can I used expanding foam round a combi flue?

[Bob]

I have recently moved into a house, where there is a large gap around the combi flue (i.e hole has been made, but never finished off). Can I used expanding foam to fill this gap, or do I need to use mortar?

[visionset]

You can, but I'd use mortar. If you do as you plan you really need to cover it with something such as a trim ring. PU Foam is not UV stabilised and will deteriorate in the sun. It is also soft and hardly a great external finishing material.

[Stuart B]

Maybe the OP meant on the internal wall.?

[John Stumbles]

On the internal wall I always use one-coat plaster. Foam would look terrible and risk over-expanding and spilling into the boiler.

Likewise I use mortar (sand & cement mix from the sheds) for outside.

[Stuart B]

But my reply was in response to

"You can, but I'd use mortar. If you do as you plan you really need to cover it with something such as a trim ring. PU Foam is not UV stabilised and will deteriorate in the sun. It is also soft and hardly a great external finishing material."