Water Ingress on New Installation

Hi this sounds like the windows have not been sealed down onto the cills so water is travelling inside down sides of external cills hope the helps thanks Dave

Based on the pattern of the issue, this appears far more consistent with water ingress than condensation. The moisture only occurs following rainfall and can be replicated with a hose test, which strongly indicates that external water is entering around the window rather than forming internally. Condensation would typically present more generally on cold surfaces (such as glass or across wider wall areas) and would not be triggered specifically by external water exposure. The fact that the damp is localised to the bottom corners of the window reveals further supports the likelihood of installation-related issues, such as inadequate external sealing, problems with drainage (weep holes), or disruption to the cavity tray/damp-proofing during installation. Given this evidence, I would ask that the installation be re-inspected with a focus on external sealing, drainage, and weatherproofing details, as the current symptoms are not consistent with condensation.

This can appear when the windows cills haven't been sealed properly especially between the window and the cill only way to check would be to take the windows out