Carpet underlay under floorboards

[EarlyInTheMorning]

Just removed 6 month old carpet in guest room and kept underlay because it was virtually new. We've put engineered floorboards on top of the carpet underlay. Have we made a mistake?

I don't know but I want to do similar so watching with interest

Can I use the existing carpet underlay with plank flooring?
We do not advise the use of carpet underlay as this is not suitable for plank flooring. It does not support the planks and would cause the boards to move, resulting in gaps in the floor.

Can I use existing carpet underlay beneath my new wood floor?
No you cannot. Carpet underlay has too much give in it. If you fit your hardwood floor on top of carpet underlay the boards will move and will either squeak or come apart - or both.

From googling "engineered floorboards on top of the carpet underlay" from your post.

I googled too but wondered whether they were just trying to get you to buy more stuff so wondered if anyone had any real experience of doing this.

To be honest, I thought I'd thought I'd missed something - I can't ever imagine thinking it'd be a good idea to put carpet underlay under an engineered floor. What would be the advantages?

Insulation? We have underfloor heating so it would never have occurred to me...

Insulation/soundproofing were what I was thinking- what would you do if your floor was concrete- would you bother with any underlay or just go straight for the flooring?

In our last house we had a concrete floor. We a sheet of something under the wood floor that went down, it was like that very slightly foamy polythene stuff you sometimes get in packaging. It was very thin,maybe 6mm? And not very expensive, less than £2 per m2. The difference is that it is much less squishy than carpet underlay and you get minimal movement between the boards. I think you can even get a sound absorbing kind, but maybe that is more expensive.

Jazz, mumsnetters, everybody, listen up
DO NOT PUT CARPET UNDERLAY UNDER FLOORBOARDS
[bitter emoticon]

Warning heeded! Dare I ask what happened?

Spongy floor, feels like laminate, creaks and moves. Will be taken up and put down all over again with proper underlay for floorboards. Darn it.