How do underlays manage to let water vapour through while keeping the rain out? Toby Cambray delves into the physics…

A while back someone asked me how breather membranes work, and I admit to being initially stumped. I’m writing this in the hope that I’m not the only one to whom the answer is not immediately obvious. To be clear, we’re talking about the sort of materials that are used as underlay in roofing, or on the outside of timber frames; they let vapour out so as to avoid moisture accumulation (or, if you insist, interstitial condensation) but by some witchcraft, they also keep the rain out. The H2O molecules are the same, so what gives? If you wanted to make a breather membrane, there are two possible approaches.

Firstly, we could make some sort of plastic sheet that’s thin enough to allow vapour to diffuse through fast enough. The drawback is that with typical materials, it would have to be really very thin – for example, common or garden 500-gauge polythene (that’s about an eighth of a mm) has an Sd value of around 50 metres – about right for a vapour barrier (though plain polythene isn’t really robust enough). But a “low resistance underlay” is, according to BS 5250, anything with a vapour resistance less than 0.05 m, so we’d need our breather membrane to be 1,000 times thinner. The cling film in your kitchen cupboard is roughly a tenth the thickness of 500-gauge, and 100 times thinner than that is really quite thin, with the obvious difficulties that brings (making it, and protecting it). Of course, if you can make a polymer that’s much more vapour open, you could produce it in more practical thicknesses.

The  second  approach  is  how  most  practical breather membranes work. If you’ve ever taken  a  moment  to  peel  a  bit  apart,  you’ll find  they  are  fluffy,  and  fray  at  a  torn edge, with  no  hint  of  a  continuous  membrane. They are made by spinning plastic into fine fibres  and  squashing  them  together  while hot  to  make  a  sheet.  This  effectively  creates a  sheet  with lots  of  small  holes.  This  means water  vapour  can  pass  through  much  more easily and we don’t have to manufacture insanely thin layers of plastic. But why doesn’t the liquid water come through? They are not working  by  osmosis,  or  behaving  like  some sort of molecular sieve.

The  answer   comes  down  to  an  important property  of  water  we  call  surface  tension. Surface  tension  is  caused  by  the  fact  that molecules  in  water  ‘prefer’  to  be  below  the surface  than  on  it.  This  manifests  itself  in several  ways;  for  example,  in  the  absence of  other  forces,  a  drop  will  form  itself  into a sphere. Although it is harder to see (apart from in special cases like the Prince Rupert’s drop – google this and thank me later), solids have surface tension too.

When a liquid interact  with  a  solid,  the relationship between the surface tensions plus that of air, sets up a contact angle, which we can  observe  in  the  shape  of  the  droplets.  If they are relatively flat, the contact angle  is high,  and  the  material  is  said  to  by  hydro-philic;  if  it  is  low,  the  water  ‘beads  up’  and the surface is said to be hydrophobic. As long as the contact angle is lower than 90 degrees, the water won’t be absorbed into the material, because the surface tension acts like a balloon, stoppin  the  liquid  ‘dropping  into  the gaps’. Incidentally, when you put a fresh coat of Nikwax on your breathable jacket,  you’re doing the same thing. And so, if the surface of our breather membrane is hydrophobic, it can have some gaps to let vapour through easily, but also prevent liquid water penetrating.

I wanted to focus on the physics of these materials  in  this  column,  but  I  should  also mention another disadvantage of most breather  membranes:  they  aren’t  good  for bats. Essentially, bats crawling up the membrane tend to pull those fibres out and can get tangled up and die.  There’s an excellent AECB webinar on the subject – do have a listen if you haven’t already. Details that avoid fibrous breather membranes are possible, and indeed bat-friendly membranes are available. Presumably these don’t rely on spun plastic fibres. I wonder how they work…

Originally published in Passive House Plus magazine

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