expanding foam, CH pipes and draughty stairs?

Our hall (well small square) has the CH pipes going up through a hole in the ceiling which is a bit draughty, there is lots or space around the pipes, can I fill the hole with expanding foam stuff or is that a no-no with the pipes?

Also our loft conversion has those half tread stair thingys, and looking at them last night, one side has quite a big gap between them and the wall (about 4mm) so was wondering if I can fill it with some silicone? Or is there something better?
Thanks :)
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