In article
s.com, Jim writes

That is to say, fix all the floorboards, then lay celotex/kingspan
(I'll have to look those up), then foil tape on top of that, and then
let the fitters lay underlay and carpet ontop of that?

Celotex is a foil-backed insulation mat, about an inch thick. It
wouldn't stand up to being walked on, even with underlay and carpet on
top. It's used for things like insulation between the roof joists
during a loft conversion, for example.

I don't agree with TNP here. There would be some heat loss through the
floor, but not much, certainly not worth the effort of following his
suggestion of lifting the boards and fitting celotex between the joists,
then re-laying. As others have said, you'd also be creating pockets of
stagnant air between the celotex and the floorboards.

My opinion.

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