Installing the window sill inside, what preparations need to be made?

Hello everyone,

we have completely replaced our facade. Previously, it consisted only of wood, thin wooden panels, and wooden windows. Now new windows have been installed and the facade has been reinforced. On the outside, an aluminum window sill was installed and underlaid with mortar, which is not even completely seal-tight. There is quite a bit of air coming through. :rolleyes:

Now we want to install interior window sills made of stone.

Currently, a wooden slat is placed on the stones and below the window the bare stone with holes can still be seen.
What precautions need to be taken?

I think just foaming it in would be the wrong approach. Should insulation such as XPS be glued in here? Can the window sill then be glued in afterwards or is more needed? I want to avoid thermal bridges and mold formation.

I am grateful for your advice.

There is quite a draft coming through here. :rolleyes:

OMG
Installed it yourself? That's just not acceptable.

Want to avoid thermal bridges and mold growth.

Then the windows should have been installed correctly in the first place.

No idea how to fix that. Air is coming in, so moisture isn't far behind.
Then the windows should have been installed correctly in the first place.

No, I don’t do that myself. There were a window fitter and a bricklayer. I also don’t think it’s ideal, but I trusted the tradesmen here. What would have been the right thing to do?

What would have been correct?

Google RAL installation, airtightness, driving rain resistance, correct window installation or similar. Then you will see a proper installation. With you = botch job.

The problem is that after the framework was installed, the windows were placed in front of the lower wall. I know that the sides were properly sealed with sealing tape. The mason then filled underneath with mortar, which is rather unsightly and certainly not according to RAL. But now the damage is done and I need a solution that does not involve - tearing everything out again. Surely something can be done afterwards with sealing tape, foam filling, or insulation boards to correct the issue to some extent.

If necessary, knock out the mortar below the windows again and put something proper in here. But before I approach it the wrong way, I wanted to ask first.