Airtightness and Windtightness Membranes

Understanding the Difference Between Airtightness and Windtightness Membranes

In energy-efficient and breathable construction, the terms airtightness and windtightness membranes often come up and understandably, they can be confused. While both contribute to the performance and longevity of the affected elements of a building, they serve each very different purposes and are used in different locations within the building.

In this article, we’ll break down what each membrane does, where it should be used including in walls, roofs, and floors; and why in the right circumstances both can be essential for high-performing, healthy buildings. 

What is an Airtightness Membrane?

Airtightness membranes are installed on the internal (warm) side of the building envelope, usually behind plasterboard or other internal finishes.

Their primary purpose is to prevent air leakage from the inside to the outside, helping to retain heat and reduce the risk of condensation within the structure. Airtight membranes can also help manage internal vapour movement, depending on their vapour permeability.

 

Key Benefits: 

• Reduces heat loss: Air leakage is one of the biggest causes of heat loss in buildings. By sealing the internal envelope, airtight membranes used in conjunction with proper insulation help maintain indoor temperatures and lower energy bills.
• Protects against condensation: When moist air escapes into walls or roofs and meets a cold surface, condensation can occur and lead to rot, mould and reduced insulation performance. Airtightness membranes help control vapour movement and help prevent this happening.

Examples of internal airtight membranes include DB3.5 and Sopravap Hygro

 

What Is a Windtightness Membrane?

Windtightness membranes – including breather membranes and sarking membranes – are installed on the external (cold) side of walls, roofs or sometimes floors. They prevent cold air and wind from penetrating the insulation layer and causing damage to the material which can affect the performance of the insulation. 

However, not all materials used for windtighness in the building industry are created equal! Some will be very effective in stopping damage to the insulation as described in the previous paragraph, but will also trap moisture within the structure because they are not vapour permeable. That’s where breather membranes come in…..

In the case of breather membranes, the windtightness membranes offer protection from the external air entering the building structure, but still allow water vapour to escape from fabric of the building. A win, win! 

Common windtightness breather membranes include PAVATEX ADB and PAVATEX UDB.

 (Both of which allow vapour to escape.)

 

Key Benefits:

• Prevents wind wash: Wind penetrating the outer layers of a building can disturb insulation and reduce its effectiveness. A windtight membrane forms a barrier against this.
• Allows outward drying (Breather Membranes only): Some types of windtightness membranes are vapour-permeable, so when installed correctly they allow any trapped moisture in the structure to escape, helping to prevent damp & mould. It is important to note that not every type available on the market is breathable however. 

 

Can Windtightness Membranes be used in Floors?

Yes, and it’s sometimes overlooked. 

In suspended timber floors, especially those with insulation between the joists, windtightness membranes can be extremely effective. Cold air circulating beneath a floor can degrade insulation performance through wind wash, particularly if the void is ventilated.

In floors, windtightness membranes:

• Act as a draught barrier beneath the insulation
• Help maintain the thermal performance of natural or fibre-based insulations
• Support breathable build-ups by allowing moisture vapour to escape (breathable membranes only) 

 

A breathable membrane like PAVATEX ADB may be installed below the insulation layer in the floor cavity (under the floorboards) to block wind without trapping moisture.

Our suspended timber floor systems use breathable membranes.