Post and beam Posted February 16 Share Posted February 16 First off i have to say i totally accept that this is true, i just cannot picture it. A membrane can be impervious to the passage of air but allow the passage of water vapour. The size of the molecules is where i cannot see this but hey ho ! Link to comment Share on other sites More sharing options...
IanR Posted February 16 Share Posted February 16 I had the same issue visualising this. I've settled on it being speed/rate of transmission. Moisture moves through very slowly, air could as well if the pressure differential was constantly +ve or -ve, but that slow movement of air is effectively airtight. I have an Egger DHF tongue and groove outer layer that is both my airtight layer (no membrane) and vapour open... Link to comment Share on other sites More sharing options...
SteamyTea Posted February 16 Share Posted February 16 (edited) It is a bit more than just the size of molecules. The 'weave' of the material will reduce the speed of the air because it has to take a longer path. So say the material is 0.1mm thick, but the weave means the shortest path is actually 1mm, for any given pressure, the time taken will be ten times greater. From that you would think that a water molecule with a slight + charge, rather than a nitrogen (charge 3-) or oxygen (charge 2-) molecule there would be no difference, but by choosing a membrane material with the right charge properties, water molecules can be attracted and made to stick in place, then normal evaporation can take place. Edited February 16 by SteamyTea Link to comment Share on other sites More sharing options...
jack Posted February 16 Share Posted February 16 Speculating, but: As a gas, water is just a bunch of free H2O molecules flying around. If they find a hole big enough, they'll work their way through. In contrast, the molecules in liquid water are attracted to each other due to their polar nature. The negative side of each molecule is attracted to the positive side of adjacent molcules. Surface tension is an example of this - the attraction causes water to form droplets rather than spread out in a molecule-thin sheet over a surface. That attraction causes liquid water to clump together in a way that impedes its ability to traverse pores that are big enough to allow individual water molecules to pass through. Link to comment Share on other sites More sharing options...
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