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vapour control layer

  • 17-01-2014 4:03pm
    #1
    tinca
    Registered Users, Registered Users 2 Posts: 29 ✭


    Hi... just wondering what the difference is in the types of this membrane available? There appears to be a massive price difference between top and bottom of the range with the top varieties claiming technologies that allow for air/moisture to pass both ways depending on internal/external temperatures and pressures. If there is a breather membrane on the external side of the structure (timber frame), is it not the case that the internal membrane should completely restrict the flow of air/moisture? The cheapest membrane available is a light duty polythene membrane, it is at it sounds, plastic and no microscopic movement allowed. I'm not suggesting it's better or worse but would like to know your opinions on what's available. Thanks!


Comments

  • #2
    MicktheMan
    Registered Users, Registered Users 2 Posts: 2,880 ✭✭✭MicktheMan


    tinca wrote: »
    Hi... just wondering what the difference is in the types of this membrane available? There appears to be a massive price difference between top and bottom of the range with the top varieties claiming technologies that allow for air/moisture to pass both ways depending on internal/external temperatures and pressures. If there is a breather membrane on the external side of the structure (timber frame), is it not the case that the internal membrane should completely restrict the flow of air/moisture? The cheapest membrane available is a light duty polythene membrane, it is at it sounds, plastic and no microscopic movement allowed. I'm not suggesting it's better or worse but would like to know your opinions on what's available. Thanks!

    There is no such thing as a complete barrier to moisture so you should always design for moisture getting past the barrier and that it can easily dry out.


  • #3
    tinca
    Registered Users, Registered Users 2 Posts: 29 ✭tinca


    so does that not imply that one type is just as effective as the other... that the structure will be ventilated from the outside (through the breather membrane) and that therefore it matters little what permeability the inside vapour control layer has?


  • #4
    sydthebeat
    Subscribers Posts: 42,172 ✭✭✭✭sydthebeat


    tinca wrote: »
    so does that not imply that one type is just as effective as the other... that the structure will be ventilated from the outside (through the breather membrane) and that therefore it matters little what permeability the inside vapour control layer has?


    "breathable" works both ways... the structure needs to be able to breathe inwards as well as outwards.

    If you place a material like polyethylene internally then you stop vapour from exhausting inwards on those hot summer days when its warmer outside than inside. This trapped vapour will condense and rot structural timbers, which is exactly what you do not want happening.


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