I’ve not field tested it yet, but “Super Strength Reynolds Wrap” aluminum foil appears to be the only aluminum foil that's usualy available that is thick enough to be impermeable to water and oxygen. Although its on backorder until next week at Alcoa’s (Reynolds) store/website ( MyBrands.com ), its usually available there at retail in the 18" x 25' rolls (it is expensive). I thought it might also be thick enough so that a reasonable amount of folding, wrinkles, wear & tear (assuming daily carry) would not create pinholes.

From Wikipedia: “Aluminum foils thicker than 0.025 mm (0.001 in) are impermeable to oxygen and water. Foils thinner than this become slightly permeable due to minute pinholes caused by the production process.”

Below are examples of aluminum foil gauges:

Inches Microns
Standard (generic) 14.000
Standard (extra thick - generic) 16.000
Boardwalk - extra standard 16.000
Standard Reynolds Wrap* .00064 16.256
Standard (generic) Heavy Duty 20.000
Boardwalk - heavy duty 20.000
Heavy Strength Reynolds Wrap* .0009 23.622
Standard (generic) Extra Hvy. Duty 24.000
Super Strength Reynolds Wrap* .00137 34.798
35 gauge aluminum pie pan .01000

***The standard tolerance for Reynolds Wrap aluminum foil is: +/- 10%.

Micron X .00003937 = inches
Millimeters X .03937 = inches
a mil is a unit of length equal to 0.001 inch
micron is one millionth of a meter, or equivalently one thousandth of a millimeter