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Housing - New Build: New Build - Need advice with flat roof insulation build up?

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    • CommentAuthorCrofty
    • CommentTimeMay 16th 2010 edited
     
    Hi

    This is my first post to please bare with me....unfortunately I have more questions that experience, but I'm keen to learn!

    My new build for which I have just received planning permission has large areas of flat roof (part height restrictions, part design) and I'm trying to get my head around the best build up for thermal performance vs build up depth.

    There were some errors on the planning application that we are trying to work around and one of the issues I have is that I believe I need a flat roof make up of 200mm joist, firings, ply deck , 200mm of insulation (woodfibre to help with decrement delay) and a EDPM single ply membrane. Unfortunately this creates a roof build up of over 400mm in depth, which is difficult to accommodate into my approved plans.

    Are there insulation options that I can use above the deck that will perform better than the options above for less thickness? If not, what options do I have to create a shallower build up? Cold roof, or a hybrid roof where I have insulation above and in between the joists?

    I think I understand the concept of cold/warm roof, but does having infill insulation prevent the deck from being warmed and in turn create condensation issues? I intend to use a vapour control membrane, but there will be places where light fittings etc will make this less than reliable. I will have the family bathroom directly below this flat roof so don't want issues with moisture and condensations.

    I presume I would still need ventilation on a hybrid roof? Excuse me as I'm not a construction professional, but how do you create an airtight structure to prevent heat leakage and still have ventilation in the roof space?

    I'd really appreciate your advice on this matter...

    Thanks
    • CommentAuthortony
    • CommentTimeMay 16th 2010
     
    I would give down-lighters or any light fitting that penetrates the insulation, vapour or air sealing barrier a big miss.

    There is a nice passivhaus detail for a flat roof with fully filled joists and apparently no ventilation gap.

    I hate flat roofs so please dont build it flat -- min fall 1 in 40.
  3.  
    Posted By: tony

    There is a nice passivhaus detail for a flat roof with fully filled joists and apparently no ventilation gap.


    Any idea where one might find that or what it might comprise... (anyone?)

    J
  4.  
    PS
    Posted By: Darth tonyI hate flat roofs...

    Hate is a bit strong... you should be careful with hate...
    Posted By: YodaFear is the path to the dark side. Fear leads to anger. Anger leads to hate. Hate leads to suffering.

    Wouldn't want any suffering would we...?
    • CommentAuthorCWatters
    • CommentTimeMay 16th 2010
     
    Perhaps someone can do a condensation risk analysis for you on something like.....

    EDPM
    5mm plywood bonded to 100mm insulation (eg from Celotex)
    firings
    100mm insulation between 200mm joists
    Vapour barrier formed by sealing the foil covered insulation to the joists.
    100mm void
    plasterboard.

    That would give a about 110mm void for downlights (low energy type hopefully).
  6.  
    Of course with the firrings the 100mm void gets bigger as yo go up hill...
    • CommentAuthorCrofty
    • CommentTimeMay 16th 2010
     
    Thanks all.

    Just to add, I'm thinking of using engineered joists (easy joists) and was planning on using the top 100mm (or whatever is required) of the joist space for running services and MVHR flat ducting, and adding the extra insulation below that, effectively creating a void above the insulation. This means I'd have to cut out for down lighters etc if used.

    The ideal would be to have all the insulation on top, but 400mm+ is hard to accommodate.

    Anybody able to a condensation check on such a build up would be great.

    I want to build to a 1:40 fall, which of course makes the roof build up even bigger above the largest firing.....duh!

    Tony - Any pointers for finding that full fill passivhaus build up?


    Thanks all
    • CommentAuthortony
    • CommentTimeMay 16th 2010
     
    DAI 05 or DAh 01
    • CommentAuthorRobinB
    • CommentTimeMay 16th 2010
     
    I'd consider insulation above and between the rafters. We had height issues too - though our roofs are between 12 and 37 degrees slope. In the end we used 75mm kingspan TP10 over and 150mm rockwool in between. My understanding is as long as the greater insulator is towards the outside there shouldn't be a condensation risk within or below the roof.

    If anyone violently disagrees please let me know while there's time to do something about it - just! By the way how much does one pay for a condensation risk analysis?

    RobinB
    • CommentAuthorCWatters
    • CommentTimeMay 16th 2010
     
    Posted By: RobinB My understanding is as long as the greater insulator is towards the outside there shouldn't be a condensation risk within or below the roof.


    I'm not an expert but I don't think it's quite that simple. I believe what matters is the temperature of the structural elements in relation to the dew point or something like that. It should be possible to plot a graph of temperature vs distance through the roof sandwich and work out if the tops of the joists are above or below the dew point when it's X degrees outside and Y degrees inside.
    • CommentAuthorTimber
    • CommentTimeMay 16th 2010
     
    How about:

    Roof membrane
    Class 3 t&g ply deck, 18 or 22 mm
    nominal 50 mm ventillation void formed with firrings
    breather membrane (optional, but I would if it were me)
    100 mm wood fibre insulation board
    200 mm joist full fill with warmcell or carfully installed mineral wool
    500 guage polythene vapour control layer
    plasterboard.

    That would shave 50 mm off your roof build up and give good insulation values.

    Oh, and please try and avoid downlighters. [mumbles 'work of the devil' etc]

    Timber
    • CommentAuthorCrofty
    • CommentTimeMay 17th 2010
     
    Thanks again.

    Dont suppose anybody on here could do a condensation risk analysis on Timber's or any other suggested build up?

    If not, where would I go to get such a check?
    • CommentAuthorCWatters
    • CommentTimeMay 17th 2010
     
    Perhaps the 30 day free trial will do it..
    http://www.builddeskonline.com/sw50865.asp
  14.  
    Posted By: RobinBWe had height issues too - though our roofs are between 12 and 37 degrees slope. In the end we used 75mm kingspan TP10 over and 150mm rockwool in between. My understanding is as long as the greater insulator is towards the outside there shouldn't be a condensation risk within or below the roof.
    This is a good rule of thumb for a pitched roof without a sarking board, i.e. where the main concern is the rafters. In that case the rafter undersides are heated to room temperature & ensuring at least half of the thermal resistance is outboard of the rafters should avoid significant risk of condensation on the rafters themselves.

    However, in a flat roof there is typically a plywood/OSB deck above the joists. Plywood is not vapour open, the roof covering is typically not vapour open & no part of the deck is heated to room temperature. So I would recommend doing the condensation risk analysis.

    David
  15.  
    Posted By: CroftyThere were some errors on the planning application that we are trying to work around and one of the issues I have is that I believe I need a flat roof make up of 200mm joist, firings, ply deck , 200mm of insulation (woodfibre to help with decrement delay) and a EDPM single ply membrane.
    If you don't have the height to ventilate above the insulation then I think you need to forget woodfibre insulation. It needs to have a vapour open path to the room or a vapour open path to the outside. Sandwiched between the vapour tight roof covering & a vapour control layer it will have neither of these paths open to it.

    Posted By: CroftyI'm thinking of using engineered joists (easy joists) and was planning on using the top 100mm (or whatever is required) of the joist space for running services and MVHR flat ducting, and adding the extra insulation below that, effectively creating a void above the insulation. This means I'd have to cut out for down lighters etc if used.
    Any service void should be below the insulation, i.e. on the warm side. Putting it above the insulation will lead to warm air leaking into this cold space from every service breakthrough greatly increasing the condensation risk.

    Even if it's below the insulation you need to be very careful to keep it airtight. So how about fitting a polyethylene vapour control layer over the joists before the deck is fitted? Draping the polyethylene over the joists & fitting 100mm of Celotex/Kingspan before the deck is fitted to the top. From the bottom:

    Plasterboard;
    100mm service void between joists;
    100mm Kingspan/Celotex supported by polyethylene draped over joists;
    18mm T&G OSB3 deck;
    100mm Kingspan/Celotex to top of deck;
    EDPM watertight covering direct to insulation.

    David
    • CommentAuthorTimber
    • CommentTimeMay 17th 2010
     
    I havn't done a condensation risk analysis on that exact makeup, but experience tells me there will be no problem! If you want I will do a U value and condensation check tomorrow.

    Timber
    • CommentAuthorwookey
    • CommentTimeMay 17th 2010
     
    Crofty - I've spent a lot of time thinking about this exact problem recently as I have essentially the same problem: Part of extension is flat roof because of height limitations. Plans define absolute max height so I'm trading off internal height vs insulation levels vs roof build thickness.

    One think you can do to improve the numbers is to use tapered insulation instead of firings. That way the insulation is thicker over at least part of the roof. It's probably easier to build too, but I assume not cheaper.

    Here is a good place to do condensation risk sums easily: http://www.vesma.com/tutorial/uvalue01/uvalue01.htm
    Fill in the build and the temps/humidities of interest (the defaults are fine for the UK) and you get a little red asterisk next to components which are likely to get condensation on them.

    It suggests that David's proposed build-up above will be fine.

    You can improve your insulation by just using PUR/PIR rather than woodfibre as it's much better insulation per thickness, but that is at the expense of decement delay/thermal mass. I'm not sure how much that matters if there is adequate mass in the building anyway (I've been meaning to start a thread about that so will do so now).

    You said you were thinking of easy joists. So far as I can tell, the way to minimise build height is good old-fashioned solid wood. All the engineered options are taller for a given strength (unless I am looking in the wrong places/at the wrong numbers)

    I've also found that doing the calcs for the joists will often allow lower-profile ones than just picking the standard items out of the BS8103-3 tables.
    • CommentAuthorSaint
    • CommentTimeMay 17th 2010 edited
     
    Crofty, think about an inverted roof system (USD). Dew points are thrown outside the structure which is on the warm side of the insulation. Simple to construct. Membrane is protected from UV. I've done one and it worked perfectly
  19.  
    Posted By: wookeyYou said you were thinking of easy joists. So far as I can tell, the way to minimise build height is good old-fashioned solid wood.
    I didn't pick-up on this originally, but are these metal web joists? If so, they are great for running services in intermediate floors, but I would not recommend them for a roof. The metal webs bridge the insulation & they are broader than solid timber or I beam joists creating large voids which are difficult to insulate & even more difficult to seal.

    I would use I beam joists, they are easier to cut around & the OSB web has much higher thermal resistance. The downside is that they typically need herringbone struts which bridge insulation placed between the joists.

    David
    •  
      CommentAuthordjh
    • CommentTimeMay 18th 2010
     
    Posted By: wookeyOne think you can do to improve the numbers is to use tapered insulation instead of firings. That way the insulation is thicker over at least part of the roof. It's probably easier to build too, but I assume not cheaper.

    When Ivan (I think it was) did his roof he found it was actually cheaper to put two tapered batts together to make flat insulation. The reason was that there are lots of remnants of pallets around so it depends on having the ability to be flexible.

    ... that is at the expense of decement delay/thermal mass. I'm not sure how much that matters if there is adequate mass in the building anyway (I've been meaning to start a thread about that so will do so now).

    I was looking at the SAP 2009 tables that were mentioned in another thread and noticed that they have tables of thermal mass etc for various constructions (blocks + wet plaster, blocks + plasterboard on dabs etc). Might be useful.

    So far as I can tell, the way to minimise build height is good old-fashioned solid wood. All the engineered options are taller for a given strength

    That would make sense. There's more material in a regular joist of the same height, even if it's not ideally distributed.

    Cheers, Dave
    •  
      CommentAuthordjh
    • CommentTimeMay 18th 2010
     
    So far as I can tell, the way to minimise build height is good old-fashioned solid wood. All the engineered options are taller for a given strength

    That would make sense. There's more material in a regular joist of the same height, even if it's not ideally distributed.

    One way to get an even slimmer buildup would be to build a stressed skin panel. Ply-joist-ply, say, using the whole cross-section of the ply as the web of the girder. Dunno what building regs have to say!
  22.  
    Posted By: davidfreeboroughIf you don't have the height to ventilate above the insulation then I think you need to forget woodfibre insulation. It needs to have a vapour open path to the room or a vapour open path to the outside. Sandwiched between the vapour tight roof covering & a vapour control layer it will have neither of these paths open to it.

    David


    I am still looking for way to get some dense insulation in there for decrement delay as per the original post. I found this on NBTs site David, it seems to contradict what you have said above, although I can understand why you said it. Any comments?

    http://www.natural-building.co.uk/PDF/Pavatex/Pavaroof/Roof_PAVAROOF_FLAT.pdf

    Peter
  23.  
    Posted By: Peter ClarkI am still looking for way to get some dense insulation in there for decrement delay as per the original post. I found this on NBTs site David, it seems to contradict what you have said above, although I can understand why you said it. Any comments?

    Interesting. Their pitched roof literature & the BBA certificate for pitched roof applications are very explicit about requiring a ventilated gap to the outside. However, this literature also makes a lot of noise about it being a breathable build-up & not requiring a vapour control layer to the inside.

    The flat roof literature on the other hand explicitly states that the build up shown is for use with vapour closed watertight layers & requires the use of a vapour control layer. They lead on decrement delay & do not mention (the lack of) breathability. Interestingly, they do not yet seem to have a BBA certificate for flat roof applications.

    My concern would be that if the woodfibre boards get wet, either during construction or subsequently due to failure of vapour barrier or watertight layer, they have no way of drying.

    David
  24.  
    Posted By: davidfreeborough

    My concern would be that if the woodfibre boards get wet, either during construction or subsequently due to failure of vapour barrier or watertight layer, they have no way of drying.

    David


    Of course that is the concern. Well spotted on the lack of BBA for flat roofs.

    What about Timber's suggestion of ventilation above the insulation using the 'firrings gap'?

    Posted By: Timber
    Roof membrane
    Class 3 t&g ply deck, 18 or 22 mm
    nominal 50 mm ventillation void formed with firrings
    breather membrane (optional, but I would if it were me)
    100 mm wood fibre insulation board
    200 mm joist full fill with warmcell or carfully installed mineral wool
    500 guage polythene vapour control layer
    plasterboard


    Peter
  25.  
    Posted By: Peter ClarkWhat about Timber's suggestion of ventilation above the insulation using the 'firrings gap'?

    Would this be 50mm average reducing to zero at one edge? If so, there would be a real risk of stagnant air where it reduces to zero.

    David
    • CommentAuthorTimber
    • CommentTimeMay 18th 2010
     
    No, the gap would be 50 mm nominal, i.e. 50 mm at the smallest end.

    A slope of 1 in 80 i think is ok, but 1 in 40 is a 'safer' option. You could work out what gap would result at the other end of the span.

    I prefer a true warm roof, but if you havn't got the space, then some form of hybrid setup would be best.

    To be honest, there are many ways in which you can build up the roof, it all depends on your prefered choice of materials and budget.

    To the OP, are you top or bottom hanging your roof joists? What is your external wall buildup? Thermal bridging around the perimeter of the roof may be as important as the overall U value of the roof itself! Have you looked at thermal bridging?

    Timber
  27.  
    Well, I dont know what Timber is thinking, I thought 80mm reducing to 30mm, but I have never built a flat roof.
  28.  
    Posted By: TimberI prefer a true warm roof, but if you havn't got the space, then some form of hybrid setup would be best.
    Makes sense. Looks like a good compromise.

    David
    • CommentAuthorCWatters
    • CommentTimeMay 19th 2010
     
    Which is kinda where we started.

    Work out how much you can have above and between the joists and get a condensation risk analysis done for that combination.
    • CommentAuthorPeter Clark
    • CommentTimeMay 20th 2010 edited
     
    Posted By: CWattersWhich is kinda where we started.


    Yes in a way, but the ventilation above the insulation seems significant to me.

    Crofty mentioned decrement delay, could a ventilation gap like Timber's help significantly with controlling the room temperature beneath the roof during summer heat?

    Also if this ventilation technique contributes to making a hybrid roof safer from a condensation point of view, we could get some decrement delay by using denser insulation between joists.

    Is it at all normal to ventilate in this way - above the insulation in a warm/hybrid roof, Timber, or anyone?

    Is it likely to get building control approval?

    Peter

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