If you have ever installed external wall insulation, you will know the key is making it waterproof. We are not talking about water going through the render itself – to be honest, all renders should prevent penetrating damp – instead we are taking about water finding a pathway behind the insulation, either at the top of the system or around windows and other openings.
There are lots of proposed mechanisms for sealing EWI systems, but in this blog we are going to look at the best options, particularly focusing on the top of the insulation system where the EWI meets the roof.
Extending roof lines to protect EWI systems
In an ideal world, the soffit of the roof will extend out far enough to mean the insulation can be completely covered and protected from water ingress, but adding 100mm+ of solid wall insulation often means this isn’t the case.
The best solution is therefore to extend the roofline – but this is also the costliest, both in terms of labour costs and materials. It does however ensure that water going behind the system will never happen, with the extended roofline coming over the new solid wall insulation system and directing water far from the walls.
Using verge trim to protect EWI systems
If this is not possible, you need to turn to verge trim; this is what is used in about 90% of cases. When it is done well and done with the right products, this can be as effective as extending the roofline, but the quality really does vary from installer to installer.
Our preferred solution is our Verge Trim 1 – this is the perfect solution for preventing water ingress at the top of the insulation system. It essentially has a ‘T’ at the back of the trim. This ‘T’ is slipped up behind the fascia board and then screwed into place through the bottom of the ‘T’.
There is a 15-degree drop on the verge as it extends away from the wall, and then it has a drip on the leading edge to help prevent water running back along it. Verge Trim 1 is available in two different sizes to house different thicknesses of insulation. The 140mm Verge Trim 1 is used when you are using 90mm or 100mm in your solid wall insulation install. The 110mm Verge Trim 1 is used for 50mm or 70mm.
The beauty of this verge trim is the fact it goes up behind the fascia board, so there is no change of water ingress when this is used, offering a completely watertight seal.
Verge Trim 3 is very similar to Verge Trim 1, although it is fixed in place by screwing through the fascia board. If you are replacing the verge trim at the same time, you can remove the fascia board completely before installing verge trim 3, and screwing it securely in place at 300mm centres. It is then possible to replace the new fascia board in place and you won’t need to screw through the fascia board at all.
Again, like Verge Trim 1, there is a 15 degree drop on the verge trim itself, helping to ensure any water falling on top of the trim is directed away from the property. It is also available in 110mm and 140mm varieties, depending on the thickness of the insulation used.
Verge Trim 2 is very popular although perhaps the most basic of our verge trim covers. This is simply attached to the wall and then where the verge travels away from the wall, it must be siliconed – ideally with CT1 or another hard-wearing silicone sealant. A combination of the verge trim and the silicone sealant will help prevent water ingress. Like the other verge trims, there is a 15-degree angle on the verge itself (which covers the insulation), and it is available in either 110mm or 140mm varieties.