There are a number of membranes now being offered to the façade industry for sealing around windows, doors and façade interfaces, both for new-build and façade remediation projects. However, due to varying product specifications, different methods used to test them, and even a degree of ambiguity around the regulatory requirements, identifying the appropriate membrane for each application can still be challenging.
There may also be a tendency to view window and façade sealing membranes as a relatively insignificant part of the fabric of a building, but the integrity of the façade, and indeed the whole building, is 100% dependent on each sealing membrane performing its specific function correctly for the lifetime of the building.
So we must ensure that the type of membrane that’s installed in the façade through-wall, is not only compliant with the Building Regulations, but has been tested to the appropriate performance standards. Also, most importantly, we need to be certain that it will perform its primary design function.
There are two main types of membrane that are installed externally within a façade – a Breather Membrane, and an Interface Sealing Membrane. The two are made of different materials and have very different performance features. So it’s critical for the integrity of the building that the two different membranes are not confused and are installed in the right places, so that they can perform the function that they are designed for.
An Interface Sealing Membrane is used to seal vulnerable points for potential moisture ingress, such as around windows, doors, and curtain walling, as well as along slab edges and sheathing board-to-concrete interfaces. For many years, this membrane has been made from EPDM, a type of rubber. EPDM has been used because of its durability, flexibility and excellent resistance to water penetration – all features which have proven essential when sealing these vulnerable points.
But, as we know, the big drawback with EPDM is that it is combustible, and Approved Document B requires membranes used above ground in the external wall on relevant buildings to be fire-classified to class B-s3, d0 minimum.
Consequently, new fire-classified alternatives to EPDM have been developed, which, crucially, still have the water and air-tightness properties of EPDM that are necessary to create an effective seal around windows and interfaces. However, if we prioritise fire classification without considering the critical performance requirements, we fail to understand the fundamental reason for using the membrane in the first place.
Some suggest that a breather membrane type material can be installed to seal a window externally, supposedly to allow any moisture to escape, following an inside-tighter-than-outside approach. A couple of points to note on this;
- The installation of a breather membrane layer within the through-wall (not as a seal around windows) is essential to provide a water-resistant barrier that protects the building, particularly during construction stage before the façade is installed. But, due to its micro-porous character, it also allows any water vapour that may be within the external wall to escape outwards. So to use the window surround as a specific point for any potential moisture to escape is therefore unnecessary.
- When it comes to a membrane that’s installed to seal around windows, it’s clearly far more important that it will prevent water penetrating from the outside, than to facilitate the movement of any potential water vapour from the inside. The breather membrane layer throughout the whole façade allows the latter anyway.
- A correctly specified and installed through-wall includes an internal Vapour Control Layer which restricts the movement of warm, moist air from inside the building entering the external wall build-up. Should any moist air find it’s way into the external wall, it will escape through the wall and out through the external breather membrane.
We need to be sure that we fully understand the purpose and function of the two very different façade membranes. There are documented occurrences of serious legacy issues due to moisture ingress because this was not understood.
So, here’s the question.
Should an external window membrane be completely watertight and airtight to prevent moisture from getting in? – or should it not be completely watertight and airtight, ie. micro-porous, to allow any potential moisture to escape…. something that a correctly designed and installed through-wall allows anyway?
Let’s get it right first time, every time.
