What’s the difference between internal and external vapour control?

If you’re building to Passivhaus standards, you will need to adopt airtightness and vapour control to ensure you reduce air leakage and thermal bridges in the building. This strategy is at the core of Passivhaus standards, but there are a few ways builders have approached this over the years.

Traditional airtightness methods have used silicone, paper and adhesive tapes. Although popular, these methods have quite a few downfalls, including labour intensity and long-term wear. While these methods have appeared cheap, their poor performance can end up costing quite a bit, in both cash and labour.

Vapour control membranes have been developed as an alternative solution to the methods above. Passive Purple is safe, sustainable, easy to apply and delivers the most consistent results that do not wear over time.

Internal vs external vapour control membrane

A vapour control membrane is used to create air tightness and control vapour, or moisture, within the building. When used internally, the goal of the vapour control membrane is to stop heat from escaping through cracks or weak points in the building. When applied externally, the goal is to reduce the amount of heat that can enter inside so that residents don’t cook themselves indoors in the summer.

While the membranes are similar, there are some differences between internal and external, primarily to do with the SD values. The internal membrane is designed to be more vapour closed, and the external membrane is designed to be vapour open.

Here’s how each helps your building to reach Passivhaus standards, reducing electricity bills and carbon footprints:

Internal vapour control membrane

Internal vapour control membrane is applied to air leakage paths to create airtightness, which only lets in a very small amount of moisture.

Internally the vapour control membrane would go on blockwork, timber frames, joints, ceilings, floor to wall connections and any other surface that could be likely to leak air. This application allows you to control the internal atmosphere, eliminate the need for heating and instead maximise the energy already being produced in the building. The bodies living and moving inside, the appliances like the fridge, toaster, kettle, and oven are all producing warmth, which is the substitute for heating in a Passivhaus.

External vapour control membrane

External vapour control membrane is applied outside of the building, on the warm side, to prevent heat from going inside and roasting those who live there. If the internal vapour control membrane is considered the winter warmer, the external vapour control membrane is what eliminates the aircon in summer.

The external vapour control membrane lets moisture come out while also being super airtight, weatherproof and watertight so that nothing can wear down its efficacy.

World's first Passivhaus certified liquid membrane

Passive Purple liquid airtight membrane

Passive Purple is the world’s first Passivhaus certified liquid airtight membrane. Unlike the methods before it, Passive Purple is a fully bonded, liquid-based membrane that can simply be brushed, rollered or sprayed onto surfaces. Just one person could cover 300m2 per day using a spray gun. All they have to do before use is stir.

Once applied, Passive Purple cures to form a rubber-like elastomeric membrane making it easy to apply over substrates, including spray applied foams and concrete, which would be difficult for alternative airtightness tools.

Both the internal and external Passive Purple liquid vapour control membranes are VOC free, fire class B, tested by the BBA, and certified by Passivhaus. All these checks mean that you are guaranteed performance.

Where to find out more

To learn more about the Passive Purple range of products and their technical details, visit the Passive Purple website or get in touch to learn more.

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