Go to Forum Home Building Refurbishment and Retrofit Wall breathability – differences between external and internal surfaces

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    • #31642
      Peter Draper

      Hi AECB,

      Does anyone know the optimum ratio of breathability between the inside to the outside of a wall?

      I know that the internal side of a solid wall should be less breathable than the external. By having a more breathable external finish it 'pulls' any moisture in the wall to the outside, thus keeping the inside drier. I also know that differences can be achieved through simple mechanical effects like surface areas, i.e. a smooth inside finish is effectively less breathable than a rough external render even if the same materials are used. It can also be achieved by using different types of lime or aggregate. Lime washed lime putty with limestone aggregate on the outside with a hydraulic lime inside, or a hydraulic with sand internally.

      Anyway, I have picked up the that outside should be at least 3 times more breathable than the inside, but cannot remember where I heard this. Does anyone have a source that they can point me towards please?



    • #38450
      Mark Siddall

      4:1 is the ratio that I know. Mentioned in The Whole House Book, By Cindy Harris & Pat Borer and some other books that I have.

      Some simple calcs should make sure that you are on the right track also – BuildDesk U does a simplified hygrothermal calculation using the Glazer method i.e. BS EN ISO 13788 -2002


    • #38451
      Peter Draper

      Thanks Mark, appreciated.

    • #38452
      Tom Foster

      5:1 AFAIK! But still, the question that no one can answer is – where did this formula originate?

      The Glaser calc seems to works adequately if the higher-resist 'inner' layer is actually not fully inboard, but up to say half way outboard thro the insulation sandwich (but don't take my word – check it for yourself).

      E.g. p'b'd + skim on 95×50 studs or 145×50 rafters filled with Warmcel, sheathed externally with 9 Smartply OSB3, 150 EPS EWI outboard of that, acrylic rendered. The OSB forms the 'inner' resistor (the Warmcel and p'b'd + skim inboard of that being near-transparent), around 5x as resistive as the EPS + acrylic render outboard of it. The OSB, glued and screwed, also forms the foolproof, easy, longterm-durable, unbroken airtight membrane (which it wouldn't be if sheathed inboard of the studs).

    • #38453
      Nick Grant

      5 is the figure in my head but it's a bit academic given all the other variables that ae also important such as eliminating air leakage, driving rain etc.

    • #38454
      Mark Siddall

      Glazer method only really suited to timber frame – and then with limited success (see GBM article by Neil May two years or ago.) WUFI and Delfin are definitely considered to be a more robust tools these days.
      Nick, is bang on when suggesting that this is a third order problem.

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