Well actually, Ventile's not TOTALLY reliant on it's swelling properties.
Ventile is weatherproof because:
a) It can be treated with Grangers or Nikwax cotton proof, allowing water to bead on the outer surface.
b) It uses 30% more yarn than conventional fabrics, making it extremely tight. Therefore, it is rather difficult for water to soak in.
c) Eventually, water will get through the above two factors. Because Ventile is an oxford weave, it swells, making the already tight weave even tigher. Uniform expansion of fibres does not work so well in twill weaves, IE: Gabardine.
d) Finally, good Ventile jackets can be made in the double layer format. This works on the principle that if the outer layer gets wet, the closely woven lining creates a barrier from the wearer's body to the outer layer. As well, save for the seams and zippers, the lining fabric does not touch the outer. This creates a very slight airspace that helps.
And there you have it, 4 reasons to buy a Ventile jacket.
(I used to think "Man, who would want a cotton jacket in the rain?" Poof. I get a double layer anorak for my birthday. Well, that soon changed my opinion!)
A.
