Exposed roof truss - Where is the vapor barrier?

If the 3-layer panels are processed airtight and connected airtight everywhere, it will probably work. Still unclear why a vapor barrier was not used.

Also incomprehensible to me ... the one who could explain it to us only writes emails telling us we are know-it-alls.

I spend a lot of money on it, so I want to know what was installed and why ... the site manager was also not very happy that we made such a fuss, after all the carpenter is not building the first roof truss....

But if something happens, then only after years and then I'M the one with the trouble ...

We have now contacted the manufacturer and our expert .....

Sleepless weekend behind me..... After we kindly inquired again with the carpenter, we received the information that the vapor barrier in our case is diffusion-open because we have diffusion-open insulation. The manufacturer of the wood fiber boards said that the 3-layer board can fulfill the function of the vapor barrier if it is airtight behind it with the Alujet. However, it depends on the SD value of the 3-layer board. According to what the company has assumed for now, we would mathematically be just below what we should have. In addition, the manufacturer’s instructions for the Alujet should have been considered in the construction, and these require the use of the nail sealing tape. The carpenter is now contacting the manufacturer of the wood fiber boards. He uses this construction method on all roof trusses with exposed sheathing. Actually, I now need a calculation to confirm whether it fits, and also an extended warranty from the carpenter. I can’t take it anymore, can’t anything just run normally one time…

Unfortunately, that is not unusual. Our architect (!) never understood the principle of dew point calculation and what it all has to do with diffusion resistances.

bei uns sei die Dampfbremse diffusionsoffen, da wir eine diffusionsoffene Dämmung haben.

By definition, there can be no diffusion-open vapor barrier. It does not help the insulation if it itself is diffusion-open, because if the dew point lies within the insulation layer, liquid water constantly forms there and the insulation soaks up the moisture. Mineral wool is also diffusion-open...

Maybe you could double-pane from the inside to increase the diffusion resistance on the interior side.

Maybe you could double-board from the inside to increase the diffusion resistance on the interior side.

But then I wouldn't have a visible roof truss anymore....


We are waiting for a statement from the carpenter and the manufacturer of the board......

Then I don't have a visible roof truss anymore....

That's a lie anyway, when a roof looks from below like it hasn't been insulated for over forty years.