Mould & condensation
Condensation control and humidity assessment
Assessment of moisture production, ventilation, heating and insulation to reduce surface condensation.
Measured, not assumed
Condensation is a relationship between moisture, temperature and air change
Air holds water vapour. The warmer it is, the more it can hold. When moist air meets a surface cold enough to bring it to its dew point, the water comes out of the air and onto that surface. Everything we assess follows from those two sentences.
It means condensation is never caused by one thing. It is the product of how much moisture is produced, how cold the coldest surfaces are, and how much of the moist air is replaced with drier air. The factors we measure and record:
Relative humidity in each affected room
Air temperature, and how the property is actually heated
Dew point, calculated from the readings taken
Surface temperatures on external walls, reveals and ceilings
Thermal bridging at lintels, reveals, junctions and floor edges
Insulation presence, continuity and condition
Background ventilation — trickle vents, air bricks, passive provision
Mechanical extract in kitchens, bathrooms and utility rooms, and whether it works
Internal doors, air movement and furniture against cold walls
Moisture production: occupancy, cooking, washing, drying, bathing, aquaria, pets
Unvented tumble dryers and portable gas or paraffin heating
Any building-fabric moisture adding to the vapour load
Cold surfaces and cold spots are the half of the equation most often ignored. A bedroom at 40% relative humidity will still grow mould behind a wardrobe on a wall whose surface sits at 10°C.
What the issue is
Condensation occurs when moist internal air meets a surface below its dew point. It is the most common cause of domestic dampness and mould, and is usually addressed through ventilation, heating and insulation rather than chemical treatment.
Common symptoms
- Water on window glass and frames
- Mould in corners, behind furniture and on ceilings
- Damp bedding, curtains or wardrobes
- Musty smells
- Worse in colder months
Likely contributing factors
- Insufficient background ventilation
- Extract fans absent, undersized or not working
- Intermittent or low heating
- Cold bridging at junctions
- High internal moisture production
Conditions that may be mistaken for it
- Rising damp on ground-floor walls
- Penetrating damp on external walls
- A slow plumbing leak
- Construction moisture in new work
How the issue should be investigated
- Record temperature, relative humidity and dew point
- Measure surface temperatures at affected areas
- Test extract fan performance
- Review background ventilation provision
- Consider occupancy and heating patterns
Potential treatment or repair options
- Improved mechanical extraction to bathrooms and kitchens
- Positive input ventilation or heat-recovery systems
- Background ventilation improvements
- Insulation improvements at cold spots
- Practical guidance on heating and moisture control
When treatment may not be appropriate
- Where mould is limited and simple ventilation changes will resolve it
- Where a building defect is the real source of moisture
Factors affecting the cost
- Number and type of ventilation units
- Electrical alterations required
- Access to lofts and external walls
- Making good and decoration
What to expect during an inspection
The surveyor will take psychrometric readings, test existing fans and discuss how the property is used and heated.
Why it keeps coming back
Cleaning the mould does not address the condensation
Wiping mould off a wall removes the visible growth. It does not change the humidity, warm the surface or increase air change, so the conditions that produced the growth are all still there — and the growth returns, usually to the same spot.
Anti-mould paint has the same limitation. Applied to a cold, regularly wetted surface, it delays the reappearance rather than preventing it. The durable fix is to change the conditions: reduce the moisture at source, raise the temperature of the cold surface, and get enough air change to carry the vapour away.
- Reduce moisture at source where it is being generated
- Increase effective extract where moisture is produced in volume
- Provide adequate background ventilation across the dwelling
- Raise the temperature of the coldest surfaces, and address thermal bridges
- Improve air circulation where furniture traps cold, still air
- Remediate the existing contamination properly once conditions are controlled
How we assess it
What a condensation assessment involves
- 1
Record the internal environment
Air temperature, relative humidity and dew point taken room by room with a calibrated hygrometer, alongside surface temperatures on the affected construction, so the risk can be calculated rather than guessed.
- 2
Find the cold surfaces
Thermal imaging where it helps, and inspection of insulation, junctions, reveals and lintels, to locate where surfaces fall towards dew point.
- 3
Assess air change and extract
Background provision, extract fans and their ducting, and how the property is used day to day — including whether the extract is actually run for long enough to clear the moisture it is meant to remove.
- 4
Separate fabric moisture from condensation
Where moisture is entering the fabric as well, that mechanism is identified separately: adding heating and airflow will not resolve a leaking gutter.
- 5
Set out a proportionate plan
Written recommendations in priority order, covering source control, ventilation, heating and surface temperature, with the remediation the existing contamination needs.
Related work
Related work and reading
Mould remediation
Safe removal of the contamination, and control of the moisture conditions that allowed it to grow.
Internal wall insulation
Cold solid walls warmed correctly, with the condensation risk assessed instead of ignored.
Damp surveys and moisture investigations
What a professional damp investigation covers, what the instruments can and cannot establish, and what your written report contains.
Penetrating damp investigations
Rainwater and external defects traced to the elevation, opening or rainwater fitting responsible.
Rising damp assessments
Capillary moisture, damp-proof courses, bridging and salt contamination assessed on the evidence rather than a meter reading.
Damp-resistant plastering
Reinstatement in the right materials once the cause has been dealt with and the wall has begun to dry.
Where we work
Condensation and humidity assessment across Bedfordshire, Northamptonshire and Cambridgeshire
Condensation and humidity assessment is carried out across our operating area. Choose your county or town to see the work we do locally, the postcode districts we cover and how to arrange an inspection.
Frequently asked questions
Next step
Not sure which inspection you need? Start with a few questions.
Tell us what you are seeing and we will set out what is likely, what is uncertain and which inspection would answer the question. Speak to us on 0333 335 6152 if you would rather talk it through.