How to properly fit almuminum glass window without leakage?

Question

I got a brick-wall house. The wall is 25cm thick. I want to add windows. There are 2 ways of adding a window:

1. Adding the window on the inside side of the wall, so that if the aluminium frame of the window is 5cm deep, it means that there will be a 20cm of the wall exposed as "outside of house area".
2. Another option is to install the window on the outside side, so that the 20cm of the wall will be inside of the house, where I can make better use of it.

I like option (2), but the problem is that I heard rumours that (2) causes water leakage and that I better follow (1).

I am really doubtful of this rumour, and I think the real issue might be that the installation is not done properly.

My questions:

1. Is the rumour correct? And why?
2. Any special installations procedures for (2) to avoid water leakage?
3. If I adopt (2), should I leave some small wall-lip on the outside? E.g. instead of making the window flush against the outer wall, I pull it inwards by, say, 2cm, so that the after installing the 5cm-thick aluminum frame, 18cm is inside the house, 2cm outside the house?

Answer 1

If your brick wall is constructed correctly it should have an air/moisture barrier inside the wall. Then you need to attach the flashings of your aluminum window to that barrier which is usually a continuous membrane by window weatherproofing tape available at home improvement centers and install a waterproof sill with proper details on the bottom of the window to shed the rain out. the location of that barrier in the wall determines where you install the window. Here is a detail

If your brick wall is not up to standards and doesn't have a moisture barrier, you have to cut a rough opening and then cut a groove inside the thickness of the wall 5cm all around the opening and insert the flashing or flange of the aluminum windows into this grove and seal it with caulking all around. Again you need to install galvanized flashing to shed the rain on the outside face of the wall both on top and on the bottom of the window over the stone/ concrete sill. You need to hire a mason with experience in these types of work, or you may end up having a permanent leaking problem. In this case, you can install the window close to the outside face of the wall.

Attention to providing an uninterrupted all-around seal with metal flashing and caulking is imperative.