ᐅ Adding an exterior windowsill after installing external wall insulation (EWI)

Good evening,

We have a bay window that extends 1.5m (5 feet) outward and has a narrow patio door on one side. This door is only opened for cleaning or ventilation, not for going outside.
There was originally supposed to be a windowsill here. For all the other windows (which are not floor-to-ceiling), we have aluminum windowsills.

Is it possible to install a windowsill here afterwards? For the others, these actually extend slightly into the plaster or the external thermal insulation composite system (ETICS). That’s difficult to do here.

The window faces north and is relatively sheltered since the house wall is directly to the left. So this window will not be exposed to much rain.
The bay is single-story, while the wall on the left side extends up two floors.

Sure thing. However, I wouldn’t recommend aluminum, as it can easily get scratches and dents. Choose something made of artificial stone, set in a mortar bed, and seal it with color-matched silicone (make sure it is suitable for exterior use, which usually means it contains silicone).

But how do you connect to/into the ETICS?

Well, we have aluminum window sills everywhere else. Since you don’t step out there anyway, I don’t really see the risk of dents or scratches.

I don’t know your specific situation, but I can tell you that aluminum is usually not used for this. However, whether aluminum or porcelain stoneware, it should fit precisely and then be sealed with silicone containing silica. I can’t recommend any specific products, but any hardware store will be able to assist you.

Short answer: no.

The insulation is neither walkable nor can you fit the connection properly since it is embedded in plaster.

L-shaped brackets with special thermal bridge screws are fixed to the masonry; the insulation goes around that, then the window sill is installed, and then it’s plastered.

This way, the load is on the masonry and everyone is happy.

The builder has already sealed the insulation on top, but I would also apply a bitumen membrane with cold bitumen adhesive. These are available in various widths as cavity membranes. Cut it to size and stick it on with bitumen cold adhesive. You can throw the brush away afterward and make sure to wear gloves.

You can then have aluminum tread plate trimmed and cut to length. However, this will look more or less semi-professional, and you probably won’t get it perfectly fitted. Seal the sides with silicone.

The problem is that the opening is not squared.

What will be placed in front of the window?

Otherwise, leave the roofing felt membrane there for now; it’s sanded and doesn’t look too bad at first, and it’s better than the current state.

No one should step on it afterward. I won’t be doing it myself, but my general contractor will. As mentioned, he wants to simply glue the aluminum window sill in place, without plastering the sides. This isn’t really the professional method, but the question is whether it might be sufficient in this case since it’s only half a meter (about 20 inches), right near the corner, and the window faces north anyway.

A flower bed will be installed in front of it later.