Are glass textiles with PTFE coating used as vapor barriers for installation under the roof?
One reason could be the important general insulation/anti-mold rule for moderate climate zones like Central Europe: 

A vapor barrier should be close to the warmer inside layers. 
The insulation layer should be close to the cold outdoor side. And the inner window glass should be the coldest surface in a room, enabling condensation on a well defined suited surface.

That way the dew point most likely is not inside the wall or on the wall paper/plaster which has organic material to feed fungi.

To get precise information, this web site may help:

Ubakus.de

It contains many construction/insulation material, foil layers can be inserted everywhere to see the difference.

Possible problems will be shown in a second. 

Experimenting/researching is easy by switching on/off of layers. 

R-value can be shown by clicking on the spanner/tool icon next to the U-value.

It is free for private use.

Fastest way is to choose an adequate example (menu) and to edit the layers.