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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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    • CommentAuthorShevek
    • CommentTimeJul 1st 2021 edited
     
    Has anybody taped joints when installing OSB T&G roof sarking? I see they do in the US with that stretchy ZIP tape system.

    If you read the likes of Smartply's roof installation instructions they say joints should be glued with PVA to ensure a snug fit and reliable distribution of strength, stiffness and spanning capacity along and across the panel.

    The project I'm currently working on is insulation between rafters only. The rafters are I-joists but only 220 mm deep so I was considering specifying taped joints as an additional measure against interstitial condensation. But the developer/client may consider this overkill, particularly given that there is also a breather membrane, which itself could have taped joints (although the project is only in zone 1 on the Wind Zone Map).

    What do you think? Also does anybody know if it's possible for a WUFI assessor to take into account OSB joints taped vs not taped when they do a dew point calc?

    This is my proposed roof build up:

    - Plain roof tiles
    - 25 mm Roof tile battens
    - 25 mm Counter battens (maybe 11 mm)
    - Proctor Roofshield OR Solitex Plus, maybe joints taped
    - 15 mm T&G OSB, joints glued, maybe taped
    - 220 I-Joists rafters, fully filled with Earthwool Rafter Roll
    - Pro Clima Intello Plus vapour control, joints taped
    - 25 mm battens/service zone
    - 12.5 mm plasterboard
    •  
      CommentAuthordjh
    • CommentTimeJul 1st 2021 edited
     
    Posted By: ShevekHas anybody taped joints when installing OSB T&G roof sarking? I see they do in the US with that stretchy ZIP tape system.

    If you read the likes of Smartply's roof installation instructions they say joints should be glued with PVA to ensure a snug fit and reliable distribution of strength, stiffness and spanning capacity along and across the panel.

    Could you give a link to the actual instructions you're referring to, please?

    I was considering specifying taped joints as an additional measure against interstitial condensation.

    I'm not clear how exactly taping joints on sheets on the cold side will help prevent insterstitial condensation. What do you think it will do?
    • CommentAuthorShevek
    • CommentTimeJul 1st 2021 edited
     
    This one Dave:
    https://files.mutualcdn.com/medite-smartply/files/SMARTPLY-ROOF.pdf

    Posted By: djhI'm not clear how exactly taping joints on sheets on the cold side will help prevent insterstitial condensation. What do you think it will do?

    By reducing air leakage, and therefore the transportation of water vapour through gaps in the building fabric.* It's the cold side but it's also the windy side.

    * and by preventing the flow of cold outdoor air into the structure, cooling interior surfaces
    •  
      CommentAuthordjh
    • CommentTimeJul 1st 2021
     
    Thanks for the link. That's very helpful.

    Posted By: Shevek
    Posted By: djhI'm not clear how exactly taping joints on sheets on the cold side will help prevent insterstitial condensation. What do you think it will do?

    By reducing air leakage, and therefore the transportation of water vapour through gaps in the building fabric.* It's the cold side but it's also the windy side.

    * and by preventing the flow of cold outdoor air into the structure, cooling interior surfaces

    You seem to be ignoring the bit where it talks about Specification and Design:

    "In order to avoid condensation on the underside of sarking
    panels, ventilation must be provided in addition to a
    correctly placed vapour control layer in accordance with
    established design guidance.

    "Where sarking
    panels are used over a ‘room in the roof’ with sloping ceilings
    and insulation between the rafters, there must be adequate
    ventilation space to the underside of the sarking over the
    insulation, ventilated from eaves to ridge, and a vapour
    control layer positioned on the warm side of the insulation."

    Since you're required to have ventilation under the OSB, the question of preventing air movement doesn't really come into it. You have to encourage air movement! You won't be able to use full-filling of the rafters either, so can you achieve the desired level of insulation?

    I'm no expert but AIUI sarking is often separate planks with gaps left between them. I take it you're planning to use the OSB as a structural member as well? Our roof is full-filled with ply on top and panelvent below. There's Intello underneath and Solitex above. Ecological Building Systems did a WUFI analysis to provide the required evidence for building control to allow us to do that. The Intello provides the vapour control and airtightness. The Solitex provides breathable watertightness.
    • CommentAuthorjfb
    • CommentTimeJul 1st 2021
     
    what is the OSB there for?
    • CommentAuthorShevek
    • CommentTimeJul 1st 2021 edited
     
    Posted By: djhYou seem to be ignoring the bit where it talks about .. there must be adequate ventilation space to the underside of the sarking over the insulation

    Yes I was ignoring that on the basis that the OSB is sufficiently vapour permeable and that a WUFI assessment will identify any problems. It's not an unusual build up. What I'm worried about is if I-Joists + Insulation are thick enough to avoid any problems.

    Our roof is full-filled with ply on top and panelvent below. There's Intello underneath and Solitex above. Ecological Building Systems did a WUFI analysis to provide the required evidence for building control to allow us to do that.

    That's handy to know they do WUFI analysis, thanks. What was the ply you used on top?

    Posted By: jfbwhat is the OSB there for?

    For racking strength (and windtightness). But perhaps we could use a board internally for racking?
    •  
      CommentAuthordjh
    • CommentTimeJul 1st 2021
     
    Posted By: ShevekYes I was ignoring that on the basis that the OSB is sufficiently vapour permeable and that a WUFI assessment will identify any problems.

    Will you also need to get Medite's agreement to waive their instructions? I don't know what the current rules are.

    What I'm worried about is if I-Joists + Insulation are thick enough to avoid any problems.

    Dunno about that. We have 450 mm of Warmcel and that's definitely enough!

    That's handy to know they do WUFI analysis, thanks. What was the ply you used on top?

    2 layers of 9 mm glued together (to allow it to curve around the barrel vault).

    perhaps we could use a board internally for racking?

    That's what our Panelvent does. But also the ply as well of course.
    • CommentAuthorShevek
    • CommentTimeJul 2nd 2021 edited
     
    Posted By: djhWill you also need to get Medite's agreement to waive their instructions? I don't know what the current rules are.

    Or just use a different product. The issue here is BS 5250. It effectively defines OSB/ply as being impermeable.
    • CommentAuthorShevek
    • CommentTimeAug 12th 2021 edited
     
    Just to give you a little update, we had Wufi calcs done on our proposed buildup and it achieved a 'very high degree of security against interstitial condensation and moisture accumulation'. 18 or 15 OSB is fine.

    - Plain roof tiles
    - 25 mm roof tile battens
    - 11 mm counter battens
    - Solitex Plus
    - 18 mm T&G OSB, joints glued
    - 220 I-Joists rafters, fully filled with Knauf 32 Glass Mineral Wool
    - Pro Clima Intello Plus vapour control, joints taped
    - 25 mm battens/service zone
    - 12.5 mm plasterboard
    • CommentAuthortony
    • CommentTimeAug 12th 2021
     
    I think that in service the OSB will at some point get a little damp and swell up a bit irreversibly- I would leave gaps between the sheets 5mm on length 2.5mm on width. It comes very dry but in a roof it will experience different and varied and varying conditions.
    •  
      CommentAuthordjh
    • CommentTimeAug 13th 2021 edited
     
    Posted By: tonyI think that in service the OSB will at some point get a little damp and swell up a bit irreversibly

    Where's the water going to come from Tony? And why did wufi say something like that wasn't even close?
    • CommentAuthortony
    • CommentTimeAug 14th 2021
     
    Try it and see, frosty night, wood gets cold, air warms up or is already warm from leaking out from house, condenses in cold wood sheet, just a little bit will cause it to swell
    •  
      CommentAuthordjh
    • CommentTimeAug 14th 2021
     
    Frosty night, in Portugal?

    FUD
    • CommentAuthortony
    • CommentTimeAug 14th 2021
     
    They get radiation frosts on clear nights in the Sahara
  1.  
  2.  
    The biggest issue here is Air-Tightness not WV Diffusion, 99 times more moisture arrives to the outer layer of a roof by leakage than by diffusion (concentrated at the OSB joints) so making the roof airtight (sealing OSB joints) significantly reduces that. What level of airtightness did WUFI assume when your calculations were done djh and Shevec? My research concluded that the roof build-up becomes irrelevant when an air'tightness level of less than 1ACH is acheved.
    Have you considered a layer of EPS on top of the OSB Shevec? The Canadian Home Builders Manual states that 10mm of EPS on top of a rafter reduces the condensation risk on the rafter by 90%. Like tony I've seen too many spongy OSB flat roofs, some Scandinavian roof companies are moving away from OSB directly beneath the Fiberglass or Parlon towards high density EPS.
    •  
      CommentAuthordjh
    • CommentTimeAug 15th 2021
     
    Posted By: Viking HouseWhat level of airtightness did WUFI assume when your calculations were done djh

    Dunno. I never saw the calculations or results. Just a guarantee from EBS for my roof. FWIW my airtightness was measured afterwards as 0.47 ACH (design value was < 0.6 ACH).
    • CommentAuthorShevek
    • CommentTimeAug 16th 2021
     
    I'm in Portugal but this project is in Watford.

    1 air change assumed, based on taping of joints and gluing OSB joints.

    Posted By: Viking HouseHave you considered a layer of EPS on top of the OSB Shevec? The Canadian Home Builders Manual states that 10mm of EPS on top of a rafter reduces the condensation risk on the rafter by 90%.

    No I hadn't, that's quite a figure.
  3.  
    Posted By: ShevekNo I hadn't, that's quite a figure.
    You may also be able to lose the Intello and service battens!
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