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How much breathing space for felt ?

  • 18-04-2013 3:12pm
    #1
    blast06
    Registered Users, Registered Users 2 Posts: 228 ✭✭


    Hi,

    2 questions here that i need help on !!
    My prelim BER advised i have 125 mm polyviso between the rafters and a 52.5mm cosy board in my rooms upstairs in dormer house. However, my rafters are only 150mm thus meaning there would only be 25mm for breathing (i had thought they were 175 all along until i measured last night !!).

    What is the minimum gap i would need to ensure the membrane can 'breath' ?.... note also that the insulation will only be fitted along the rafers for ~1.6 metres (the 'knee' wall is quite high at 1.5m). I have normal sized batons on the outside of the membrane.

    Cheers.


Comments

  • #2
    muffler
    Registered Users, Registered Users 2 Posts: 46,555 ✭✭✭✭muffler


    blast06 wrote: »
    What is the minimum gap i would need to ensure the membrane can 'breath' ?
    50mm is recommended


  • #3
    sydthebeat
    Subscribers Posts: 42,172 ✭✭✭✭sydthebeat


    blast06 wrote: »
    Hi,

    2 questions here that i need help on !!
    My prelim BER advised i have 125 mm polyviso between the rafters and a 52.5mm cosy board in my rooms upstairs in dormer house. However, my rafters are only 150mm thus meaning there would only be 25mm for breathing (i had thought they were 175 all along until i measured last night !!).

    What is the minimum gap i would need to ensure the membrane can 'breath' ?.... note also that the insulation will only be fitted along the rafers for ~1.6 metres (the 'knee' wall is quite high at 1.5m). I have normal sized batons on the outside of the membrane.

    Cheers.

    50mm is require, by regulation.

    It has nothing to do with the memebrane "breathing" but everything to do with condensation exhausting away to the external.

    if its only a small section what you can do is add a batten under the rafter in the section required, use 125mm , and fix the insulated plasterboard to the battens.


  • #4
    blast06
    Registered Users, Registered Users 2 Posts: 228 ✭✭blast06


    Thanks.
    I see that 100mm plus 62.5 cozy board (for 1 particular product!) actually gives me the required .16 value in any case so will double check with guy that done BER report and hopefully go with that.
    Cheers.


  • #5
    Certified
    Registered Users, Registered Users 2 Posts: 147 ✭✭Certified


    Kooltherm is a very expensive product if that is what your thinking?


  • #6
    blast06
    Registered Users, Registered Users 2 Posts: 228 ✭✭blast06


    Ah crap .... i was looking at the Kooltherm but thinking polyviso sheets and thats what i just got the builders suppliers to price. Lucky enough i said hold off on delivering yet.
    I see though 75mm Kooltherm with 62.5mm insulated slab would get u value of 0.16 (which is what i need). However, i can't seem to find u-values for polyviso for rafters and studded walls .... or is this not recommended ??
    Cheers


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  • #7
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,146 Mod ✭✭✭✭BryanF


    Breathable felt and double batten above?


  • #8
    sydthebeat
    Subscribers Posts: 42,172 ✭✭✭✭sydthebeat


    BryanF wrote: »
    Breathable felt and double batten above?

    breathable felt doesnt let moisture through it, only vapour ;)
    it wouldnt be very good roofing felt if moisture was allowed through it

    If the dew point in under the felt ie a cold roof, then 50mm gap is required between insulation and felt...

    otherwise build a warm roof and negate the 50mm gap.


  • #9
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,146 Mod ✭✭✭✭BryanF


    sydthebeat wrote: »
    breathable felt doesnt let moisture through it, only vapour ;)
    it wouldnt be very good roofing felt if moisture was allowed through it

    If the dew point in under the felt ie a cold roof, then 50mm gap is required between insulation and felt...

    otherwise build a warm roof and negate the 50mm gap.
    bang-on! miss read the OP


  • #10
    Atlantic1
    Registered Users, Registered Users 2 Posts: 368 ✭✭Atlantic1


    blast06 wrote: »
    Ah crap .... i was looking at the Kooltherm but thinking polyviso sheets and thats what i just got the builders suppliers to price. Lucky enough i said hold off on delivering yet.
    I see though 75mm Kooltherm with 62.5mm insulated slab would get u value of 0.16 (which is what i need). However, i can't seem to find u-values for polyviso for rafters and studded walls .... or is this not recommended ??
    Cheers

    Hopefully, these will help you in that department.

    http://www.idevelop.ie/iproducts/prd/3005/rigid_board_insulation/insulation_boards/

    http://www.idevelop.ie/iproducts/prd/199/rigid_board_insulation/thin-r_xt,pr/

    http://www.idevelop.ie/iproducts/prd/197/rigid_board_insulation/thermapitch_tp10/



    And Page 5 here will tell you exactly what you want to know about U values with polyiso between the rafters and insulated plasterboards underneath.

    It's the Ventilated Roof one you need to look at, not the Warm Roof one. ;)


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