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    • CommentAuthormarsaday
    • CommentTimeMar 8th 2017
     
    I want to create a warm roof. Is what i propose a warm roof ?

    125mm joists laid over oak purlins. Insulation of 125mm kingpin between the joists. OSB sheets over the top. Fibreglass system to finish.

    I am doing a new extension on the rear of my house. I think 125mm is ok for building regs, but do you think it is worth going for a bit more over the top of the joists and then laying the OSB. Say maybe 25mm extra.
  1.  
    Posted By: marsadayI want to create a warm roof. Is what i propose a warm roof ?

    125mm joists laid over oak purlins. Insulation of 125mm kingpin between the joists. OSB sheets over the top. Fibreglass system to finish.

    My understanding of a warm roof is that all the timberwork/roof structure is on the warm side of the insulation, so if your insulation is between the joists then it is not a warm roof. (so ventilation is needed above the insulation) I am not sure that an extra 25mm over the joists would be enough.
    It is always worth putting in more insulation than building regs require if you can!
    Put all the insulation over the joists - then that's a different matter. (assuming you have the available height).
    • CommentAuthoradi
    • CommentTimeMar 8th 2017
     
    I would go with the insulation above the joists must be equal or greater than between other wise you have a risk of condensation on the timber joists which will dramatically reduce there life span.

    Adrian
  2.  
    I want to create a warm roof. Is what i propose a warm roof ?

    No!

    125mm joists laid over oak purlins. Insulation of 125mm kingpin between the joists. OSB sheets over the top. Fibreglass system to finish.

    Warm roof has insulation over the rafters/joists.

    Kingspan has a lambda of approx 0.022 (maybe 0.021)W/mK.

    Approx U value calc: 0.125(m)/0.022 =R=5.618m2K/W

    Plus 'base case' approx R value (0.5) = 6.18m2K/W.

    1/6.18 =0.161W/m2K

    'Thin that out' a bit for the interruption of the joists (which you won't have if you have a true warm roof) and you are only just (perhaps) sliding in at 0.18W/m2K.

    Took long time to type this, so apols for any cross-posting!
    • CommentAuthormarsaday
    • CommentTimeMar 8th 2017
     
    Right thanks all, but i see the 125mm as wasted space if left empty. So would prefer to fill with 125 insulation.

    So how about i do this plus add 50mm over the top. This must be a warm roof surely ?

    Would build control be ok with this solution ?

    My height of the structure could be out if i do 125mm joist and then 125 odd kingpan on top.
  3.  
    The rule of thumb I have seen suggests that you should have 2/3rds above and 1/3 between, so maybe 150 over and 75 between.
    • CommentAuthormarsaday
    • CommentTimeMar 8th 2017
     
    Ok, so if i am using , say 175 overall it will be 75 between and 100 over. This isn't exactly 33:66, but not far off.

    I just can't seem to see the problem with filling the space full and then having 50mm on top.

    I will speak to BC tomorrow to see what issues they may have. If they say it is ok, i will go with this.
    • CommentAuthormarsaday
    • CommentTimeMar 8th 2017
     
    Just watched a video showing a hybrid warm/cold roof wrongly made. The joists have broken down after 4 years due to condensation. Looks like they placed insulation just between the joists.
  4.  
    Insulation just between the joists is not a hybrid warm/cold roof. It is a cold roof.
    • CommentAuthormarsaday
    • CommentTimeMar 8th 2017 edited
     
    http://www.diydoctor.org.uk/project_images/building_a_flat_roof/warm-flat-roof.jpg

    I hope this link works, but it shows a warm roof with the joists packed with insulation. It says this:


    Warm Flat Roof: By fully packing a warm flat roof with insulation there can be no air to condense. However this type of warm roof relies on there being absolutely no voids whatever otherwise interstitial condensation will occur and this can go unnoticed for many years.
    •  
      CommentAuthordjh
    • CommentTimeMar 9th 2017
     
    That image does not show a warm roof. On the Internet nobody knows you are a dog.
  5.  
    Posted By: djhThat image does not show a warm roof.

    And doesn't even show a VCL
  6.  
    ''That image does not show a warm roof. On the Internet nobody knows you are a dog.''

    What?!

    Are we talking warm woofs here?

    But, to be more helpful:

    "A warm roof and a cold roof are two different ways of insulating a roof. A warm roof building will make the entire structure of the building warm in an attempt to avoid any cold bridging. It is where the insulation is located 'above' or 'above and between' timber rafters.30 Jan 2013
    What is the difference between a warm pitched roof and a cold pitched ...
    blog.kingspaninsulation.co.uk/what-is-the-difference-between-a-warm-pitched-roof-and...''

    ''Warm flat roof construction Explained. - YouTube
    Video for warm roofâ–¶ 2:52
    https://www.youtube.com/watch?v=WZ6Ng6YI9OA
    1 Jan 2012 - Uploaded by Steve Roofer
    Hopefully, this video will help explain the difference between warm flat roof construction and cold flat roof ...''
  7.  
    Fwiw, when our flat roof felt was replaced with a plastic membrane system, the installer insisted the system requied 50mm PIR board _over_ the ply deck, directly below the membrane. He thought it protected the ply from thermal movements on hot afternoons or frosty nights which would strain the membrane.

    As the roof structure is cold/ventilated with ceiling insulation, the PIR would not save much energy, but his price was good so I didn't argue.
    • CommentAuthormarsaday
    • CommentTimeMar 9th 2017
     
    Spoke to BC today and he seems happy with 125mm insulation between 125mm joists plus 50mm laid above joists, then OSB.

    Just a phone convo, but he thought it was fine.
    • CommentAuthorDarylP
    • CommentTimeMar 9th 2017
     
    .. get it in writing, or any telephone conversations count for nothng!:shamed:
    •  
      CommentAuthordjh
    • CommentTimeMar 9th 2017
     
    .. and bear in mind that you don't get to sue him if it doesn't work, you get to pick up the costs yourself. He just makes sure you tick boxes, he doesn't accept responsibility for whatever you do.
    • CommentAuthormarsaday
    • CommentTimeMar 10th 2017
     
    Kingspan do a condensation calculation apparently. This is free, so i will call them monday to see if what i propose will be ok.
    • CommentAuthorEd Davies
    • CommentTimeMar 10th 2017
     
    If you want to play with different conditions I did a little tool for simple cases: https://edavies.me.uk/2014/05/cra/ Won't deal with your mix of joists and insulation, though. I did put your data into it to have a play with varying the temperature ranges but then distracted myself wondering about a basic assumption it makes: temperature and actual vapour pressure are presumably linear through layers of uniform material but it doesn't follow that the equilibrium vapour pressure is linear. I had been assuming that if there was a risk it would appear at the interfaces between materials but now I'm not so sure.
    • CommentAuthormarsaday
    • CommentTimeMar 11th 2017
     
    Hopefully kingpan will helpful.

    Thinking about it all these last few days i am tempted to go 100mm between and 100mm above. The cost is not so important, but i would prefer to keep the height lower if i can.
  8.  
    +1 I wondered that too.

    Actually at the first 'dew' point where (actual vapour pressure) exceeds (eqbm vapour pressure) there will not necessarily be condensation, so long as the whole water flux can continue to move outboard, to an even colder point with even lower actual vapour pressure. (Down the gradient to lower potential). It will stop and condense when it finds a spot where the gradient is insufficient to move the whole flux through the vapour resistance.
    Which ideally would be outside, but could be at the interface with the next impermeable layer of the construction. It won't be within a uniform material with linear vapour pressure gradient.

    Pictures here: https://buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

    The waterproof layer in a warm flat roof is outboard of the insulation. Condensation Risk Assessment should predict condensation at the inboard face of the cold waterproofing layer, which is where the ply/OSB deck normally sits. So according to CRA: a warm roof should be at high risk of rot. This is contradicted by real world experience... A case where simple CRA calculation is not helpful.
    •  
      CommentAuthordjh
    • CommentTimeMar 11th 2017
     
    Posted By: WillInAberdeenThe waterproof layer in a warm flat roof is outboard of the insulation. Condensation Risk Assessment should predict condensation at the inboard face of the cold waterproofing layer, which is where the ply/OSB deck normally sits. So according to CRA: a warm roof should be at high risk of rot. This is contradicted by real world experience... A case where simple CRA calculation is not helpful.

    Not always true; inverted roofs have the waterproof layer inboard of the insulation. But yes, condensation analyses only make sense on at least a year's timespan, ideally a multi-year analysis. And maybe using something like a glassfibre deck instead of ply or OSB is also a useful precaution.
    • CommentAuthormarsaday
    • CommentTimeMar 13th 2017
     
    Kingpin said it needs to be 50mm between the joist and 100 over it. The max i would like to go to is 100 over it. Seems a shame i can't get more between.
    • CommentAuthorringi
    • CommentTimeMar 13th 2017
     
    With a warm roof, you need a VERY good VCL on top of the roof boards below most of the insulation. And/OR use glued OSB as your airtight-less layer than paint the OSB unless you are using the airtight OSB, as normal OSB will let water vapor get past.

    So as to get more then 100mm over the roof, can you use smaller joists if you put a steal in? Have you check if SIP would work?
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