How to waterproof the roof to prevent mold inside?

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We have an addition to the house that needs to be converted into a living space (kitchen). So far, the roof has been built with a simple boardwalk structure and roofing felt (double layer). Now we have installed exterior insulation on the roof, including everything that goes with it (photo 1).

Now we're wondering how we should/could seal everything from the inside so that we don't have a mold problem. Up until now, this has been an unheated room, and the roof itself has been well ventilated – the unplastered part + the wood (see photo 1) was only covered with wood boards on the outside. We would like to hang the ceiling inside again on the surrounding bottom beam (green) (picture 2). But now we still need to ensure that the roof structure is airtight. Currently, between the roof beams at the front we can still see the exit to the outside (picture 3).

Does anyone have any advice on the best way to build the ceiling from the inside, including sealing (key word: vapor barrier)?

  • But before cutting anything off, make sure the overhang is not a support for the roof.
  • If you saw it off flush, it will wear down just like it does now.



I would put wool in the gaps and then put vapor barrier (climatic membrane) underneath, glue it to the mvt, and then plaster it. Plasterboard underneath, with a counter-batten.


I'd trim it with wood boards, one at the front, one at the bottom.

A little fancier, but it would be more labor intensive to saw off the overhangs, put rock wool in between, and then plaster with fabric to the top.


The first thing that comes to mind goes in a completely different direction. Will this addition also be insulated from the outside? The pictures don't show very well, but the wall construction consists of only one layer of bricks, doesn't it? Then the problem ultimately isn't the roof, but perhaps the transition to the walls, because they will probably always be significantly colder after the insulation work above

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