Hello

I am new. I am working on a moisture diffusion problem. I am trying to discover how important barrier tickness is to the rate that water vapour can get across a barrier.

Suppose I have a 1m x 1m barrier of say PE (polythene) sheet and it lets in X grams of water per hour through water diffusing through the plastic itself. Now suppose I make the barrier of polythene four times thicker. Does the rate of water transfer go down to 1/4 of what it was, or a 1/16?

i.e. Is the rate of water vapour transfer:

A. Inversely proportional to the just thickness of the barrier OR

B. Inversely proportional to the SQUARE of the thickness of the barrier?

Also does it matter if the water is transferring under pressure or by simple passive diffusion?

My thinking is that when under pressure it would flow much like electrical resistance - in which case to get the total resistance you simply add up the resistance of each bit of stuff in the way.

HOWEVER it seem to me that passive diffusion is different because the water molecules at any point are just as likely to do their random walks in the wrong direction - in which case the square of the distance seems more intuitive!

Either way we need to find a way to explain how ice not that far down in the Antarctic is exactly the composition that is was 100s of thousands (millions?) of years ago.

Many thanks

J