Damp, Mould and Condensation: A Complete Guide for Retrofit Professionals

Why Moisture Science Keeps Appearing in Retrofit Exams

Damp, mould, and condensation are not just homeowner complaints — they are the central risk that PAS 2035 exists to manage. When a retrofit adds insulation and airtightness without addressing ventilation, moisture that once escaped through a leaky fabric has nowhere to go. Understanding exactly how moisture behaves is essential for Retrofit Assessors, Coordinators, and Installers alike, and it is one of the most heavily examined areas across every qualification route.

This guide works through the moisture cycle from source to symptom, using the kind of real-dwelling evidence that makes the theory stick.

Tip: Examiners love scenario questions about moisture. If you can explain why condensation forms on one wall and not another, you can answer almost any variant they throw at you.

The Five Sources of Household Moisture

Every dwelling generates moisture continuously. The main sources are:

1. Cooking and kettle boiling
2. Bathing and showering
3. Washing and drying clothes indoors
4. Breathing — roughly half a litre per person per day, more with exertion
5. Moisture ingress through basements, crawl spaces, and walls

These sources run all year, but winter is when problems surface. Occupants close windows to keep warm, ventilation drops, and rain and groundwater increase ingress. Less moisture leaves the dwelling, so relative humidity climbs — exactly the condition assessors must recognise during a dwelling survey.

Relative Humidity: The Concept You Must Be Able to Explain

Humidity cannot be discussed in isolation, because air’s capacity to hold moisture depends on its temperature. Relative humidity (RH) is the amount of moisture in the air relative to the maximum that air could hold at its current temperature. Warmer air holds more moisture; a drop of roughly 10°C halves the air’s moisture capacity.

This produces a counter-intuitive effect worth remembering: turning the heating on lowers the RH reading without removing a single gram of water. The moisture is still there — the warmer air simply has more capacity. Heating alone masks a moisture problem rather than solving it.

RH Range	Condition	Consequence
Below 30%	Too dry	Chapped lips, dry skin, irritated eyes
40–60%	Healthy target zone	Comfortable, mould-suppressing
Above 60%	Too humid	Dust mites thrive; mould risk rises
Around 80%	High risk	Mould grows readily; fabric decay begins

Dust mites flourish at 70–80% RH but stop growing and die off below 60%. Sustained high humidity also rots plasterboard, corrodes electronics, and aggravates allergies — the health and fabric consequences that make ventilation a mandatory consideration in any retrofit design.

Dew Point and Why Condensation Targets Cold Surfaces

Condensation forms when moist air meets a surface colder than its dew point — the temperature at which RH reaches 100% and the air can hold no more moisture. Single glazing and uninsulated walls are the classic cold surfaces. The glass cools the thin layer of air beside it, that air’s capacity drops, local RH hits saturation, and water is deposited on the surface.

A cold window is effectively an accidental dehumidifier. This is why a dwelling can show a healthy whole-room RH of under 50% yet still stream with condensation on cold mornings: when outdoor temperature falls near freezing, the glass surface drops well below the indoor dew point and condensation is inevitable. Even closed curtains make it worse, because they stop warmer room air from washing over the glass and keeping the local RH below saturation.

This mechanism — surface temperature versus dew point — underpins interstitial condensation risk, thermal bridging questions, and the reasoning behind insulation details in both PAS 2035 assessments and PAS 2030 installation practice. If this area feels shaky, our building physics revision guide covers the fundamentals in more depth.

Controlling Moisture: The Three-Part Strategy

1. Reduce Moisture at Source

Occupancy behaviour has a huge impact, and it is a legitimate part of retrofit advice:

• Always run the cooker extract fan when cooking
• Fit humidistat-controlled extract fans in bathrooms — they run until moisture falls to a set level and cannot be switched off prematurely
• Ventilate during and after showering; squeegeeing a shower enclosure can remove around 200 millilitres of water that would otherwise evaporate into the room
• Avoid indoor drying racks where possible, or contain them in a room served by a dehumidifier or extract ventilation

2. Ventilate

The gold standard is mechanical ventilation with heat recovery (MVHR): multiple extract points, filtered supply air, and a heat exchanger retaining around 95% of the heat in the outgoing air. In existing dwellings the ducting is often impractical, which is where positive input ventilation (PIV) enters the picture — a loft-mounted unit pushing filtered air into the dwelling to dilute moist air. PIV avoids ducting but has real limitations: the supply air is only tempered, the area around the unit can feel chilly, closed internal doors block circulation, and siting matters enormously. These trade-offs between ventilation strategies appear regularly in Coordinator exam scenarios — the Level 5 topic map shows where ventilation sits in the wider syllabus.

Trickle vents, kept open through winter, provide useful background dilution where mechanical options are not viable.

3. Remove Moisture Mechanically

Dehumidifiers split into two families:

Type	How It Works	Best Suited To
Desiccant	Air drawn over a moisture-absorbing wheel, regenerated by a heater	Cold, unheated spaces such as garages; not temperature-dependent
Compressor (refrigerant)	Air drawn over cold coils below dew point	Heated dwellings above roughly 3°C; generally lower running costs

A well-controlled compressor unit in a typical winter dwelling can extract one and a half to two litres per day. But note the crucial finding from real monitoring: extraction volume does not correlate directly with window condensation, because surface condensation is driven by outdoor temperature and dew point, not just average indoor RH.

Did you know? Setting a dehumidifier’s target RH too low wastes energy pulling moisture out of furniture, walls, and floors — unnecessary drying that costs money without improving comfort.

Insulation: Part of the Strategy, Not the Whole Answer

Insulating a wall raises its internal surface temperature above the dew point, so condensation and mould on that surface stop. That is a genuine win — but insulation should be treated as one element of a moisture strategy, never a substitute for controlling humidity and ventilation. Insulating without addressing moisture generation risks masking the problem or displacing condensation to the next coldest surface. This is precisely the whole-dwelling reasoning PAS 2035 demands: heat, air, and moisture must be considered together, with ventilation assessed whenever fabric measures change the balance.

Turning This Into Exam Marks

Moisture questions often come as multi-answer items — “select all factors that increase condensation risk” — so practise recognising complete answer sets, not just single facts. Our guide to multi-answer retrofit questions explains how to approach them without second-guessing. Keep sessions short and topic-focused: ventilation and moisture one day, fabric and heat loss another.

How RetrofitReady Fits Into This

Ventilation, moisture risk, and building physics run through every RetrofitReady app, matched to your role:

• RetrofitReady: Coordinator — 600 PAS 2035 questions across domestic retrofit, risk, fabric, services, ventilation, MTIP, and monitoring, ideal for the whole-dwelling moisture reasoning tested at Level 5.
• Retrofit Assessor — 480 questions across eight syllabus areas, including multi-answer and diagram-based practice covering condensation, ventilation assessment, and dwelling surveys.
• RetrofitReady: Installer — 480 questions across the two core units and six fabric measures, with multi-answer and diagram-based practice on the installation details that keep moisture out.

Across the full suite you get 1900+ questions across 4 apps, each with explanations you can use as revision notes. Start with the free quizzes or browse the complete RetrofitReady app suite.