Humidity & Condensation On Board: Diagnostic Protocol
Fogging on windows, droplets on diffusers, persistent musty smells, dampness, condensation in cabins or technical areas — on a yacht, humidity rarely has a single “fix”. The right approach: a fact-based diagnostic rooted in dew point, temperatures, airflow rates and infiltration sources.
Condensation occurs when the temperature of a surface (bulkhead, grille, duct, ceiling, portlight, evaporator) drops below the dew point of the surrounding air. In other words: it is not about being “too cold” or “too humid” in isolation — it is a mismatch between air humidity and surface temperature.
- Air temperature and relative humidity (%RH) in the affected zone.
- Dew point (calculated or read from a hygrometer/probe).
- Surface temperature of bulkheads / grilles / ducts (IR thermometer recommended).
If surface temp < dew point → condensation is likely (or inevitable) until one of the two values changes.
- Cabins / suites: fogging, musty smell, damp textiles, condensation on ceilings.
- Corridors / interior areas: water running near grilles, localised cold spots.
- Ducts / technical ceilings: hidden condensation → corrosion, delayed dripping.
- Fresh air zones: humidity spikes during boarding, open doors, galley activity.
- Glazing / superstructure: thermal bridges, cold surfaces.
The goal is not to have “a perfect measurement”, but a coherent chain of measurements that explains the phenomenon.
Map it
Where, when, and under what conditions?
- Precise location (cabin, side, ceiling, grille, duct).
- Timing (night/day, underway/at anchor), weather, sea state, occupancy.
- Events: doors, showers, galley, laundry, boarding.
Measure air & dew point
Air temp + %RH → dew point (per zone).
- Measure at occupant level and near grilles.
- Compare healthy zone vs humid zone.
- Does the humid zone have a higher dew point?
Measure surface temperatures
Identify surfaces below the dew point.
- Ceilings, external bulkheads, glazing, grilles, ducts.
- Locate “cold spots” (thermal bridge / missing insulation).
- Is condensation concentrated on those areas?
Check airflow rates & distribution
Humid or poorly circulated air condenses faster.
- Insufficient flow → stagnant air, %RH rises, dew point rises.
- Supply/return imbalance → warm/humid air infiltration.
- Dirty filters/coils → poorer heat exchange, humidity remains.
Identify the moisture source
Fresh air, infiltration, occupancy, internal leaks.
- Untreated outside air (high latent heat loads).
- Infiltration through doors/openings, seals, hatches.
- Internal sources: shower, galley, laundry, water leak.
Link to setpoints & controls
Poor settings can actively create condensation.
- Chilled water setpoint too low vs actual dew point.
- Dehumidification settings absent or inactive.
- Drifting %RH/temp sensors → incorrect control decisions.
- Dew point too high (humid air ingress) → high %RH + diffuse condensation.
- Surfaces too cold (thermal bridge / insulation / duct) → condensation localised on “cold” areas.
- Insufficient flow / poor balancing → zones that “go mouldy” because air is not renewed.
- Fouled coils / drainage → ineffective dehumidification + water not properly drained away.
- Poorly positioned return → humid air remains trapped (stratification).
- Overly aggressive setpoints → chilled water too cold + surfaces drop below dew point.
Simple rule of thumb: localised condensation = usually surface/duct/insulation issue. Diffuse condensation = usually high dew point (humidity load).
- Clean accessible filters/coils + check drainage (slopes, traps, blockages).
- Check airflow rates (supply/return), valves, dampers, blocked grilles.
- Simple adjustments: avoid overly low setpoints when dew point is high.
- Reduce obvious infiltrations: seals, hatches, doors, duct penetrations.
- Redo insulation on cold areas / ducts (thermal bridges).
- Review fresh air system: treatment, dehumidification, control logic.
- Rebalance the network (air and/or chilled water) after refit/modifications.
- Instrumentation: reliable %RH/temp sensors in the right locations + trend monitoring.
“Condensation on board is not a comfort problem — it is a technical signal. Treating it without an objective diagnostic is addressing a symptom without understanding the cause.”
Why does condensation appear mainly in the morning or at night?
Changing loads, cooler zones, reduced ventilation, surfaces cooling down (thermal bridges). If the dew point remains high, even a slight drop in surface temperature triggers condensation.
Will lowering the setpoint fix the problem?
Not necessarily. Lowering the setpoint can make condensation worse by cooling surfaces below the dew point. You first need to understand whether the issue is a “dew point too high” or a “surface too cold” problem.
What is the simplest test?
Measure temp + %RH and calculate the dew point, then measure the temperature of the condensing surface. If the surface is below the dew point, you have objective proof of the mechanism.
RivierX Engineering conducts measurements, identifies the root cause and delivers a prioritised action plan.