How diffusion-tight around windows?

[Ta i trä]

How can I reasonably manage to make it diffusion-tight around the windows in my new construction (see attached image).

The gap between the window and the frame, filled with foam sealant, is about 10 mm. The reveal will be 12 mm thick so, in my opinion, there is no chance to tape the plastic to the window with 2 mm to spare (even though my house supplier thinks it should be done that way!?).

Is there any other way to solve it? Is it even worthwhile & important to try to make it tight?

Thanks in advance.

 

[huggan]

Doesn't matter if there's tape behind the moldings, does it?

 

[Ta i trä]

It doesn't matter that there is tape behind the smygen, but if I am going to tape over the 10 mm gap and cover it with a 12 mm smyg, I have about 2 mm of space to attach the tape to the window frame so that the tape is not visible at the side of the smygen. My "experience" tells me that it won't succeed...

 

[huggan]

No, it won't work.

 

[Ta i trä]

No, exactly what I thought.

Suggestions on how to solve it instead?

 

[huggan]

The foam spray might be diffusion-tight.

 

[Ta i trä]

So there is a difference in the density of expanding foam when it comes to sealing? Then there will be some exposed wood too if I don't cover it with plastic/tape. That wouldn't count as sealed, right?