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Breather felt for roofs

  • 09-05-2009 11:01pm
    #1
    cork1
    Closed Accounts Posts: 443 ✭✭


    What difference will putting breather felt on a roof make over non breathable felt?


Comments

  • #2
    Poor Uncle Tom
    Registered Users, Registered Users 2 Posts: 14,547 ✭✭✭✭Poor Uncle Tom


    Breathable felt does what it says on the tin, it allows the roof/roofing timbers to breath and allows moisture through to be dispersed in the outside air. Using non breathable felt (or polythenes) here would keep the moisture in which would turn to condensation when the weather cools (night time) and over time would promote the growth of moulds and be generally unhealthy.


  • #3
    cork1
    Closed Accounts Posts: 443 ✭✭cork1


    thanks for that. it was just pure curiosity. ive used both but never asked the question.


  • #4
    Builderfromhell
    Registered Users, Registered Users 2 Posts: 1,570 ✭✭✭Builderfromhell


    Ì seem to remember that the Building research establishment (BRE) in the UK did some research on breathable roofing felts foir under slates etc. and found that most were not satisfactory for dispersion of moisture. I don't remember the details but I think they recommended always ventilating roof space as a precautionary measure. For that reason I always do even when using breathable roofing felts.


  • #5
    cork1
    Closed Accounts Posts: 443 ✭✭cork1


    well at work we always use the breathable felt and we always put in the slate vents and soffit vents aswell.


  • #6
    sydthebeat
    Subscribers Posts: 42,165 ✭✭✭✭sydthebeat


    cork1 wrote: »
    well at work we always use the breathable felt and we always put in the slate vents and soffit vents aswell.

    building regs require ventilation even when breather felt is used...

    they only exception is a warm deck flat roof...


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  • #7
    Carlow52
    Closed Accounts Posts: 2,389 ✭✭✭Carlow52


    Cork1 and BFH Interesting indeed:

    Taking the following example warm roof construction, can u explain the air path for the soffit vents and slate/ridge vents if u dont trust the breathable felt

    From the top down

    Slates/concrete roof tiles
    40 mm slating battens
    breathable membrane
    40 mm counter battens
    50 mm rigid insulation across rafters
    150 mm rigid insulation between rafters
    vapour barrier below rafters
    plasterboard, joints taped
    skim

    The reason I ask is that some people reading this might get the impression from this post that if you don't trust the breathable then why incur the cost.

    If u use unbreathable then the air path is different and the roof is generally referred to as a cold roof.


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