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Ventilation Gap Between Felt and Insulation

  • 30-12-2011 11:38pm
    #1
    Closed Accounts Posts: 4


    Hi All,

    I have a question about the ventilation space between the felt and insulation in a cold pitched roof. Reading the building specs you need to leave a 50mm gap to provide adequate ventilation. However I have been informed by an engineer that if you use a quality breathable roofing membrane this can be reduced to 25mm. I'm just wondering if this is a standard recommendation before going ahead with this in my house spec? Thanks


Comments

  • Moderators, Home & Garden Moderators Posts: 10,146 Mod ✭✭✭✭BryanF


    owenmclove wrote: »
    Hi All,

    I have a question about the ventilation space between the felt and insulation in a cold pitched roof. Reading the building specs you need to leave a 50mm gap to provide adequate ventilation. However I have been informed by an engineer that if you use a quality breathable roofing membrane this can be reduced to 25mm. I'm just wondering if this is a standard recommendation before going ahead with this in my house spec? Thanks
    id ask for the breathable felt IAB or BBA certificate, a google of the name should find this.
    why not double batten above the breathable felt and create a warm roof.
    why not tell us the roof build-up from the inside out, we' d get a better Q&A going


  • Closed Accounts Posts: 4 owenmclove


    Thanks for the reply. To be honest I had not thought of the roof build up until now and this is what the engineer proposed in his specification. He says that with the below build up I should get a u-value of about 0.14.

    The roof is a dormer type roof and proposed build up is a 52.5mm insulated plaster board with air-tightness membrane then 150mm rafterloc type insulation between 7" rafters. For the roofing membrane either a Tyvek or Siga breathable membrane have been recommended.


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


    My guess is your roof is already a warm roof.

    Whats the plan for fitting light fittings in this space?


  • Closed Accounts Posts: 4 owenmclove


    My understanding of a warm roof is that the rafters are not exposed to the outside ambient temperature? If thats the case then my roof has been specified as a cold roof.

    Sorry should have mentioned that on the flat of the roof the plan is to use rock wool at about 300mm deep. I don't plan on using recessed lighting upstairs to minimise breaking the air-tightness barrier and all of my cabling etc will be traveling on the ceiling under the rock wool if that makes sense?

    I'll be looking into a warm roof now, but I'm also interested in knowing if what I've been told is a standard detail as I'd like to be able to trust my engineer in these things!


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


    I believe you have a warm roof, I dont thing warm roofs are limited to your description.
    My understanding of a warm roof is that the rafters are not exposed to the outside ambient temperature?

    I offer the link below to show the general ideas and not any product endorsement
    http://www.buildbase.co.uk/buildbase_roofersguide/pages/warmroof.html

    If you or ur advisors have a different idea then please post the links


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


    The OP's post got me looking at this in a bit more detail.

    The standard PH roof designs all seem to use OSB as the sarking board layer.

    [ In fact the OSB layer is used on both internal and external retro wall insulation projects as well as ceilings in the SEAI document

    Reading around it seems that the standard OSB is 9 mm and that 18mm OSB is considered non-breathable.

    In any of the PH roof details there is no ventilated air-gap between the insulation and the membrane

    However from here:
    http://www.buildbase.co.uk/buildbase_roofersguide/pages/warmroof.html
    I have extracted the following reference which invokes a British Standard for the 50 mm ventilation if ply or OSB is used as opposed to 150 mm sarking board with 2mm gaps between them.

    < ...However, where using sarking boards comprising sheets of plywood or OSB, this type of warm unventilated roof configuration is not recommended. In a warm roof where the membrane is supported on a plywood or OSB sarking board, ventilation of the mandatory 50mm airspace below the sarking board is required, in accordance with BS 5534 (BS 5250: 2002) ...>
    [image to follow once wthax.org gets the image processed]


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