Installing Foam roofing insulation directly over joists?

In my local B&Q last week I was reading the packaging of their 'Space Board'
product, sheets of foam insulation made by Knauf. I am assuming, never
having seen any, that this is similar to Kingspan or Celotex?
Anyway, since I'm thinking of properly insulating, and maybe boarding, the
loft, at some point soonish, I read the installation instructions.
They say, and the website confirms:

that you can put the board directly *over* the existing joists, and then put
chipboard on top, screwing through the chipboard and foam into the joists.
Does this work? It seems an attractive method to get a decent total
thickness without fitting additional 4x2s across the joists etc., as I've
seen recommended here. But I'd worry that the foam would deform under the
chipboard, leading to a horrid result in time.
And if it does work ... would it also work with Celotex/Kingspan?
Thanks for your thoughts
Jon N
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The Night Tripper

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