My 1978 house has a brick chimney from a boiler cupboard out through the roof.  Inside the brick chimney is a 6” concrete flue.

The boiler has long been decommissioned.  The cupboard isn’t used, but as we’re having a new kitchen, we’re intending to put a proper door on it, and use it as a “pantry”.

The chimney was for an old gas boiler, previously oil.  The top has a metal top, but moisture does find its way in when it rains.

I intend to put a circle of celotex top and bottom of the flue, and was hoping to cap it with something like the below, but can’t find it in 6”.

   

This isn’t my chimney, but it’s the same.

   

What’s available to cap and prevent rain on windy days entering?

Is capping with celotex top and bottom ok? Thinking of sweating?

Cheers.