Threads on the old forum indicate that cement render is an effective air barrier when trying to achieve air tightness.
Would this apply to lime render as well and how would it compare with cement render?
Does anybody have any data to quantify how effective these renders are as an air barrier?

If your render has hairline cracks every 12mm you've got serious problems - I suggest you hack off and ask for your money back.

Terry - I seem to remember someone doing succesful tests on lime plaster with straw bales but can't remember the details - I think there might have been a story on Plant Fibre Technologies website? But its down at the mo. Lime allows water vapour transmission on a microscopic scale - it will still block out draughts and air circulation - just like a windproof jacket will keep the wind off you, but let moisture escape.

>The principle is water easy-in, just like every kind of render, but then water easy-out as soon as the sun shines
Don't confuse water permeability and water vapour permeability. Having hairline cracks in your render will significantly increase your water permeability,as they will be big enough to soak up water droplets, (prob. under capillary action if they're hairline) but not water vapour permeability. Thats why under the British Standard for test for renders there are is a test for each, with each figure being stated seperately.

>unlike render that cracks in bigger panels, which obstructs re-evaporation thus acts as a one-way pump
How does a big crack obstruct water re-evaporation? Water vapour permeability is determined by the microscopic pore structure of the render - not by cracks. All a crack can do is increase the surface area exposed to air, but it doesn't change the pore structure. Thats why cement/acrylic renders are very "un-breathable" - even though they often crack badly due to poor elasticity. The acrylic blocks the pore structure.

>Don't think that render won't crack - it starts shrinking from the moment it's applied
Cement and lime both shrink as they dry and set, it varies depending on the precise material, even where it comes from. There are ways around it.

>You're lucky if you haven't got serious problems, if your cracks are much more than 12mm apart
Cracking can be determined by a number of factors - I'm not sure if you're talking about shrinkage drying (cure - reduces fines or cement content, reduce water addition with plasticisers) or cracking after the render has set (poss cures include stabilise background before rendering, use a more elastic render).

Humble apologies - I seem to have got all confused.
I was infact thinking about internal plaster rather than external render as Tom suspected.

In answer to the original question yes lime systems when used as internal plasters are just as effective as cement (or gypsum) systems. It is however very not the plaster that is the problem but how it is (or is not) joined to the many other parts of the building fabric that is a much greater source for concern.