Silicone sealant, often referred to as mastic, is a sealant used in hundreds of different applications. You will probably find it around the house sealing pipes and plumbing, gaps around doors and windows and perhaps around your bath and shower.

Silicone sealant, not to be confused with Silicone Render, is also used to seal solid wall insulation on some properties.

Where is Silicone Sealant used in EWI?

There are several areas where an installer might use silicone. For example, you may see it around windows where the render meets the frame, on oversills where it abuts the wall and around verges where the trim meets the wall or the fascia.


This isn’t ideal because most silicone will not bond with cement very well. On a job where the expected lifespan of the system is 30 years or more, you can be sure that the sealant will need replacing a lot more frequently than that. In an exposed position, this sealant may even perish within a year! That means water ingress behind the insulation and damp inside the property.

How Can I Avoid Using Sealant?

If that sounds like a problem, you might want to consider alternatives to sealant. The best option is to design your project to minimise the use of sealant. Beading is really important around windows, under window sills and at joints. Beading not only creates a good seal, but it also means you get a nice straight edge – and a professional finish. For example, our Reveal Beads come with either a sponge or rubber seal to minimise the use of mastics.

Expanding foam tape is also another option to consider for minimising the use of silicones. The expanding foam tape can be used to create a weather-tight seal in structural and expansion joints in walls and around door and window frames. When installed into a joint, the tape will begin to expand to fill up the gap and smooth over minor imperfections and irregularities.

At the roof line where the verge meets the fascia, it is usually possible to create a joint that is not fully exposed to the elements. You will probably still need to use some sealant, but creating a joint that is protected will mean the sealant lasts a lot longer.

What Sealant Should be Used for EWI?

If you have no choice but to use sealant, we recommend that it is kept to a minimum, and that you use a good quality sealant that will offer a long term solution. One of the best sealants on the market is CT1, which we stock. The difference between CT1 and standard silicone sealants is that this sealant adheres to plastic as well as to brick and cement. This means you will get a quality seal between areas like verge trims and walls.

Whilst we would recommend trying to achieve a sealant-free profile, especially in areas where water is likely to interact with the joint frequently, we understand that this is not always possible, and in these cases a good quality sealant is a good alternative.

Most importantly, don’t be put off insulating a property just because you are worried about sealant. If you are struggling to work out how to seal your job, just get in touch and we would be happy to advise about sealant, beading and verges, and how to create a profile that is protected from the elements.

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