Depending which type of Mylar foil is choosen, the transparency for oxygen and water vapor
can differ a lot.

So far it seems to me that Mylar GX-P from the G series has the lowest transparency.

Note: metallizing the film with e.g. aluminium will improve these properties,
      at least it will not worsen it

see http://www.dupontteijinfilms.com/FilmEnterprise/FilmTypeSelector.asp?Version=US

Property  	              Thickness 	Value  	Units    	Test
BARRIER                       (Gauge)   
Gas Permeability -O2, 24 hr  	48  	         0.02  	cc/100 inē  	ASTM D1434 30°C/70% RH/1 ATM 
WVTR  	                        48  	         0.005  g/100 inē/day  	ASTM F1249 40°C, 90% RH 


FYI:  4 Gauge = 1 micometer -> 48 Gauge = 12 micrometer , stupid unit ...

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