Condensation, Damp, Mould & Beer

Let’s do an experiment.

Take a cold can of beer out of the fridge (alternatively, a cold bottle of wine works just as well). Take it outside on a warm summers day and drink it.

Easy hey?

The Condensation forming outside the can explains relative humidity.

Relative humidity is the amount of water vapour present in the air compared to the maximum amount the air could hold at that temperature, expressed as a percentage.

For example, 50% relative humidity means the air contains half the water vapour it could theoretically hold at that temperature.

It measures how close the air is to being saturated with moisture, with higher values indicating more humid conditions. Temperature affects it because warmer air can hold more water vapour than cooler air.

The warmer summer air outside in Nottingham contains water vapour at around 70% relative humidity. 70% of the moisture it could theoretically hold. when this air hits the can the temperature drops so the relative humidity of the air reaches the limit of what it can hold (Relative Humidity =100%). At a relative Humidity of 100%, condensation forms.

The Dew Point defines the temperature at which this condensation forms.

https://www.accuweather.com/en/gb/nottingham/ng1-7/current-weather/330088

Today's weather forecast indicates that the dew point is 10°C (a warm beer). This means that condensation can form on any surface below 10 degrees.

In winter, Nottingham's average temperatures range from 2°C to 7°C. Due to the damp winter conditions, relative humidity is often high, around 80-90%. The dew point, which depends on temperature and humidity, is usually close to the external air temperature in such conditions.

Condensation & Mould in the house

It doesn’t really take condensation on a surface to grow mould. You just need high relative humidity, which easily happens at colder spots in a room. Always remember the maxim: cold spots equal mould spots. If a surface is colder than the room, the air right at the surface is colder and thus has high relative humidity. The nice, warm & dry air inside a typical home in Nottingham in the winter is 20°C and 50% relative humidity (RH). If a section in the corner of a room is 12°C, then the relative humidity is slightly above 80%, which is definitely high enough for potential mould growth.

Calculating whether or not you will get mould growth is complicated because it involves the materials, the amount of time spent in high relative humidity, and the moisture storage of the surface. But generally, if you avoid cold spots, you can avoid mould from interior moisture.

The way we heat our homes also affects this. If you heat your home intermittently, only when you are there, the risk of condensation and mould increases.

The tips to avoid condensation, damp and mould are:

• If possible, you should heat your home to always be above 16 degrees

• Ventilate your home - get lots of fresh air in

• Extract moisture at the source - kitchens and bathrooms in particular

Any condensation analysis assumes you are:

1. Ventilating the building properly, i.e. a mechanical ventilation system with heat recovery or at least continuous extract is required and

2. no bulk water leakage into the assembly.

3. A constant temperature/humidity (20%°C 80%° RH)

Otherwise, it is impossible to predict what will happen, as conditions would depend on how the house is lived in.

• if the entire building is always very humid, then mould can grow, no matter how well-designed the building.

• If the roof/wall leaks, it doesn’t matter what you picked for assemblies – you will have issues.

• If appliances or your shower leak, or the moisture is not extracted, you will have issues.

You should also consider that condensation does not just occur on surfaces. It can occur with a wall or roof, which is called interstitial condensation. As this condensation is hidden, it poses particular challenges in diagnosis and treatment.

Analysis Options

The Glaser Method

The Glaser method is a simplified, standardised procedure used in building physics to assess the risk of interstitial condensation (condensation within building components like walls or roofs) due to water vapour diffusion. Developed by Dr. Helmuth Glaser in 1959, it is outlined in standards such as DIN 4108 and BS EN ISO 13788:2012.

The Glaser method calculates temperature and vapour pressure at each interface for a wall with layers (interior plaster, insulation, brick). If the vapour pressure exceeds the saturation pressure in the insulation layer during winter, condensation is predicted. The method then checks if this moisture evaporates in summer to ensure the wall remains safe.

The method evaluates whether moisture accumulates in a construction under steady-state conditions and whether it can dry out, helping to prevent issues like mould growth or structural damage.

The Glaser method is rough and ready, Suitable for simple, low-risk constructions or preliminary assessments. It is best used as a screening tool to identify potential issues or confirm low-risk designs

BS 5250:2021 guides when Glaser is sufficient versus when WUFI (per BS EN 15026) is needed.

WUFI

We input the wall or roof assembly, internal conditions, external weather, and many other parameters, such as angle of inclination (or colour), into the simulation software. We usually simulate at least five years and estimate the moisture behaviour hourly. We can extract graphs showing the progression over time of the moisture content of specific elements, as well as temperature and humidity. We assess risks from moisture accumulation, such as mould, rotting of organic materials, corrosion and spalling.

Unlike the Glaser method, the software does not provide a simple pass/fail result.

Wufi is suitable for predicting where moisture might accumulate within a construction. It details what happens between layers of material, i.e., at the interface between insulation and a breather membrane.

The Glaser method considers what happens at each interface, but Wufi also simulates what happens throughout each layer. The effects of this can be significant where there are materials that can store moisture (and heat) and transport liquid water by capillary action (and other mechanisms). This might sound exotic and only for niche products, but it applies to standard materials such as brick and timber.

To summarise.

Damp and mould problems usually occur when internal conditions are cold and moisture isn’t managed effectively. There are analysis tools that can help us understand why condensation and mould issues occur despite a warm and dry interior.

The type of tool to be used depends on the situation