2026

These are trade supplier figures, not government ones, and we have named the supplier for each so you can weigh them accordingly.

Fan costs are from eFans Direct Ltd’s 2026 bathroom extractor fan installation guide, which also puts electrician labour at £40 to £70 per hour with a £50 to £75 call-out. MVHR costs are from eFans Direct Ltd’s 2026 MVHR cost guide, which separates ductwork and ancillaries at £800 to £1,500 or more. PIV costs are from Dry Basements UK’s 2026 PIV installation cost page, covering unit, ducting, ceiling diffuser, fused spur and commissioning. That last one needs a caveat the others do not: Dry Basements UK sells and installs PIV, so treat its £900 as a vendor’s starting price rather than a market rate, and read it alongside the compliance column rather than against it.

A second illustrative calculation, again with stated assumptions and no real property behind it. Take the Category B semi above, 90m² internal floor area across two storeys, three bedrooms. If the owners choose continuous mechanical extract, paragraph 1.20 and Table 1.3 set the whole dwelling ventilation rate as the higher of 0.3 l/s per m² of internal floor area, which is 27 l/s here, and the bedroom-count minimum, which is 31 l/s for three bedrooms. So 31 l/s governs, and the combined continuous extract across kitchen, bathroom and WC must reach at least that. If instead they choose MVHR at the middle of the quoted range, £3,750 installed, and run it for ten years at £60 a year in electricity and £40 a set of filters twice a year, the ten-year cost is roughly £5,150, about £515 a year. Your own quotes will vary with duct routes and how many wet rooms need serving.

The cheapest fix on the list, and the one everybody skips

Paragraph 1.21 requires internal doors to allow air to flow through the dwelling, with a minimum free area equivalent to a 10mm undercut in a 760mm wide door, which works out at 7,600mm², measured 10mm above a fitted floor finish. New carpet with a thick underlay routinely wipes that out.

The PAS 2035 design guidance treats it as a pass or fail item. A complete ventilation system, it says, should include extract from wet rooms, background ventilation to habitable rooms, undercuts beneath all internal doors, and purge ventilation in every room. Where any of those is not available, not in correct working order, or not to the correct specification, the system is to be considered inadequate. A house can have a brand new £3,000 MVHR unit and still fail because the bedroom doors sit flush on the carpet.

If your insulation came through ECO4 or GBIS, someone else pays to fix it

Government statistics published by DESNZ on 3 November 2025 should change how anyone in this position behaves. Ofgem commissioned random audit samples in April 2025: 378 external wall insulation properties from a population of 24,600 installations, and 380 internal wall insulation properties from a population of 36,100. The results: 92% of external wall insulation installations and 27% of internal wall insulation installations had at least one major technical non-compliance affecting system performance. Counting Category 1 defects as well, the figures rise to 98% of EWI and 29% of IWI. Category 1 health and safety risks were found in 6% of EWI and 2% of IWI installations.

DESNZ announced remediation action on 23 January 2025. Around 65,000 households have had external or internal wall insulation under ECO4 and the Great British Insulation Scheme, counting measures fitted to the end of November 2024, so the true number today is higher. Defects found by TrustMark checks on work fitted since 2022 include “missing or incomplete paperwork, insufficient ventilation, or missing or exposed insulation, which if left unchecked could lead to damp and mould”. Insufficient ventilation is a named defect. 39 businesses were suspended from installing new solid wall insulation, and the government has demanded that installers fund any repair work themselves.

If your wall insulation was installed under ECO4 or GBIS since April 2022, gov.uk’s guidance page on poor quality wall insulation sets out what to do:

1. Contact Ofgem’s ECO help line on 0808 169 4447, Monday to Friday 9:30am to 4:30pm, or email [email protected]. That guidance states that “all properties which had external wall insulation installed will be offered a home check”.
2. Ask specifically whether the ventilation assessment required by PAS 2035 was carried out, and ask for the retrofit assessment and design documents.
3. Do not accept an offer to pay towards remediation. The published position is that you should not be asked to pay any money for this work, and a faulty installer will be asked to fix the issue.

The scheme dates moved in 2026. ECO4 was extended by nine months from 31 March 2026 to 31 December 2026, and DESNZ gave three reasons in its consultation response. The extension gives obligated suppliers more time to meet their targets. It ensures an orderly close to the scheme. And it supports consumer protection by letting the supply chain focus on remediation of non-compliant installations. Remediation is one purpose among three, not the headline one. The Great British Insulation Scheme closed as planned on 31 March 2026 with no extension and no carry-over.

One thing to be clear about, because it is easy to assume the opposite: nothing in the published guidance ties your home check or your right to remediation to the ECO4 end date. That date governs when suppliers must have delivered against their targets. Do not let anyone use it to rush you into a decision or to suggest your entitlement expires.

Grants that pay for ventilation alongside insulation

The Warm Homes: Local Grant is the England-only route where ventilation upgrades get funded as part of a package, and eligibility is narrow: a postcode in Indices of Multiple Deprivation income deciles 1 or 2, means-tested benefits or ECO4 Flex Route 2, or gross household income of £36,000 or less. DESNZ policy guidance sets cost caps of £16,600 per home for energy performance upgrades and £16,600 for low carbon heating, uplifted from £15,000 and averaged across a project rather than applied house by house. Owner occupiers are not expected to contribute. Delivery runs to 31 March 2028.

The PAS 2035 requirement attached to it matters more than the money. Under section 15 of the design guidance, a retrofit designer must assess the existing ventilation and specify an upgrade where the package of specified and existing measures falls into Category B or C, or where there is evidence of condensation or mould growth recorded in the retrofit assessment, regardless of category. Mould on its own is enough to trigger it even if no insulation is being fitted.

If you rent, the timescales your landlord has to meet

Awaab’s Law came into force for the social rented sector on 27 October 2025. For significant damp and mould hazards, MHCLG guidance gives a social landlord 10 working days to investigate, 3 working days after that to issue a written summary, and 5 working days to complete relevant safety work, with preventative work starting within 5 working days or 12 weeks where circumstances prevent an earlier start. Emergency hazards must be investigated and made safe within 24 hours. Private renters are not covered yet: MHCLG’s guide to the Renters’ Rights Act says the government will consult on how to apply Awaab’s Law to the sector and will set out detail in due course, so no timescales exist. The reformed Decent Homes Standard adds Criterion E, a standalone requirement that homes are free from damp and mould, and treats mechanical ventilation as a component that must be repaired or replaced if it fails, from 1 April 2035.

Five ways this goes wrong

• Counting only the current job. Paragraph 3.8 requires measures fitted since the dwelling was built. An installer who counts only the work they are quoting for will under-report your category.
• Treating trickle vents as always available. Table 1.6 restricts natural ventilation to less airtight dwellings. In a Category C house it needs expert design advice, and if the house measures as highly airtight under paragraph 1.46 it needs continuous mechanical extract instead.
• Closing the vents in winter. Paragraph B10 sets the Table B2 surface water activity limits for all dwellings, new and existing, and evaluates them during the colder months of the year, as moving averages over a month, a week and a day. Winter is the season the criterion is tested in, and it is the season people shut the vents. Paragraph 1.29 requires ventilation to be controllable. Controllable does not mean shut.
• Buying a dehumidifier instead. A dehumidifier manages a symptom and adds electricity cost. Approved Document F treats the problem as a balance of extract from wet rooms, background supply to habitable rooms, and a path between the two through door undercuts. A missing element in any of the three is what the PAS 2035 guidance fails you on.
• Blaming the household. The joint OHID, DHSC, MHCLG and UKHSA guidance states: “We are absolutely clear that it is totally unreasonable to blame damp and mould in the home on lifestyle choices.” It attributes cause to building deficiencies, inadequate ventilation, inadequate heating and poor energy efficiency, and it estimated that in 2019 damp and mould in English residences was associated with around 5,000 cases of asthma and around 8,500 lower respiratory infections among children and adults, with the true burden possibly 3 to 8 times greater.

What to do, and by when

Work through this in order.

1. This week. List every energy efficiency measure your home has had since it was built. Deeds, EPC history, old invoices and the previous owner’s paperwork all help. Score each against Table 3.1 and read your category off the grid above.
2. This week, if you have mould. Take surface readings at the affected walls with a surface hygrometer, and take room readings in two or three other rooms. A surface figure above 0.75 in multiple rooms points to whole-house ventilation. A single band or reveal points to a cold bridge. If anyone in an at-risk group sleeps in that room, move them out of it now.
3. As soon as you can, with no deadline attached. If your wall insulation came through ECO4 or GBIS since April 2022, call Ofgem on 0808 169 4447 and ask for the home check. The 31 December 2026 ECO4 end date is a scheme administration date, not a cut-off for your remediation, and you should not be asked to contribute.
4. Before you order replacement windows. Confirm in writing which paragraph 3.15 route the quote follows, and what equivalent area you are getting: 8,000mm² for habitable rooms and the kitchen and 4,000mm² for a bathroom on the default route in England and Wales, or 5,000mm² and 2,500mm² in Northern Ireland. Equivalent area, not free area.
5. Before 31 March 2028. If you are in England, on a low income and in an EPC D to G property, check whether your local authority is delivering the Warm Homes: Local Grant. PAS 2035 compliance means the ventilation assessment comes as part of the package.
6. Before signing anything for PIV. Ask the supplier to point to the clause in Approved Document F that names it. There is not one, and the PAS 2035 design guidance excludes it by name. If they say it is compliant under Building Regulations, ask them to confirm in writing that they will obtain building control acceptance under paragraph 3.4 as an alternative provision.

If your count puts you in Category B or C and you have no mould, you have time to plan and to get three quotes. If your count puts you in Category B or C and you already have mould on external walls, the ventilation upgrade is the fix and the surface readings will tell you whether it is the whole house or one cold spot. Either way, the count comes first, because it is what a building control officer or a retrofit designer will use, and doing it yourself costs nothing but an evening with the paperwork.