Greetings!

How much factory soundproofing is there in the roof of a fabia? Anybody ever removed the headliner and took a lookt? Is it reasonable to put some extra sound/heat insulating mat in there?

Thanks!

The headliner IS the insulating layer.

It's quite chunky, but if you pull back all the plastic trims around the base of this layer, you can get above it if you wanted to shove more foam up there. I wouldn't use too much though as you may get bulging in places. Also, it won't take much weight to make the stock mat balloon downwards. You might be able to use double sided tape to stick to the roof itself, thereby negating any worries about the weight on the original headlining.

There are usually a few sound insulating pads stuck to the roof skin but they aren't all that substantial.