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Condensation Control

  • 14-01-2011 1:59pm
    #1
    Just Bob
    Closed Accounts Posts: 18 ✭


    Simple question -
    Is 50mm gap between insulation and standard membrane in sloped roofs the same as having a a 50mm gap on the external of a breathable membrane and allowing insulation to be placed against this breathable membrane?


Comments

  • #2
    DOCARCH
    Moderators, Home & Garden Moderators, Science, Health & Environment Moderators Posts: 18,747 Mod ✭✭✭✭DOCARCH


    No. With breathable roofing felts it is recommended you leave a 25mm gap between the insulation and (under) the felt. The insulation should never be placed against the breathable felt.


  • #3
    sydthebeat
    Subscribers Posts: 42,581 ✭✭✭✭sydthebeat


    see here

    and DOARCH, would you accept when the manufacturers recommend over what the building regulations require??

    id be very wary of 25mm when taking into account the natural 'bow' of the felt to create a drainage channel for any water....


  • #4
    DOCARCH
    Moderators, Home & Garden Moderators, Science, Health & Environment Moderators Posts: 18,747 Mod ✭✭✭✭DOCARCH


    sydthebeat wrote: »


    and DOARCH, would you accept when the manufacturers recommend over what the building regulations require??

    id be very wary of 25mm when taking into account the natural 'bow' of the felt to create a drainage channel for any water....

    True. Point taken.

    I would always also recommend always venting the eaves and ridge (despite what manufacturers might say).


  • #5
    L driver
    Registered Users, Registered Users 2 Posts: 196 ✭✭L driver


    DOCARCH wrote: »
    No. With breathable roofing felts it is recommended you leave a 25mm gap between the insulation and (under) the felt. The insulation should never be placed against the breathable felt.

    What does the air venting under the felt do that the air over the felt doesn't?


  • #6
    sydthebeat
    Subscribers Posts: 42,581 ✭✭✭✭sydthebeat


    L driver wrote: »
    What does the air venting under the felt do that the air over the felt doesn't?

    condensation will form between the insulation and the felt...

    if the vent space is between the felt and the insulation, this condensation is vented away, thus significantly reducing risk of rot damage to structural timbers and reducing risk of wet insulation which seriously impacts on performance.

    if he vent space is over the felt, it cannot achieve the above.


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  • #7
    L driver
    Registered Users, Registered Users 2 Posts: 196 ✭✭L driver


    Syd,
    Why is there full fill systems so? Why will the vapour only condense where the insulation touches the membrane, would it not condense in the last mm of the insulation? Where can the vapour be vented to if it is inside the membrane?


  • #8
    sydthebeat
    Subscribers Posts: 42,581 ✭✭✭✭sydthebeat


    L driver wrote: »
    Syd,
    Why is there full fill systems so? ?

    "full fill" systems are, in the vast majority, specified in 'warm roof' construction and not 'cold roof' construction which we are talking about here.

    The only full fill solution i see applicable here is the spray-on open cell foam based system, which i have reservations about as well.

    L driver wrote: »
    Syd,
    Why will the vapour only condense where the insulation touches the membrane, would it not condense in the last mm of the insulation?

    it wont.... there should be an air gap.. thats what im saying... it will form on the cold face of the insulation.. the gap is required to vent it away.
    L driver wrote: »
    Where can the vapour be vented to if it is inside the membrane?

    see above... you need the vent space between the insulation and the membrane... whether breathable or not, in my opinion.

    The only situation where you can get away without a vent space is where the construction is a 'warm roof'. In this, the condensation point occurs outside the structural members layer (rafters) .. and thus negates the risk of damage.


  • #9
    L driver
    Registered Users, Registered Users 2 Posts: 196 ✭✭L driver


    Syd,
    I see your points but wonder about this topic a lot, and I am not disagreeing but probably not understanding. I have seen many type of roof insulation, I have heard everything from needing 2'' vented to air will make it's way into a roof anyway. How long does it take a roof to fail because of lack of 2'' air gap? If the house's RH is managed with humidity fans in bathrooms does this help. This is a big issue... How have dormer houses been insulated for the past years, including those before breathable membrane? Packed with fibreglass mostly. Lots of habitable attics have insulation between the rafters to the wallplate and then insulated board fixed onto the rafters. I was at a talk where a man from a very reputable company, not Irish, said with breathable membrane full fill is no problem!? I have seen many recommend a roof build up of, from outside in, slates, counter batten, batten, membrane, full fill HDfibreglass/rockwool/cellulose then air tight layer, counter batten with more insulation. This must be madness so.
    If a roof is to have a habitable space should our regs specify only warm roof construction? I am a bit confused and would appreciate more info and thoughts


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