Has anyone put insulation under carpet and underlay?

[dancerdog]

Just that really. I have a really cold room, above a garage. Can't insulate from underneath, can't take floorboards up to insulate between the joists.

I was planning to use the thickest underlay I can find, but have now seen a product called Floorfoam which is supposed to be used under the underlay. Just wondering if anyone has experience of this, and if so, did it make the carpet move about?

I have insulated underlay. (Not sure what the brand name is). It doesn't move but that's probably due to the fact it's stapled to the floorboards and the carpet over it is fitted using good gripper strips. It does make a slight crackling noise when you walk over it though.

Could you use celotex insulation board then board on top then underlay and carpet? You’d lose circa 10cm height in the room but it would be noticeably warmer.

Thanks for your comments.