In roofing systems, it is important to consider the risks of condensation and mould, and how to prevent that as much as possible. Flat roofs are usually insulated very well. However, if the roof is not designed well or constructed poorly, the consequences can be severe.

In this article, we will cover:

• The Glaser Method
• Vapour Control Layers
• Wonderbuilds Vapour Control Layer

The Glaser Method

The Glaser Method (BS EN ISO 13788) is a method that can be used to calculate the likelihood of condensation inside building structures. It can be used to compare different roof or wall designs and seeing how design changes affect performance.

However, this method has limits. It does not consider:

• Air movement through materials
• Sunlight and heat loss at night when there is no sunlight
• Wind and rain
• Moisture already in materials or rainwater absorbed during the construction phase
• Junctions and interfaces between different parts of the building

Therefore, the Glaser Method cannot give a fully accurate picture of what happens to moisture inside buildings in real-life conditions.

What Designers Need to Consider

Designers should look at other factors that could affect condensation, such as:

• Sun exposure and night cooling
• Wind and rain
• The way a building is used and how much water vapour is created inside

Too much moisture indoors can cause condensation and mould. This can damage the structure and affect the health of people living in the building.

Managing Moisture

To manage moisture risks, designers need to understand the link between heat, air, and water vapour. They must think about:

• Where moisture comes from (construction materials and building use)
• How it moves through the building
• How to remove it before it causes problems

Many construction materials (like concrete, plaster, screeds, and mortars) contain water when mixed. This water is only needed for workability and curing — it is not part of the finished material. If this excess water or later moisture from building use is trapped, it can create long-term problems. It is important to minimise trapped moisture during construction.

Vapour Control Layers

A Vapour Control Layer is a roofing felt that can be used to prevent moist air from entering the structure of a building. It is installed on the warm side of the insulation layers. To ensure that the VCL performs well, side and end joints should be kept to a minimum.

Wonderbuilds Vapour Control Layer

At Wonderbuilds, we offer SA VB600, which is an effective vapour control layer that can be used in built-up roofing systems.

Wonderbuilds SA VB600 needs to be used in conjunction with Wonderbuilds SA Primer.