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Warm roof - rafters, insulation and airtightness

  • 11-09-2014 4:18pm
    #1
    shell820810
    Registered Users Posts: 50 ✭✭


    Can anyone provide details of best/most cost effective method for warm roof.

    Architect has specified u value 0.12 for roof
    100mm PIR between rafters, 50mm ventilation gap (150mm rafters), 82.5mm insulated board under rafters.

    I have no idea where vapour barriers or airtightness barriers etc come into play here.

    Also, i have read earthwool/rockwool is better between the rafters?

    Any diagrams, recommends for products etc??


Comments

  • #2
    Phirstclass
    Registered Users Posts: 14 ✭Phirstclass


    What you have described above is not a warm roof construction and will not give you your desired or calculated u-value.
    Although 100mm PIR board has a good thermal conductivity, it will be difficult to fit effectively in between rafters without leaving gaps.
    A more effective way of achieving a high performance warm roof would be to insulate over your rafter. Wood fibre boards are good in this respect as these will act as an insulator to eliminate thermal bridging at each rafter and also act as your wind tight layer. On top of this would go cross battens for ventilation below your roof finish. Then you could fully fill your rafter depth with earth wool/cellulose etc.on the underside of your 150mm rafters would be where you would want to fit your air tight membrane. Insulation needs an effective wind tight and air tight barrier each side of it for optimum performance.


  • #3
    shell820810
    Registered Users Posts: 50 ✭✭shell820810


    Phirstclass wrote: »
    What you have described above is not a warm roof construction and will not give you your desired or calculated u-value.
    Although 100mm PIR board has a good thermal conductivity, it will be difficult to fit effectively in between rafters without leaving gaps.
    A more effective way of achieving a high performance warm roof would be to insulate over your rafter. Wood fibre boards are good in this respect as these will act as an insulator to eliminate thermal bridging at each rafter and also act as your wind tight layer. On top of this would go cross battens for ventilation below your roof finish. Then you could fully fill your rafter depth with earth wool/cellulose etc.on the underside of your 150mm rafters would be where you would want to fit your air tight membrane. Insulation needs an effective wind tight and air tight barrier each side of it for optimum performance.

    Thanks, just to be clear, as there have been some issues with definitions for warm roof etc. What I mean is "room in a roof" type setup, so that we can easily convert the attic to living space down the line.

    I assume to insulate over the rafters requires a different type of roof construction to normal - have you anywhere I could reference this?


  • #4
    shell820810
    Registered Users Posts: 50 ✭✭shell820810


    I also have a sunroom and part of an open plan kitchen with a pitched ceiling, and architect had specified same setup for all three?


  • #5
    sydthebeat
    Subscribers Posts: 41,830 ✭✭✭✭sydthebeat


    Insulation over and between the rafters Is considered a "warm roof".

    Insulation between and under rafters is considered a "cold roof".


  • #6
    shell820810
    Registered Users Posts: 50 ✭✭shell820810


    sydthebeat wrote: »
    Insulation over and between the rafters Is considered a "warm roof".

    Insulation between and under rafters is considered a "cold roof".

    It's a warm roof space I want, is this the same thing?


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  • #7
    sydthebeat
    Subscribers Posts: 41,830 ✭✭✭✭sydthebeat


    shell820810 wrote: »
    It's a warm roof space I want, is this the same thing?

    Probably most commonly referred to as a dormer roof


  • #8
    shell820810
    Registered Users Posts: 50 ✭✭shell820810


    sydthebeat wrote: »
    Probably most commonly referred to as a dormer roof

    Ok. So a dormer roof. what are the best details and products for this?


  • #9
    sydthebeat
    Subscribers Posts: 41,830 ✭✭✭✭sydthebeat


    shell820810 wrote: »
    Ok. So a dormer roof. what are the best details and products for this?

    the "best" solution is to do as advised above and create a warm roof construction using the wood fibreboard as the air tightness layer. special attention must be paid to the eaves to ensure continuity of air tightness.

    constructing a warm roof means everything inside the roof plan is heated.

    dormers are notoriously draughty using common practices and materials. People use the small areas behind the stud walls for storage which means every door into that space is both a hole in insulation and also a demon for draughts.

    the exact make up and details should be advised by your design professional, as it has a knock on effect on your part L compliance


  • #10
    ferryman35
    Registered Users, Registered Users 2 Posts: 85 ✭✭ferryman35


    We're at an advanced stage of building a warm roof and the one piece of advice that I would give you is that with the exception of some very valuable affirmations that I got from contributors here it is very hard to find anyone with either the interest or the understanding to give you a viable solution. All I got most places was "what do you want to do that for?"

    The availability of suitable materials here is much more limited than you'll find elsewhere - so you'll have to bear that in mind when getting your design done - you and your professional need to be well informed - it doesn't mean it can't be done - just you need to know what your options are!

    The solution needs to be breathable - though you will find unbreathable solutions (and worse!) offered in Ireland, and despite all the talk of regulations, these would appear to be legal despite being inadequate, (though they wouldn't be approved in other EU states). It takes a greater depth of breathable material to achieve a similar u-value to something unbreathable - but clever use of membranes can significantly enhance the performance of the breathable insulation. You need a vapour barrier on the underside of your rafters - and breathable materials from there out - breathable insulation between and on top of the rafters and covered with a vapour permeable membrane

    The insulation is fixed to your rafters with counterbattens - these have a sealing tape applied on the bottom - these are structural members - so you need professional input here - and then your roof proceeds as normal. You can get a perforated band to run along the eaves to prevent unwanted tenants from availing of the space created by the counterbatten / batten build up.

    The fun starts when you try to get someone to do this for you! if you want more information on the materials I used feel free to PM me.


  • #11
    Jonathan222
    Registered Users Posts: 91 ✭✭Jonathan222


    Hoping this is the right forum, but I have a pretty old house, with a slate roof and nothing and I mean nothing between the slates and the eves. The eves themselves are insulated but the rafters of the roof, i.e. directly under the slates have nothing so winter is an experience.

    Was hoping someone could advise what would suit best for me to do rather than get a 'professional' in.


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  • #12
    MicktheMan
    Registered Users, Registered Users 2 Posts: 2,858 ✭✭✭MicktheMan


    Jonathan222 wrote: »
    Was hoping someone could advise what would suit best for me to do rather than get a 'professional' in.

    Why not insulate on the flat and leave the rafters alone? Make sure you do not interfere with the ventilation channels

    Or (not what you want to hear but ...)

    Advice: get a professional to advise.


  • #13
    Jonathan222
    Registered Users Posts: 91 ✭✭Jonathan222


    MicktheMan wrote: »
    Why not insulate on the flat and leave the rafters alone? Make sure you do not interfere with the ventilation channels

    Or (not what you want to hear but ...)

    Advice: get a professional to advise.

    Thanks.

    The flat is already insulated but with the slates so unprotected there may as well be no roof. Ventilation won't be a problem I'd say. Just looking for something to help, even a tiny bit.


  • #14
    MicktheMan
    Registered Users, Registered Users 2 Posts: 2,858 ✭✭✭MicktheMan


    Jonathan222 wrote: »
    Thanks.

    The flat is already insulated but with the slates so unprotected there may as well be no roof. Ventilation won't be a problem I'd say. Just looking for something to help, even a tiny bit.

    In that case, the flat is your thermal boundary and there is no point in doing anything on the slopes (rafter level).
    What do you mean by "with the slates so unprotected"?
    What are you trying to achieve / improve?


  • #15
    Jonathan222
    Registered Users Posts: 91 ✭✭Jonathan222


    MicktheMan wrote: »
    In that case, the flat is your thermal boundary and there is no point in doing anything on the slopes (rafter level).
    What do you mean by "with the slates so unprotected"?
    What are you trying to achieve / improve?

    Thanks for the comment.

    I'm just trying to insulate the 'very old' roof slates better. The slates are tacked onto the wooden roof frame with nothing underneath so the draft in the eves, which then feeds into the attic bedroom, is unreal in the winter. The eve doors are also insulated but still there is a considerable draft.


  • #16
    893bet
    Registered Users, Registered Users 2 Posts: 9,149 ✭✭✭893bet


    Jonathan222 wrote: »
    Thanks for the comment.

    I'm just trying to insulate the 'very old' roof slates better. The slates are tacked onto the wooden roof frame with nothing underneath so the draft in the eves, which then feeds into the attic bedroom, is unreal in the winter. The eve doors are also insulated but still there is a considerable draft.

    Sounds like it's airtightness work you need rather than insulation. More insulation won't stop the draft.


  • #17
    Jonathan222
    Registered Users Posts: 91 ✭✭Jonathan222


    893bet wrote: »
    Sounds like it's airtightness work you need rather than insulation. More insulation won't stop the draft.

    Cool, any suggestions for that?


  • #18
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,141 Mod ✭✭✭✭BryanF


    Jonathan222 wrote: »
    Hoping this is the right forum, but I have a pretty old house, with a slate roof and nothing and I mean nothing between the slates and the eves. The eves themselves are insulated but the rafters of the roof, i.e. directly under the slates have nothing so winter is an experience.

    Was hoping someone could advise what would suit best for me to do rather than get a 'professional' in.
    please be careful here as you NEED air-flow into the attic space.


  • #19
    MicktheMan
    Registered Users, Registered Users 2 Posts: 2,858 ✭✭✭MicktheMan


    Jonathan222 wrote: »
    Cool, any suggestions for that?

    You need to stop the air from traveling from the ventilated attic into / the attic room and vice versa. So go into the room and look at all the surfaces (floor, walls, ceilings etc). What you are looking at is essentially your air tightness barrier.
    Now, look at all junctions between floor, walls, ceiling and all holes in the floor, wall, ceilings such as electrical sockets, piping, light cables, attic access doors etc. All these are leakage points and need to be sealed. Insulation won't do this, proprietary ait tightness products (properly applied) will.
    This is not an easy job to get right but significant improvements can be achieved with experience and attention to detail.


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