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Building Regulations and Standards: Roof: ventilation through osb

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1 to 23 of 23

  1.  
    • CommentAuthorchc
    • CommentTimeJun 10th 2010
     
    Hi

    Our new shallow pitched roof is currently:

    Ventilated ridge
    Plastisol coated corrugated profile steel sheets with ventilated eaves
    50 x 50mm protimised purlins @ 600mm centres
    Tyvek Supro breather membrane
    counter battens to provide drainage channel for condensation on the breather membrane
    min 9mm OSB
    47 x 195mm C24 rafters @ 600mm centres, & between them:
    . 95mm air gap
    . 100mm Celotex GA3100
    35mm Celotex TB3035
    12.5mm plasterboard

    I understand that we need ventilation through the OSB for vapour to escape, and "holes" have been suggested. Any suggestions of where I might find reputable advice on size & placement to present to our BCO?

    Huge thanks in advance!
  2.  
    Is the OBS open at the ridge and wall plate? Do you have a Vapour Control membrane under the rafters?
    • CommentAuthortony
    • CommentTimeJun 10th 2010
     
    Cant see why holes are needed??
    • CommentAuthorchc
    • CommentTimeJun 10th 2010 edited
     
    Posted By: Viking HouseIs the OBS open at the ridge and wall plate? Do you have a Vapour Control membrane under the rafters?


    I believe it'll be open at the ridge insofar as it's specified as racking so presumbably won't have joints sealed (?), but I'm afraid no idea as to the wall plate.
    No vapour control membrane specified as such. Perhaps the Celotex TB3035 + plasterboard do this?

    Posted By: tonyCant see why holes are needed??

    To allow vapour out?

    Thank you for your replies. My ignorance is probably very obvious. Am open to all comments & suggestions!
  5.  
    I can see why you need to ventilate below the non-vapour open profile steel sheets, but why the 95mm air gap below the OSB? Is this just to save insulation? Or did the designer want to keep some of the insulation below the rafters to combat cold bridging?

    Air gaps are a bad idea unless deliberately sealed or ventilated. On the cold side of the insulation they can lead to condensation when the outside air temperature drops. It seems ridiculous to pay for expensive 100mm Celotex insulation & then leave a 95mm air gap. Why not use a considerably cheaper mineral wool insulation to fill the void, such as 200mm of Knauf Rafter Roll 32? As long as you keep the 35mm Celotex below the rafters to limit cold bridging then performance will not be affected.

    Limit vapour transfer by taping the joints of the 35mm Celotex below the rafters or fitting a polyethylene or Tyvek SD2 vapour control layer. As long as you do this then the Tyvek Supro breather membrane will be able to dissipate any water vapour reaching the OSB into the ventilated cavity between the counter battens. You do not need to cut holes in the OSB & to do so is only likely to lead to additional air leakage.

    David
  6.  
    If you have a 7-10mm gap at the ridge it should let enough vapour out I would think but it seems an odd build up. What is the room used for? Is the room ventilated?
    • CommentAuthorchc
    • CommentTimeJun 10th 2010
     
    David,

    Thank you for your excellent reply!

    The air gap was originally 50mm as the rafters were originally 50 x 150mm, but since then the rafters were upgraded to 195 - hence the larger gap.
    Forgive my stupid question, but isn't 200mm Knauf Rafter Roll 32 too thick to fit between 195mm rafters?

    If I understand correctly you're proposing <i>preventing</i> vapour transferring from the rooms through to the OSB, which does sound better to me than enabling it, I think.
    • CommentAuthorchc
    • CommentTimeJun 10th 2010
     
    Posted By: Viking HouseIf you have a 7-10mm gap at the ridge it should let enough vapour out I would think but it seems an odd build up. What is the room used for? Is the room ventilated?


    Office, kitchen, bathroom. All with windows/ trickle vents, the latter 2 with mechanical extraction.

    All suggestions gratefully received!
    • CommentAuthortony
    • CommentTimeJun 10th 2010
     
    What is 5mm between friends? and it is lots cheaper so you might have a rethink?
    • CommentAuthorchc
    • CommentTimeJun 10th 2010
     
    Oh yes I'm definitely rethinking!
  11.  
    Posted By: chcForgive my stupid question, but isn't 200mm Knauf Rafter Roll 32 too thick to fit between 195mm rafters?
    It's not a stupid question. It will fit; it arrives in roll form & its very squashy. It will expand to fill the space & a tight fit is better for limiting thermal bypass. However, it will exert some pressure on the 35mm Celotex below the rafters, so these need boards to be well fixed or secured by the plasterboard.

    Posted By: chcIf I understand correctly you're proposing preventing vapour transferring from the rooms through to the OSB, which does sound better to me than enabling it, I think.
    Yes, because the OSB is on the cold side of the insulation, there is a risk that excessive vapour transfer from the room will lead to condensation on the OSB. As long as the transfer rate is low enough, the OSB & breather membrane will be able to dissipate it under most conditions, so there'll be no long term build-up.

    David
    • CommentAuthorchc
    • CommentTimeJun 10th 2010
     
    Thank you, I'm totally convinced and hope our BCO will be too. So this is what I'll present:

    Ventilated ridge
    Plastisol coated corrugated profile steel sheets with ventilated eaves
    50 x 50mm protimised purlins @ 600mm centres
    Tyvek Supro breather membrane
    50 x 38mm counter battens to provide drainage channel for condensation on the breather membrane
    min 9mm OSB
    47 x 195mm C24 rafters @ 600mm centres, & between them:
    . no air gap, thus avoiding condensation under the OSB
    . 195mm Knauf roll 32 (2 * 100mm thicknesses, squashed a bit)
    35mm Celotex TB3035 with joints taped to control vapour
    12.5mm plasterboard

    I hope the Knauf roll is easy to cut lengthwise as the rafters are 600mm apart.

    Do we now still want a gap in the OSB at the ridge?

    Huge thanks for your help!
  13.  
    No gap needed. Keep the OSB windtight & just ventilate above the breather membrane.

    David
    • CommentAuthorchc
    • CommentTimeJun 10th 2010
     
    Thanks!

    For "windtight" should we be taping the all OSB joints, just the ridge (as there's bound to be something of a gap), or not taping at all?
    • CommentAuthorCWatters
    • CommentTimeJun 10th 2010
     
    Why do you need the OSB? Racking?
    • CommentAuthorchc
    • CommentTimeJun 10th 2010
     
    Posted By: CWattersWhy do you need the OSB? Racking?

    correct
  17.  
    Posted By: chcFor "windtight" should we be taping the all OSB joints, just the ridge (as there's bound to be something of a gap), or not taping at all?
    Fitting the breather membrane direct to the OSB & securing with the counter battens should ensure the combination is windtight. The gap at the ridge should be covered with a roll of breather membrane centred on the ridge line.

    David
    • CommentAuthorchc
    • CommentTimeJun 11th 2010
     
    Posted By: davidfreeboroughFitting the breather membrane direct to the OSB & securing with the counter battens should ensure the combination is windtight.

    The breather membrane is currently above the counter battens as I understood it would need to sag a little to avoid condensation potentially wetting the counter battens.
    ?
    • CommentAuthorMikeRumney
    • CommentTimeJun 11th 2010
     
    same here
  20.  
    Posted By: chcThe breather membrane is currently above the counter battens as I understood it would need to sag a little to avoid condensation potentially wetting the counter battens.
    Some perforated polyethylene type membranes will wet materials with which they're in contact, this is called "tenting". It is not recommended to fit tenting membranes direct to the OSB or other sarking boards. So they need to be draped over the counter battens. However, I would not use these membranes.

    Tyvek Supro is defined in its BBA certificate as a "non-tenting" membrane. It can be used in direct contact with OSB or other sarking boards. This achieves far better windtightness than draping it over the counter battens. It is normal practice for timber framed walls, I see no reason to do it differently in the roof, especially where doing it differently reduces energy efficiency & creates additional condensation risks.

    David
    • CommentAuthorMikeRumney
    • CommentTimeJun 11th 2010
     
    Hi Davidfreeborough,
    this was a useful post for me as I'm thinking of using the space made by the counterbattens as part of the airflow space in an "envelope" set up ... and having the membrane flat and tight to the back of this space just feels right compared to a "hung" one.
    Wondering now though whether there's a membrane that does the same job as Supro but with better heat reflecting qualities?
    The other thing you might be able to tell me is whether my plan to put Supro around the wall frame is a bad one.
    (It would be lying against the outer face of Thermafleece, with vertical battens over to continue the envelope space, then OSB+render for racking & weather ... ?
  22.  
    There are metalised breather membranes like Tyvek Enercor which reduce radiative heat transfer. However, all the ones I've seen are designed to have an airgap below them, so you're back to draping the membrane over the counter battens. It's not clear whether this is just to maximise the insulative benefit (so that the air gap's thermal resistance adds to that of the low emissivity surface) or whether the membrane can be used upside down where the main concern is reflecting summer heat.

    For my point of view these membranes are not worth the additional money. A draped membrane is difficult to seal, so it's difficult to seal the air space below the membrane. If this air space is not sealed the thermal benefits can be completely negated by air movement & the thermal performance of any insulation below can also be reduced. I would use a conventional breather membrane, fit it direct to the OSB & concentrate on making it windtight.

    It's normal to put the breather membrane outside the OSB in a timber framed wall, but if you need OSB to support the render then your build-up makes sense. Just make sure the membrane is pulled tight & taped, & that the gap between it & the OSB is well ventilated. Alternatively, could you use something less vulnerable to decay than OSB? How about calcium silicate boards?

    David
    • CommentAuthorMikeRumney
    • CommentTimeJun 11th 2010
     
    Davidfreeborough:
    Thanks for this Dave.

    I also wonder whether reflective membrane would be the outlay ... and I haven't looked into it yet but I'm not expecting them to have the best "green" record in terms of embodied energy or materials used.
    Will find out if I can.

    The gap between membrane and OSB should end up well ventilated as it's supposed to be part of the circulating air "loop" for AGS ...

    Will also look into the Calcium Silicate boards. They'll also have to stack up well on "£" and "eco" fronts ... I've used OSB3 in various situations and have got used to it's workability, so will be comfortable working with it, but I wouldn't be on here if I was closed to alternatives!

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