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General: Green roof ventilation

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  1.  
    • CommentAuthorrsk1
    • CommentTimeMar 19th 2021
     
    I've read a lot of past threads on the topic of moisture management under green roofs; generally inconclusive so i'm bringing it up again.

    Here is my roof build up for a low angled dual pitch roof, for a non domestic building that may only be used intermittently:

    turf + 100mm substrate
    1.2mm epdm
    18mm exterior ply
    150mm rafters, 400 centres with 100mm wood fibre ins between
    11mm osb3 with taped edges as vcl
    18mm batten as service void
    plasterboard

    100 insulation between 150 rafters leaves 50 void: keeping that void toward the warm side should be better in terms of reducing condensation?
    Plenty of people build "garden room" timber buildings along these lines. But the issue of course is condensation under the impermeable epdm. Would introducing a vb at the osb layer make this build up better or worse?

    One alternative is to leave 50mm ventilation gap above the insulation , from eaves to eaves. My concerns are that the green roof is then "removed" from the roof buildup and serves no insulation/ thermal mass function. Also, wind wash directly over the top of wood fibre (or any) insulation can't be good.

    I feel like i'm re-inventing the wheel getting my head round this stuff: green roofs are pretty common, what is the "normal" way to do it??
    • CommentAuthorrsk1
    • CommentTimeMar 19th 2021
     
    Or is wind wash only a problem with loose fill insulation such as rockwool, not with high density such as wood fibre?
    •  
      CommentAuthorfostertom
    • CommentTimeMar 19th 2021
     
    "100 insulation between 150 rafters leaves 50 void" sounds same to me as "One alternative is to leave 50mm ventilation gap above the insulation"?

    Either way, it's a waste of time without copiously venting the void, from eave to eave. That's how it should be. Yes, "the green roof is then "removed" from the roof buildup and serves no insulation/ thermal mass function" tho it does shield the insulation top from the radiant component, both 'from' mid summer solar heat and 'to' night time esp winter clear-sky cold. A simple tin-roof cover instead of the soil-on-ply would approx halve those radiant effects; the massiveness will further improve on that quite a bit.
    • CommentAuthorJonti
    • CommentTimeMar 19th 2021 edited
     
    Is there the same condensation problem with a green roof? or does the dew point occur in the growing medium? I have lived in two green roofed buildings and neither had moisture problems in the roof structure. Admittedly both had quite steep pitches.
    •  
      CommentAuthordjh
    • CommentTimeMar 19th 2021 edited
     
    Posted By: rsk11.2mm epdm

    I am not an expert, nor a lawyer. I believe in EPDM roofs - I have a couple. I think EPDM is OK under a green roof without a separate root barrier, but I also think it depends entirely on the quality of installation (particularly how the seams are sealed).

    150mm rafters, 400 centres with 100mm wood fibre ins between

    100 insulation between 150 rafters leaves 50 void: keeping that void toward the warm side should be better in terms of reducing condensation?

    One alternative is to leave 50mm ventilation gap above the insulation , from eaves to eaves.

    Only the last option is sensible, and in fact legal AFAIK. The gap needs to be ventilated as Tom says, and yes that means the impact of the green roof on thermal properties is reduced. My EPDM roofs are ventilated like that.

    11mm osb3 with taped edges as vcl

    AFAIK 11 mm OSB is not a vapour barrier. You need at least 18 mm and it has to be particular brands (the only one I know is Smartply Propassiv but I'm sure there are others). A separate vapour control membrane is likely a better bet. A cheap PE membrane should be enough.

    Wind wash on the outside of insulation is normal (and isn't normally called wind wash in that situation). That's why the external surface resistance (0.04) is less than the internal surface resistance (0.13)

    edit: As Jonti says, if the growing medium is thick enough and depending on the location, it may be possible to do without the gap. In any event you will need an expert condensation analysis and installation so hire a good expert.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 19th 2021 edited
     
    Posted By: djhAFAIK 11 mm OSB is not a vapour barrier. You need at least 18 mm and it has to be particular brands (the only one I know is Smartply Propassiv but I'm sure there are others)
    That's right - and Propassiv only because it's plastic paint coated. OSB is in fact usefully vapour-open, while with taped joints (I use 11 OSB3 gapfilling glued-and-screwed at all edges/joints) being usefully airtight (tho some disagree on 'how useful').

    Can you not increase the soil depth to 300mm or more, with structure to suit? Then the alleged benefits of a green roof can really kick in, subject as the guys say to very expert advice that the ventilation can be omitted. That calc will be more to do with the soil's thermal decrement/delay, than its insulation, which will change widely with variations in the soil's moisture content.
    • CommentAuthorrsk1
    • CommentTimeMar 19th 2021
     
    Thanks folks, good info. Possibly I was confusing airtight with vapour tight, but i defintely thought OSB was used for vcl's.
    300mm of substrate not an option unfortunately...
  8.  
    We're just in the process of having a sedum (extensive) green roof on top of low-angled (10-12 degree) single-pitch intermittently-used building in the garden. The structure is SIPS, then there's a membrane, then battens, then OSB, then GRP, then the cassette-system sedum.

    It's got no windows yet, and 2nd fix hasn't started, but hopefully there'll be no condensation.

    We ended up with extensive/sedum due to the weight/pitch/maintenance of other types. I think the only other point is that we needed a "flat-roof" skylight, rather than one for a "normal" roof angle.
    •  
      CommentAuthordjh
    • CommentTimeMar 21st 2021
     
    Posted By: rsk1i defintely thought OSB was used for vcl's

    It is/can be; just not the 11 mm kind from a random manufacturer.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 21st 2021
     
    I don't think it can be, at any thickness, unless plastic/paint coated?
  11.  
    If the insulation is below the ply roof deck, then it is a 'cold roof' with risk of condensation under the deck unless well ventilated. However if enough of the insulation layer can be positioned above the deck, then it is a 'warm roof' safe from condensation.

    There are options whether the watertight membrane goes above or below the insulation, depending if the insulation material is water resistant.

    In Scotland, cold roofs are now banned in legal building standards, because of the risk of condensation. The legal standards for new buildings also requires insulation value U=0.11 which is considerably better insulation than 100mm of woodfibre.

    What are the building regs/stds where you are, Rsk1? Could you insert some more insulation into your buildup, maybe above the roof deck and below the soil?

    For typical room air the dewpoint can be around +12degC so you always want the unventilated deck and any organic insulation to be warmer than that. I've no experience of green roofs, but I think that a few hundred mm of damp soil would not be enough insulation to do that, either cyclically or long-term.
    • CommentAuthorCWatters
    • CommentTimeMar 21st 2021 edited
     
    Posted By: WillInAberdeenIf the insulation is below the ply roof deck, then it is a 'cold roof' with risk of condensation under the deck unless well ventilated. However if enough of the insulation layer can be positioned above the deck, then it is a 'warm roof' safe from condensation.


    +1

    With anything that falls between these two camps a proper condensation risk analysis should be done.
    • CommentAuthorrsk1
    • CommentTimeMar 21st 2021
     
    Posted By: WillInAberdeen

    What are the building regs/stds where you are, Rsk1?


    As it is a small, detached building (<30m sq floor area), its exempt from BR as far as i understand.

    Another question: i'm considering a breathable membrane as a secondary watertight layer. Does it matter whther this BM is tight to the ply deck (ventilation void is underneath BM), or whether BM is tight to the insulation (ventilation void is above BM) ?
    •  
      CommentAuthordjh
    • CommentTimeMar 21st 2021
     
    Posted By: rsk1As it is a small, detached building (<30m sq floor area), its exempt from BR as far as i understand.

    That's certainly the case in England, dunno about elsewhere. But BR are there for a reason so they can certainly provide useful guidelines sometimes.

    Another question: i'm considering a breathable membrane as a secondary watertight layer. Does it matter whther this BM is tight to the ply deck (ventilation void is underneath BM), or whether BM is tight to the insulation (ventilation void is above BM) ?

    You'll perhaps have to remind us of your proposed buildup. Last I remember there was an EPDM membrane sitting on the plywood? Or do you mean the membrane is underneath the plywood? What would be the point of that?
  15.  
    Hi Rsk1, as it is a small building, it's even more important to insulate it properly, because the surface area is greater in proportion to the volume of the building. You'll only build it once, but if you don't insulate it properly when you build, you'll be wasting energy all through the life of your building. I's much more difficult to retrofit decent insulation later if your building is cold and damp!

    You could lay say 200mm slab polystyrene on top of the deck before you lay your green roof? It's cheap as chips. You could offset the cost by halving the thickness of woodfibre inside the deck, which would make the deck warmer and safer from condensation while still retaining enough woodfibre for thermal and humidity buffering. You don't want/need the VCL and services void then.

    In the UK, people complain that our buildings standards are very heat-leaky compared to other countries, which is true, but it's surprising how many designs are discussed on the Green Building forum which don't come close to meeting even the poor standards that we do have here! It's true there are a number of get-outs which mean people are not compelled to meet the requirements, but many folks do feel it's in their own interests to build warm dry buildings.
    • CommentAuthorCWatters
    • CommentTimeMar 22nd 2021
     
    Posted By: rsk1Another question: i'm considering a breathable membrane as a secondary watertight layer. Does it matter whther this BM is tight to the ply deck (ventilation void is underneath BM), or whether BM is tight to the insulation (ventilation void is above BM) ?


    For some reason not all membrane manufacturers approve their product for use in contact with insulation. I'm not sure why. I believe Kingspan make one that they say can be in contact.

    One issue on conventional pitched tiled roofs is that water blown under the tiles must be allowed to run down to the gutter. The membrane must not be pressed to the underside of the tile battens by the insulation as it could prevent this and allowing water to pool on the uphill side of the battens. This can be solved with counter battens.
    • CommentAuthorCWatters
    • CommentTimeMar 22nd 2021
     
    Posted By: WillInAberdeenYou could lay say 200mm slab polystyrene on top of the deck before you lay your green roof? It's cheap as chips. You could offset the cost by halving the thickness of woodfibre inside the deck, which would make the deck warmer and safer from condensation while still retaining enough woodfibre for thermal and humidity buffering. You don't want/need the VCL and services void then.


    +1.

    Making it a warm deck construction (all the insulation above the rafters) gets rid of the need for a ventilated void but raises the height.
    • CommentAuthorrsk1
    • CommentTimeMar 23rd 2021
     
    Posted By: WillInAberdeen
    You could lay say 200mm slab polystyrene on top of the deck before you lay your green roof? It's cheap as chips. You could offset the cost by halving the thickness of woodfibre inside the deck, which would make the deck warmer and safer from condensation while still retaining enough woodfibre for thermal and humidity buffering. You don't want/need the VCL and services void then.



    The ply "deck" is on top of the insulation, just below the green roof. So its at the cold outer edge of the build up. Not sure where you'd add 200mm polystyrene : above the epdm would make it part of the green roof, potentially submerged in water much of the time
    • CommentAuthorrsk1
    • CommentTimeMar 23rd 2021 edited
     
    Posted By: WillInAberdeen
    it's surprising how many designs are discussed on the Green Building forum which don't come close to meeting even the poor standards that we do have here!


    I take your point, but in this case it's a weighing up of: a small budget, desire to favour natural materials (eg.wood fibre vs polystyrene), and uncertainty over how and how much this building will end up be used (possibly not very often)
    • CommentAuthorJonti
    • CommentTimeMar 23rd 2021
     
    Just out of interest, why are you going for a low pitched roof on such a small building a steep pitch will allow for a much deeper root zone without much extra height.
  21.  
    Hi Rsk1,
    "Not sure where you'd add 200mm polystyrene "

    It can go between the ply and the membrane (a warm roof) or on top of the membrane (an inverted warm roof)
    - see eg here
    http://www.superhomes.org.uk/resources/insulate-a-flat-roof/

    Polystyrene doesn't mind being underground (some people use it under their foundations) but obviously some other insulants would prefer to be inside the membrane.

    A suitable amount of polystyrene is greener than an inadequate thickness of woodfibre (been a few historic threads about this) but by all means use thick woodfibre if you prefer, insulation values are similar.

    Understand you have a budget but skimping on the insulation isn't the way to save money - you just pay for it with heating bills and repairing condensation! But don't worry, you can certainly insulate well, for less money than the original buildup, see the website how.
      Screenshot_20210323-215000_copy_360x280.png
    • CommentAuthorrsk1
    • CommentTimeMar 25th 2021
     
    Thanks for the link and comments Will

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