Damp, Mould and Humidity: What Eight Real Fixes Teach Retrofit Professionals

A Case Study That Reads Like a Retrofit Assessment

A homeowner buys a 1930s bungalow that has grown by several extensions over the decades. Winter humidity readings across the rooms average between 77% and 91%. Mould covers the back walls of the fitted wardrobes and the wooden window frames are thick with black growth. Three years and eight interventions later, the average sits around 63%.

Strip away the DIY framing and this is a textbook PAS 2035 problem: a dwelling with poor air movement, no meaningful background ventilation, high internal moisture generation, and cold surfaces waiting to collect condensation. Every retrofit professional will meet this house. The useful part is not that the problem was solved — it is which fixes moved the number and which only cleaned up the symptom.

Tip: When a client describes “damp”, your first job is to work out whether you are looking at rising damp, penetrating damp, or condensation. The remedies are completely different, and only one of them is primarily a ventilation problem.

The Eight Interventions, Ranked by Retrofit Value

Intervention	What it addresses	Retrofit relevance
Humidity sensors in each room	Measurement	Evidence for assessment and post-install monitoring
Air bricks into wardrobe voids	Local air movement	Targeted fix for a cold, enclosed, unventilated space
Opening windows twice daily	Purge ventilation	Occupant behaviour — cannot be relied on alone
Trickle vents to every window	Background ventilation	Directly relevant to Approved Document F compliance
Positive Input Ventilation in loft	Whole-dwelling supply	The single largest measured improvement
Dehumidifier for drying washing	Moisture source control	Addresses generation, not removal path
Bathroom extractor, ducted to roof	Wet room extract	Core compliance requirement
Kitchen extractor, ducted out	Wet room extract	Core compliance requirement

Notice the shape of that list. Three items control the moisture source, four control the removal path, and one is pure measurement. That trio — source, path, surface temperature — is the framework you should be carrying into every survey.

Measurement Before Measures

The first thing the homeowner did was buy cheap hygrometers for every room and log readings over several weeks at different times of day. That instinct is exactly right, and it is the part most under-done in practice.

Single spot readings tell you almost nothing. Humidity swings enormously with cooking, showering, drying laundry, occupancy and outdoor conditions. A reading taken at 11am on a dry Tuesday in a room nobody has slept in will look fine. The same room at 7am after two people have slept in it with the door closed will look very different.

• Log readings across a full daily cycle, not one visit.
• Record temperature alongside humidity — relative humidity is meaningless without it.
• Identify which rooms differ, as the office and kitchen did here versus the bedroom.
• Note enclosed spaces — wardrobes, behind furniture, under stairs, in cupboards on external walls.

The bedroom wardrobe in this case is the perfect illustration. A cold external wall, a closed door, no air movement, and clothing that traps moisture. The surface temperature drops below dew point and mould has everything it needs. No whole-house strategy fixes that specific void; it needed its own airflow path.

Did you know? Mould does not need liquid water. It needs a surface relative humidity of roughly 80% sustained over time, which happens on cold surfaces long before you see visible condensation running down the glass.

Background Ventilation and the Window Trap

Every window in the bungalow received a trickle vent. This is the detail that should make retrofit professionals sit up, because it is where a great deal of real-world non-compliance hides.

When windows are replaced, the new units are dramatically more airtight than the draughty timber frames they replace. Building Regulations Approved Document F expects background ventilation to be maintained when you replace windows — but on countless jobs, vents are omitted because the homeowner does not want them, the installer does not price for them, or nobody thinks about it. The dwelling then gets warmer, tighter and wetter simultaneously.

The same logic applies to every fabric measure you touch. Insulate a wall, seal a floor, upgrade a roof, and you have changed the moisture balance of the dwelling. That is precisely why PAS 2035 requires ventilation to be assessed as part of the retrofit assessment, and why a ventilation shortfall must be dealt with within the measures package rather than left as someone else’s problem. Our complete guide to damp, mould and condensation works through the diagnostic side in more detail.

Remember: “The occupant will open the windows” is not a ventilation strategy. Purge ventilation depends entirely on behaviour, and the previous occupants of this bungalow had left most windows jammed shut for years.

Why the PIV System Moved the Number

Of the eight measures, one produced the measured shift: installing Positive Input Ventilation in the loft dropped the whole-dwelling average from around 84% to around 63%.

That is worth understanding mechanically. PIV draws air through filters from the loft space and gently pushes it into the dwelling, slightly pressurising it so moisture-laden air is displaced out through the fabric and background openings. It works well in exactly the situation described here — a single-storey dwelling with poor natural air circulation and no meaningful cross-ventilation path.

Two caveats belong in your professional judgement:

1. Incoming air is at loft temperature, so it can feel chilly. Units with integral heaters exist but consume more energy. The homeowner here noted the chill directly.
2. PIV does not replace wet room extract. It is a whole-dwelling supply strategy. Kitchens and bathrooms still generate concentrated moisture that must be removed at source — which is why the bathroom and kitchen extractors were installed as well, the bathroom fan ducted up through the loft and out through the roof because there was no external wall available.

That bathroom detail matters for installers. Ducting through a loft void introduces condensation risk in the duct itself; runs need to be short, insulated, sloped correctly and terminated properly. A powerful fan connected to a poorly executed duct will underperform badly, and the flow rate you commission is the flow rate that counts — not the figure on the box.

Moisture Source Control Is Half the Job

The dehumidifier deserves more credit than its position in the list suggests. Drying washing indoors on radiators dumps litres of water into the air of an already humid dwelling. It is one of the highest-impact behaviours in UK homes, and it is entirely invisible on a survey visit unless you ask.

Practical questions worth adding to your assessment conversation:

• Where does laundry dry, and how often?
• Is the tumble dryer vented or condensing, and where does it discharge?
• Are lids used on pans, and does the extractor actually get switched on?
• How many occupants, and are bedroom doors closed overnight?
• Is any unflued gas or paraffin heating in use?

None of that appears in an RdSAP data set, but all of it shapes the ventilation strategy you specify. The PAS 2035 and PAS 2030 six-stage overview puts this in the context of where assessment findings feed into design and installation.

The Honest Ending

Three years on, the average is 63% — outside the 30–60% band the homeowner was aiming for. But there is no mould, no condensation on the windows, and the air feels markedly better.

That is a genuinely useful lesson for retrofit professionals. Target bands are guidance for design, not a pass/fail test applied to a solid-walled 1930s dwelling in a British January. The meaningful outcome measures are whether surface temperatures stay above dew point, whether mould has stopped growing, and whether the ventilation system delivers its designed flow rates. Chasing a number while ignoring those three is how people end up over-specifying plant that occupants switch off.

Tip: In exam answers and in real reports, tie humidity findings to consequences — surface condensation risk, mould growth, health impact — rather than quoting a percentage in isolation.

How RetrofitReady Fits Into This

Ventilation is one of the highest-yield revision areas across every retrofit route, because it cuts through fabric, moisture, health, compliance and installation quality all at once. It also generates a lot of exam questions.

• RetrofitReady: Coordinator — 600 PAS 2035 questions across domestic retrofit, risk, fabric, services, ventilation, MTIP and monitoring. The ventilation and risk strands cover exactly the decision-making in this case study: assessing existing provision, recognising when fabric work changes the moisture balance, and specifying a proportionate response. Pair it with the Level 5 topic map to plan your sequence.
• Retrofit Assessor — 480 questions across eight syllabus areas, including multi-answer and diagram-based practice. This is where you drill the survey-side skills: identifying condensation versus penetrating damp, recording existing ventilation provision accurately, and capturing the occupant behaviours that drive moisture generation.
• RetrofitReady: Installer — 480 PAS 2030 questions across the two core units and six fabric measures, with multi-answer and diagram-based practice. Useful for the practical detail: duct runs, terminations, background ventilation openings, and not undoing a ventilation strategy while installing a fabric measure.

Across the suite there are 1900+ questions in 4 apps, so you can revise a single strand like ventilation and then widen into mixed practice. Short, focused sessions work better than marathon ones — the approach in using short practice sessions applies well here.

Suggested Ventilation Revision Routine

1. Answer a topic set on ventilation without notes.
2. Read every explanation, including the ones you got right.
3. Sketch this bungalow’s eight measures and label each as source control, removal path, or measurement.
4. Return to the same topic later in the week, then move into mixed practice.

Start with the free quizzes or browse the full RetrofitReady app suite. The recommended books and tools page also lists ventilation and building physics references worth having on the shelf.