DuPont website about Kapton: http://www.dupont.com/kapton/

Kapton is indeed very transparent to humidity and oxygen ... see http://www.dupont.com/kapton/general/spchemical.html 

It seems that Mylar-G??? foils are a mucg better choice .

Mainz claimed that normally they are using the TX-1 Aramid foil as a gas foil, but this foil is only available to a width of 80cm. 
However Mainz decided to use Kapton instead of Mylar ...

Attachment
#1 Main Kapton catalog
.