Weather proofing under the window cills
November 6, 2014
We have now completed the weather proof layer that goes under our aluminium window cills. We used the Pro Clima tape 'Extoseal Encors' which is a butyl flashing tape. This tape has an acrylate additive which means that it is also easy to install in both low temperatures and humid conditions - we used it over the course of a few days with very varying weather, from sunshine one afternoon to cold and light rain the next, and it was just as easy in each scenario. It is incredibly malleable and as such can be pressed, moulded, and stretched to some extent, around both internal and external corners. As the tape can tolerate some standing water we felt it was ideal for protecting the Gutex under the powder coated aluminium window cills. It's high adhesion qualities meant that the Gutex didn't need to be be totally dry as it can also bond onto a slightly damp surface, although we did find it stuck much better when the Tescon Primer RP was left to dry for a little while.
I have shown some photos below that take a step by step look at the process we went through on each window...
Firstly we primed the base and sides of the reveals, and a short distance down the front of the elevation both below and to the sides of the opening - basically everywhere that was going to come into contact with the Extoseal tape. You can just see the primer where it has already dried on the surface of the Gutex in a slightly darker colour and the whiter patches where it is still drying...
We masked the window frame that will ultimately be exposed to make sure we didn't get any primer on it as it is extremely sticky stuff! As you can see we also had to add a 20mm thick layer of insulation to the base of the window reveals and we used this additional piece t0 create a slight fall to help shed any standing water from the Extoseal.
Next we added two small patches, one on each side, behind where we were going to have to make a relief cut in the top layer of tape - this will become clearer later on!
Then we measured the main piece of tape, allowing an additional 200mm up each side of the opening. We sat this centrally in the opening ready to be stuck in place. The Extoseal tape we used here is 300mm wide and has 3 release strips on the back which is really useful as it allows you to use the smallest one (approx. 20mm wide) to stick the tape to the window frame first, and then you can incrementally move along the tape pressing it into place and removing the next two strips separately to make sure that it doesn't just get stuck to everything in sight! You can see the tape folded back at the line of the smallest release strip here:
As we have a larger amount of frame exposed at the base compared to the sides of the window we had to fold back and trim the first 20mm of the tape down to 10mm wide so that we got a nice neat corner when the two were folded one on top of the other. We worked this out after the first window which was a bit of an experiment! However, once we had established that this technique worked best for our scenario it was pretty straightforward to apply. (The narrower distance at the sides of the window frames is a result of us only having a limited space left at the side of the frames due to the over-insulating, and an additional cill piece that the manufacturer added.)
The next photo shows the side piece folded back and the edge that will go up the side of the window frame trimmed back to 10mm wide:
We basically then worked from the centre out, pressing the first 20mm of tape firmly down and back against the window frame, towards the corner. When we reached the corner the base was stuck first, and the trimmed-down side piece overlapped and stuck on top. One thing to look out for: due to the section of our window frames it is possible to feel up and underneath the projecting piece of frame to make sure that the tape is not obscuring the drainage holes - this will be different on varying window frame sections but it is important to make sure that any drainage slots or holes are not covered by tapes etc. as the installation progresses.
After the tape is securely stuck to the window frame we folded it back and cut the release film from the next layer in the centre. Then, again, working from the centre out we stuck the tape down pressing firmly all the way and making sure there were no air bubbles along the way. Finally, the last release strip can be cut in the centre and pulled back to allow the last section of tape to be stuck down, as shown below.
Once this was done we made a relief cut in the tape to allow it to work around the external corner and stick firmly over our earlier patches. As we were initially unsure exactly how malleable the tape was we decided this was a good 'belts and braces' approach to the waterproofing at these corners. In the bottom of these two photos you can see the fold we achieved at the internal corner: again for a 'belts and braces' approach we added a secondary strip over this junction to make sure there were no gaps (not shown here for clarity).
The next three photos show the before and after of the aluminium cill installation. These were made for us by Spectrum Installations to one of their standard cill patterns: they have a 3 degree fall and a 25mm upstand at the rear which is screwed through the Extoseal into the timber window frame. The Extoseal is 'self-sealing' when fastened through as it is so stretchy, and this renders the holes waterproof.
Finally, in case you are wondering why the aluminium cill stops behind the cladding batten, we are planning to take our cladding into the window reveals and over the cill to give the building a more solid and sculptural appearance when complete!