What Sort of Moisture Analysis Should I Have?

Moisture analysis is a crucial part of designing buildings that perform well over time. But when you’re working with internal wall insulation, especially on solid masonry walls, using the wrong method can lead to serious problems down the line. So which type of moisture analysis should you be using, and when?

In the UK, there are two common approaches. One is based on BS EN 13788 and is known as the Glaser method, or sometimes referred to as dew point analysis. The other is based on BS EN 15026 and is used in more advanced simulation tools like WUFI and Delphin. Both have their uses, but they work in very different ways.

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How the Glaser method works

The Glaser method is a steady-state calculation that looks at vapour pressure on either side of a wall and identifies where condensation might occur. It assumes that moisture moves in one direction, from the warm interior to the cooler exterior, and that it travels only by vapour diffusion.

This type of analysis is straightforward and widely used, particularly for lightweight constructions. But it was never intended for use with solid masonry walls or vapour-open materials. It also leaves out many key factors that affect moisture behaviour in real buildings, including wind-driven rain, solar gain, air leakage and daily or seasonal changes in humidity.

Because of this, it can be misleading, especially when applied to more complex build-ups.

What makes WUFI different

WUFI is a dynamic hygrothermal simulation tool that follows the BS EN 15026 standard. Unlike Glaser, it models changing conditions over time using real weather data. It also takes into account a much broader range of variables, such as:

• Rain penetration and capillary action
• Solar radiation and drying
• Internal humidity changes and air movement
• Moisture storage within building materials
• The orientation and shading of walls and roofs

This gives a much more realistic picture of how a building element behaves through the seasons and over multiple years. It allows designers to test different insulation strategies, check for moisture build-up and identify any long-term risks to the structure.

The problem with vapour barriers

One of the most common mistakes that comes from relying on Glaser is the assumption that adding a vapour barrier on the warm side of the wall will solve any condensation problems. While that might work in theory, in practice it can often cause more harm than good.

By blocking inward drying, a vapour barrier can trap moisture inside the wall. Over time, this leads to a gradual build-up of moisture, which can be particularly damaging if there is timber embedded in the masonry. WUFI analysis helps to uncover these risks early in the design process so that you can avoid them.

When is Glaser still useful

Glaser analysis can still be appropriate in simpler situations. Lightweight, ventilated timber frames or pitched roofs with low exposure to wind and rain are good candidates. We also find it useful for some flat roof build-ups, where conditions are more predictable and easier to model.

But when it comes to internally insulating solid masonry walls, Glaser simply doesn’t give enough information. It doesn’t reflect the complexity of how moisture really moves in and out of materials over time.

Getting it right from the start

The best approach is to choose your analysis method based on the construction type and the level of risk involved. For anything involving solid walls, internal insulation or natural materials, a dynamic simulation like WUFI gives you the insight needed to design safely and confidently.

At Back to Earth, we use WUFI to help our clients understand what’s happening inside their walls before any work begins. That means fewer surprises later and buildings that stay dry, healthy and robust in the long term.

If you’re not sure what kind of analysis your project needs, we’re here to help.