Some thoughts about which material should be used
for the gas window:

"In the tests ... all the operating conditions were under 
control except for the amount of water and oxygen. Kapton is very
transparent to H2O; and water present in the
ambient atmosphere therefore easily permeates
through the walls"

Taken from: http://www.hep.lu.se/staff/eerola/aging-2003.pdf
(Aging studies for the ATLAS Transition Radiation Tracker (TRT))

Well, as I was told by Tang about the HDC performance of the HKS chambers,
the air humidity is a problem (at least to the HV leakage currents).
In addition H20 in the drift chamber can have a negative influence
as a gas additive ... 

Used gas foil for VDCs build in Mainz, Germany:

Source: http://wwwa1.kph.uni-mainz.de/A1/publications/doctor/
(Disseration of Peter Sauer 5/1995 , Mainz, Germany)

Torayfan , 6um thickness, producer: Toray (Japan)

Torayfan is an Aramid film superior to Kapton: lower penetrability
for oxygen and water, higher tensile strength
see http://www.toray.co.jp/english/films/products/torayfan/index.html