Increasing joist height to match other floors

[kangarolf]

We are creating an larger open plan space upstairs.

One room (older) has joists that are 18mm lower.

We also want to fit UFH using spreader plates between the joists.

I could fit 18mm T and G across the whole floor but then I've got double the wood width so the room will heat and cool more slowly that the other areas.

Is ok to glue/screw 18mm timber on top of the joists only to raise em?

Thanks for the sanity check.