Introduction

Vapour pressure is the pressure exerted by the vapour phase of a substance on its surroundings. It is an important factor in determining the behaviour of water vapour in building fabrics. The distribution of vapour pressure through building fabrics can have significant implications for building performance, energy efficiency and indoor air quality. In this article, we will discuss the distribution of vapour pressure through building fabrics and its impact on building performance.

What is vapour pressure?

Vapour pressure is the pressure exerted by the vapour phase of a substance on its surroundings. It is the measure of the tendency of a substance to escape into the air as a gas. The vapour pressure of a substance is affected by factors such as temperature, humidity, and pressure. When a substance evaporates, its vapour pressure increases. Conversely, when a substance condenses, its vapour pressure decreases. This is why in winter months, you might notice condensation forming on windows and remaining in place for a period of time because the temperature conditions are not adequate to help evaporate the condensation and turn it into vapour.

If you boil a kettle, you will notice steam or vapour releasing from the spout. The vapour rises initially and then becomes invisible but if you were to hold a plate a few inches above the spout, you’ll notice that it condensates as the vapour pressure becomes trapped and cools. This happens on a larger scale within our homes when we shower or bathe, cook, clean and breathe – the vapour we omit is mostly invisible until it cools as it rises. Imagine vapour migrating to the structural building assembly and becoming trapped by non-breathable materials. It will condense as it reaches a cooler temperature and then sit within the building fabric. If this happens regularly and compounds over time, its possible for the structural assembly to become sodden with moisture.

Vapour pressure and building fabrics

Building fabrics are materials that make up the envelope of a building, including walls, roofs, floors, and windows. These materials can be classified as either permeable or impermeable to water vapour. Permeable materials allow water vapour to pass through, while impermeable materials do not.

The distribution of vapour pressure through building fabrics depends on the permeability of the materials. Permeable materials allow water vapour to pass through, so the vapour pressure is distributed evenly throughout the material. Impermeable materials, on the other hand, do not allow water vapour to pass through, so the vapour pressure is concentrated on the surface of the material.

The distribution of vapour pressure through building fabrics can have significant implications for building performance. If the vapour pressure is concentrated on the surface of an impermeable material, it can lead to the accumulation of moisture, which can lead to mould growth and other moisture-related problems. If the vapour pressure is distributed evenly through a permeable material, it can help to prevent moisture-related problems by allowing moisture to escape.

Does ventilation solve the problem?

Good ventilation is vital in achieving good indoor air quality and management of moisture within buildings. Older methods of construction focused on passive ventilation in the form of wall cavities, attic/loft eaves and other openings within the property whereas modern methods of construction focus on airtightness and insulation. When a property is insulated to an airtight standard, the offset is to increase the ventilation by implementing extraction or heat recovery systems such as MVHR. 

Part F of the Building Regulates stipulates the requirements for ventilation in “wet zones” such as bathrooms, kitchens and utility rooms and this is why a cooker and bathroom is likely to have an extractor fan that runs when switched on. A robust ventilation strategy will help to manage or eliminate moisture within homes, stopping vapour pressure from building and attacking the structural building assembly. Even in vapour-open assemblies, too much vapour may struggle to pass through the building efficiently and may become trapped.

To find out more about the requirements for ventilation in dwellings, it is worth reading Part F of the Building Regulations here: https://assets.publishing.service.gov.uk/media/61deba42d3bf7f054fcc243d/ADF1.pdf

Conclusion

In conclusion, the distribution of vapour pressure through building fabrics is an important factor in determining building performance, energy efficiency, and indoor air quality. Permeable materials allow water vapour to pass through, so the vapour pressure is distributed evenly throughout the material. Impermeable materials, on the other hand, do not allow water vapour to pass through, so the vapour pressure is concentrated on the surface of the material. Building designers and engineers should consider the permeability of building fabrics when designing and constructing buildings to ensure that they perform well and provide a healthy indoor environment. A dynamic hygrothermal analysis using WUFI software can generally identify whether a building is able to cope with vapour pressure and condensation. Speak to our technical team on 01208 832602 if you require any information about vapour in buildings.