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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorCranbrook
    • CommentTime5 days ago
     
    So would you say the information on the website below is not ideal

    https://www.rubber4roofs.co.uk/epdm-warm-deck-roof-design
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    The information on that website isn't ideal, no. It gives an incorrect expanation of the reason for preferring warm roof construction; the suggested description of a mechanically fixed roof doesn't meet building regs (at least as they were 8 years ago) - don't know about the adhesive construction; and the video it links to is about refurbing roofs (L1B) whereas the article and your situation are both new construction (L1A).
  1.  
    The manufacturers of the different flat roof membranes give fitting instructions, which need to be followed for the warranty and the BBA cert to be valid.

    All the ones I've seen require a build up similar to the link that Cranbrook posted, including a deck, VCL, insulation, membrane; but without the OSB the link shows between insulation and membrane. The VCL is fitted (as usual) on the warm side of the insulation but is there to control condensation on the cold side of the insulation, on the underside of the membrane, so best not put OSB there.
  2.  
    Posted By: CranbrookSo would you say the information on the website below is not ideal

    https://www.rubber4roofs.co.uk/epdm-warm-deck-roof-design" rel="nofollow" >https://www.rubber4roofs.co.uk/epdm-warm-deck-roof-design


    That page is a right mess! It looks like it has been cut & pasted together from other sources by someone who doesn't understand it. I would not trust any advice from this company.

    There's a fair bit of muddle generally about what "warm" and "cold" roof buildups mean - they seem to mean different things in different contexts. For example that article starts off talking about loft spaces being inside or outside of the insulation layer, which is a whole different subject and not really relevant to EPDM flat roofs.

    I've also seen roofs where the insulation is above the deck, but then there's ventilation between the insulation and the metal covering, described as "cold" in comparison to the same buildup without the ventilation which is then described as "warm". In both instances the structural deck is "warm".

    My solution is just to try and avoid using the terms altogether; they seem to cause more confusion than clarity.
    • CommentAuthorCranbrook
    • CommentTime4 days ago
     
    As I plan to use ‘classic bond’ epdm, I called their technical department today to Try and get a full spec from them for warranty purposes, unfortunately they don’t give full build up designs (maybe to avoid warranty claims) but he did say that they would advise having a vapour control layer immediately above the lower osb3/ply deck
    • CommentAuthorborpin
    • CommentTime4 days ago
     
    Posted By: Cranbrookbut he did say that they would advise having a vapour control layer immediately above the lower osb3/ply deck
    Usually this is because they don't understand the physics. As said elsewhere this concept is to stop vapour diffusing and reaching a cold part of the construction. So standard old style timber frame - insulation between studs, external OSB is on the cold side so if vapour can get from inside through the rockwool and reach the OSB, it will condense and cause issues. I shudder to think what my first house looks like.

    The vapour barrier is there to stop vapour from inside reaching the underside of the insulation and condensing on it. If the insulation is thick enough that the dew point is always inside the insulation it is unnecessary.

    Where you could come a cropper, is by putting insulation on the inside of the studs (as opposed to between the studs) effectively cooling the studs and the outer OSB. I used Iceyene between the studs as it helps to limit vapour movement and improves air tightness.

    Metal roofs are slightly different in that you might trap air between the insulation and the metal roof covering and that metal surface can change temperature quickly. if the air does not get moved, the vapour will condense on the metal surface, but that has nothing to do with interstitial condensation risk.

    Posted By: WillInAberdeenThe VCL is fitted (as usual) on the warm side of the insulation but is there to control condensation on the cold side of the insulation, on the underside of the membrane, so best not put OSB there.
    I'd argue that, assuming the condensation risk analysis says the dew point is inside the insulation (i.e. the insulation is thick enough) it doesn't matter as it would soon dry out on the odd occasion the conditions caused it. OSB does add to the structural strength.
  3.  
    Don't think so. You never get condensation inside or within an insulation layer, despite where the dewpoint lies, the condensation always collects on the outside of the insulation IE the inside face of the membrane.

    This is widely misunderstood but there's a nice explanation here with photo
    https://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

    In short, when the vapour reaches the dewpoint within the insulation, it has a 'choice' to condense there, or to carry on diffusing to an even-lower-energy place on the cold side of the insulation. So it carries on moving, until it hits the inside of the membrane, where it has to stop.
    • CommentAuthorCranbrook
    • CommentTime3 days ago
     
    If I wasn’t confused enough before, I most certainly am now 🙃

    So.. as standard I have an 18mm OSB fixed in to my flat roof joists (on fittings), I wish to use an EPDM rubber roof, can somebody give me a suitable spec to carry on?
    • CommentAuthorlineweight
    • CommentTime1 day ago edited
     
    Posted By: CranbrookAs I plan to use ‘classic bond’ epdm, I called their technical department today to Try and get a full spec from them for warranty purposes, unfortunately they don’t give full build up designs (maybe to avoid warranty claims) but he did say that they would advise having a vapour control layer immediately above the lower osb3/ply deck

    Yes, that would be the standard way of doing it. VCL above the deck.

    You ought to be able to find a supplier who will give you a spec. However their warranty may depend on it being installed by an "approved installer".

    See if you can get anything out of Classicbond for example

    http://classicbond.co.uk/downloads/
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