Hubble et. al. report Diffusivities for water in these polymers:

Mylar A (poly (ethylene terephthalate) ):      5.45 x 10-9 cm2/s at 30°C.

Kapton H (polyimide):                                3.45 x 10-9 cm2/s at 30°C.

Parylene C (poly(chloro-p-xylylene)            2.60 x 10-9 cm2/s at 30°C.                             

 

Crank, in his "Diffusion in Polymers", surveys diffusivity D of water in a wide range of polymers (in his Table V starting on page 274):

T

D x 109

ED

°C

cm2/s

kcal/mole

Polyvinyl alcohol

25

0.051

14

Wool

35

0.1

Vinylidene chloride (81) acrylonitrile (19) copolymer

25

0.32

20.2

Rubber hydrochloride

25

0.41

14

Nylon 6

25

0.97

Regenerated cellulose

25

<1

Polyethyleneterephthalate

25

3.9

10.4

Polyvinylbuteral

25

13

10.9

Polyvinylchloride

30

16

10

Cellulose Acetate

30

17

5.6

Polyvinylchloride (plasticized)

30

17

Polyvinylchloride

30

23

42

Cellulose Acetate (plasticized)

25

30

Polyvinyl acetate

25

43

12.5

Polyvinylchloride(87)-acetate(13) copolymer

32

60

7.7

Polyethylmethacrylate

25

105

8.7

Polymethylacrylate

25

120

16

Polymethylmethacrylate

50

130

11.6

Ethyl cellulose

24

180

6.3

Polyethylene (d=0.922)

25

230

14.2

Polyethylene (d=0.907)

25

240

16.4

Polydimethylsiloxane

35

~70000

3

Here we see that the diffusivity of water in all three of the film forming polymers Hubble et al. studied, Mylar A, Kapton H and Parylene C, are toward the lower end of the range of values for D in the Crank survey, although they are otherwise unexceptional.