How to seal gap between sliding window frame and window sill?

[st22]

In this colder than usual winter I started to seal doors. And now windows. Turns out for all windows in this apartment and also all windows in the common area staircase there is a gap between window and window sill. It doesn't leak, just I am concerned about drafts and heat loss. Is it because such window is old?

 

Outdoor view, there is some resin flap , it is flexible at the bottom and attached along its top long side  probably allows water to go out but not in:

The question is, is this gap by design or is it safe to seal it with silicone?

Thanks

[EricL]

Hello @st22

Welcome to the Bunnings Workshop community. It's fabulous to have you join us, and thank you for sharing your question about filling the gaps in your window sill.

The good thing about silicone gap fillers is that they can be removed. As a rule of thumb if the gap is not interfering with the operation of the sliding window you can seal it with a gap filler. If you're worried about condensation getting trapped, you'll have to keep a bit of the window open when that happens. If you happen to start getting a puddle observe carefully where it occurs and see if gathers into a particular part of the window. I recommend removing a bit of the silicone if the water needs an exit point.

I suggest using Selleys 475g No More Gaps Interior Multipurpose Gap Filler it works on a variety of surfaces and has superior flexibility. 

If you have any other questions we can help with, please let us know.

Eric 

[st22]

Awesome, sealed one window. When sealing a second window, found it is leaking 5L of water per day, from the top - completely unrelated - going to get that fixed first - with strata most likely...

I will let you know if there will be issues with the sealing later on. Thanks for your advice.

[EricL]

Hi @st22

I'm glad to hear that the first window got sealed properly. I'm sorry to hear about the other one that is leaking 5L of water. That would definitely be a strata issue. There are products you can apply even if the surface is wet, but I suggest following your idea first and that is to have it repaired.

Please keep us updated with your progress, we look forward to seeing the window sealed and the leak repaired.

If you need further assistance, please let us know.

Eric