Spray Foam Insulation & Vapour Barriers

Spray foam insulation is a popular choice for homeowners who want to improve the energy efficiency of their homes. It is known for its ability to seal gaps and cracks in walls, ceilings, and floors, reducing air leaks and preventing heat loss. However, one question that often arises when it comes to spray foam insulation is whether or not it needs a vapour barrier. In this article, we will explore the answer to this question and the reasons behind it.

First, let’s define what a vapour barrier is. A vapour barrier is a material used to prevent water vapour from moving from one side of a wall, ceiling, or floor to the other. Water vapour can cause problems in homes by creating condensation, which can lead to mould growth and other issues. A vapour barrier is especially important in areas with high humidity levels, such as bathrooms, kitchens, and basements.

Now, back to the question at hand: does spray foam insulation need a vapour barrier? The short answer is that it depends on the type of spray foam insulation being used and where it is being installed. There are two main types of spray foam insulation: open-cell and closed-cell.

Open-cell spray foam insulation is less dense and has a higher thermal conductivity than closed-cell spray foam insulation. It is typically used in areas where moisture control is less of a concern, such as attics and ceilings. Because it is less dense, open-cell spray foam insulation can allow some air and moisture to pass through it, which means that a vapour barrier may be needed in certain circumstances.

Closed-cell spray foam insulation, on the other hand, is denser and has a lower thermal conductivity than open-cell spray foam insulation. It is more commonly used in areas where moisture control is a concern, such as basements and crawl spaces. Closed-cell spray foam insulation can act as a vapour barrier on its own and may not require an additional vapour barrier to be installed.

The requirement for a vapour control layer is based upon a hygrothermal assessment of the structure to ensure that any condensation risks are identified and managed accordingly. In vapour-open building assemblies where the roof, wall or floor structure incorporates vapour permeable materials, it may be possible to omit a vapour barrier if there is confidence that any moisture can migrate through the building assembly suitably. 

Where non-breathable building assemblies are used, for example, a bitumen felt roofing underlay, moisture may migrate through the insulation and be blocked from escaping from the structure by the non-breathable roofing membrane. This can be particularly dangerous if there is a compound moisture drive into the spray foam insulation during winter months that does not suitable dry out during the summer. In these scenarios, a vapour barrier is recommended.

As an alternative to a vapour barrier, smart vapour control layers are becoming a more popular choice in new-build construction. These membranes adapt to the environment, opening to allow moisture to escape where it is suitable to do so whilst closing to prevent moisture from entering the insulation when there is a compound moisture drive into the building assembly. Before choosing the right vapour control system for the property, we recommend having a static condensation risk analysis carried out and where results are inconclusive or borderline, a dynamic WUFI-based hygrothermal assessment may be required.

Traditionally, vapour barriers are made from plastic and come in different gauges and thicknesses. It is important to choose the correct thickness of vapour barrier to ensure that it is robust enough to withstand constant vapour drive. When used, it is essential that vapour barriers are made airtight, using tape at the seams where the layers overlap. This prevents moisture from penetrating beyond the vapour barrier and into the insulation. 

In conclusion, whether or not spray foam insulation needs a vapour barrier depends on the type of insulation being used and where it is being installed. If you are unsure whether a vapour barrier is necessary for your particular situation, it is best to consult with a professional insulation contractor who can help you make the right decision. By taking the time to properly install your spray foam insulation, you can help ensure that your home is comfortable, energy-efficient, and free from moisture-related problems. If you would like to discuss a project and the requirements to manage the impacts of moisture, give our technical team a call on 01208 832602.