Understanding Relative Humidity and Dew Point in Housing

Relative humidity and dew point are fundamental concepts for understanding condensation and mould risk in buildings. Yet they're often misunderstood, even by housing professionals. This guide explains what these metrics mean in practical terms.

What is Relative Humidity?

Relative humidity (RH) measures how much water vapour is in the air compared to the maximum amount the air could hold at that temperature. It's expressed as a percentage.

• 0% RH: Completely dry air (theoretical, rarely seen in practice)
• 50% RH: Air holding half its potential moisture capacity
• 100% RH: Air fully saturated—condensation begins

The Temperature Connection

Here's the crucial point: warm air can hold more moisture than cold air. This is why relative humidity changes throughout the day even when the actual amount of moisture stays constant.

Consider a room with a fixed amount of water vapour in the air:

The moisture content hasn't changed—but relative humidity has risen from 45% to 80% simply because the temperature dropped.

What is Dew Point?

Dew point is the temperature at which air becomes saturated (100% RH) and condensation forms. It's a more stable measurement than relative humidity because it doesn't change with temperature—only with actual moisture content.

Practical Example

If a room has a dew point of 12°C:

• Any surface at 12°C or below will have condensation
• Windows, external walls, and cold corners are most at risk
• The air temperature doesn't matter—only surface temperature vs. dew point

This is why condensation often appears on windows first. Glass is a poor insulator, so window surfaces are often the coldest in a room.

Why This Matters for Housing

Mould Growth Conditions

Mould spores are everywhere—they only become a problem when conditions allow growth. The key factors are:

• Surface humidity above 80%: Even if room RH is lower, cold surfaces can have higher local humidity
• Sustained conditions: Brief spikes are less problematic than persistent high humidity
• Temperature range: Most moulds grow between 10-35°C
• Organic material: Dust, dirt, wallpaper paste, paint—all provide nutrients

The 65% RH Threshold

Building science commonly uses 65% RH as a threshold for concern. Above this level, risk increases:

• Below 65%: Generally safe, mould growth unlikely
• 65-75%: Elevated risk, especially on cold surfaces
• Above 75%: High risk, conditions favour mould growth
• Above 85%: Very high risk, growth probable within days

Surface Temperature Differential

The temperature difference between room air and surfaces determines where condensation occurs. Problem areas include:

• Single-glazed windows
• Cold bridges (where insulation is interrupted)
• External wall corners
• Areas behind furniture against external walls
• Unheated rooms

Monitoring Strategies

Why Temperature Alone Isn't Enough

Some providers monitor only temperature, assuming it indicates adequate heating. But a warm room with high moisture production can still have severe condensation on cold surfaces.

Why Humidity Alone Isn't Enough

A room showing 60% RH might seem safe—but if the temperature is 14°C and walls are at 10°C, condensation is occurring. Context matters.

The Combined Picture

Effective monitoring requires both temperature and humidity to calculate:

• Dew point: The temperature at which condensation will form
• Risk assessment: How close current conditions are to condensation threshold
• Trend analysis: Whether conditions are improving or deteriorating
• Mould risk index: Probability of mould growth given sustained conditions

What the Data Tells You

Pattern Recognition

Continuous monitoring reveals patterns that inspections miss:

• Overnight humidity spikes: May indicate inadequate ventilation or heating
• Post-cooking/bathing peaks: Normal, but should dissipate within 2-3 hours
• Persistently high baseline: Suggests underlying moisture source or building issue
• Sudden changes: May indicate leak, behavioural change, or system failure

Distinguishing Causes

Environmental data helps determine whether damp is caused by:

• Condensation: High RH, humidity varies with occupancy, worse in winter
• Penetrating damp: Humidity correlates with rainfall, location-specific
• Rising damp: Ground floor, persistent regardless of weather
• Leaks: Sudden onset, may correlate with plumbing use

Comfortable and Safe Ranges

For UK housing, target conditions are:

Key Takeaways

• Relative humidity changes with temperature—even when moisture content stays the same
• Dew point indicates condensation risk—compare it to your coldest surfaces
• 65% RH is a useful threshold—above this, risk increases significantly
• Context matters—brief peaks are different from sustained high humidity
• Monitoring both metrics gives the complete picture