MVTR
Moisture vapour transmission rate is how much water vapour passes through one square metre of the material in a day, in grams. It is measured at a set temperature and humidity, usually hot and humid, so results can be compared.
Products that go soft or lose their crunch, such as chips, biscuits, and wafers, need a low MVTR. So do powders that cake in humid air.
OTR
Oxygen transmission rate is how much oxygen passes through one square metre in a day, in cubic centimetres, also at stated test conditions.
Oxygen turns fats rancid and dulls aroma and colour, so fried snacks, nuts, coffee, and dairy powders need a low OTR. Nitrogen-flushed packs only stay protected if the film keeps oxygen out.
How films compare
Plain BOPP is a good moisture barrier but lets oxygen through. Plain PET is better on oxygen. Metallising either film improves both numbers a great deal. Aluminium foil, when intact, is the strongest barrier of all.
Always compare figures measured at the same conditions. A number without its test temperature and humidity can't be compared with another supplier's.
Using the numbers
You rarely need to specify MVTR and OTR yourself. Tell your supplier the product, the shelf life you need, and where it will be sold, and they will pick films whose barrier fits.