Sensible Topic New double extension - wall insulation suggestions please

[shabbaranks]

Hi guys - especially any builders out there.... Im currently building a 2 storey extension and with the current costs of materials was wondering what people are using as insulation for the walls? Previously Ive used PIR\celotex but noticed there are other solutions out there (Im not a builder) and with the costs of raw materials at the moment thought I would see if there are cheaper but just as good alternatives?

Also any links to suppliers would also be appreciated - we need a cavity filled so 110mm

Thanks!