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Housing - New Build: Ventilation gap under sarking board

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  1.  
    • CommentAuthorEd Davies
    • CommentTimeApr 17th 2014
     
    22 x 150 whitewood sarking boards over timber 235 mm I-beam rafters. Sarking boards are separated by a couple of mm to be vapour open. There will be a membrane, counter battens, battens and steel roof on top.

    The question is, is it OK to full-fill between the rafters up to the sarking with glassfibre or is a ventilation gap above the glassfibre, below the sarking, needed? My intuition is that the glassfibre is sufficiently vapour open that the ventilation gap is redundant but it'd be nice to be a) sure and b) able to prove it to a Scottish BCO.
    • CommentAuthortony
    • CommentTimeApr 18th 2014
     
    Is it a warm roof or a cold roof :confused:
    • CommentAuthorEd Davies
    • CommentTimeApr 18th 2014
     
    All the real structure (post and beam) is on the warm side so I suppose it's a warm roof. The I-beam OSB web passes through the insulation so the top flange is on the cold side. Second drawing here:

    http://edavies.me.uk/2013/09/roof-rethink/

    Going back to the designer to discuss whether using OSB for the sarking layer would eliminate the need for this ventilation layer.
  4.  
    You could probably get away with full fill IF the space under the metal roof sheeting is well ventilated, eaves, ridge and in between.
  5.  
    I would ventilate on top of the breather membrane & sarking boards. I would also use T&G OSB as the sarking board & fully fill with insulation below it. That way, you're using the breathability of the the breather membrane & usign it & the sarking boards to keep wind out of your insulation.

    As bot says, with this arrangement & a non-vapour open roof covering, you need to ventilate at eaves & ridge.

    David
    • CommentAuthorEd Davies
    • CommentTimeApr 18th 2014
     
    Thanks chaps - yes, would be fully ventilated at the eaves and ridges with standard profile steel foam filler bits.

    T&G OSB was my original thought, too. House designer originally suggested 9 mm OSB (which I'm pretty sure you can't get as T&G, right?) so I was a bit surprised when the spec came back with the sawn timber sarking boards. Didn't argue until I realized the implications of the air gap.

    Haven't heard back from him but it's a lovely day and he's probably busy working on his own house's extension today.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 19th 2014 edited
     
    Posted By: Ed Daviesfully ventilated at the eaves and ridges
    that implies through ventilation above the insulation, below the impermeable covering. Otherwise, why ventilate?

    Have we done this one before? Despite what metal roof cos say, WUFI says it's sure death without that through ventilation. Facing a clear sky, even daytime sometimes, the metal roof is a super-cooled condensation plate, and all that permeability just feeds more and more vapour up to be condensed. The metal roofing will be literally dripping - the cause of every caravan's rapid rot fate.

    Whereas with copious through ventilation allowing the fabric to dry diurnally and seasonally in both directions, it's very robust and tolerant of that condensation water, even of moderate leakage. No need for vapour barriers/checks anywhere, even 'intelligent' ones - in fact performs better without.

    As usual, don't take my word for it (unless paying for consultancy!)- check it out to your satisfaction.
    • CommentAuthorEd Davies
    • CommentTimeApr 19th 2014
     
    Thanks Tom. I don't think the need for ventilation above the sarking/membrane is even slightly controversial.

    The question is, would I need another gap below the membrane and timber, but above the insulation? Presumably not open to the world but allowing vapour from under the middle of a sarking board to go 75 mm to the nearest gap between boards. What I was hoping for was an example of timber sarking board of this type (presumably in Scotland as it's a bit of a Scottish habit) with vapour open insulation like mineral wool right up against it. Absent that I'll want to change to OSB but we're getting a bit late in the design process for that.
    • CommentAuthortony
    • CommentTimeApr 19th 2014
     
    I would say that so long as you have a vb on the warm side of the insulation then no need for a void below the breathable sarking.
  10.  
    Hi Ed, I personally would go with 18mm t+g OSB, the traditional sarking board and penny gaps in combination with a membrane outside I would be concerned more about too much air movement getting through both, you are asking a lot of a site fitted membrane to stop wind washing your mineral wool insulation. I don't doubt it "would do" and be passed off by all and sundry but that's just standard for a cold roof on a timber frame bungalow, not an efficient A frame house which is all roof.

    There should be no problem with fully filling between the I beams and up to the inside of the sarking as others have said. Your roof as currently proposed is not a true warm roof as the rafters aren't fully warm, it is more a hybrid, the first drawing you have is a proper warm roof with the rafters inboard.

    I don't think anyone will have a problem with changing to OSB instead of 6" sarking boards at this stage and it should be a bit quicker to install and get properly air tight, especially if you are looking to minimise external labour in getting wind and water tight.
    • CommentAuthorEd Davies
    • CommentTimeApr 19th 2014
     
    Thanks willie, yes, t+g OSB is my preference. I am right in thinking 18 mm is the thinnest t+g, isn't it?

    With sarking board I'd do a tony and fold the membrane over at the join but that'd be a big faff on 8 metres of roof on an site where it's breezy more often than not. With t+g I'd just make sure there's a counterbatten over the overlap.
  12.  
    Yes the 18mm is the thinnest commonly available t+g, I think you are right to try and simplify the membrane installation as much as possible.
    • CommentAuthorEd Davies
    • CommentTimeApr 20th 2014 edited
     
    Posted By: fostertomDespite what metal roof cos say, WUFI says it's sure death without that through ventilation. Facing a clear sky, even daytime sometimes, the metal roof is a super-cooled condensation plate, and all that permeability just feeds more and more vapour up to be condensed. The metal roofing will be literally dripping - the cause of every caravan's rapid rot fate.
    Going off-topic on my own thread, why can't you think of it as the metal skin acting as a dehumidifier for the roof structure below? Water condenses on the metal then runs down to the bottom of it or drips onto the membrane and runs down that. So long as it gets out at the bottom, without being dammed by battens or anything, then all is well. All extra ventilation serves to do is to bring in more water vapour.

    (Sort of like how the single glazed windows in the static caravan I'm staying in act as a dehumidifier so long as I wipe them dry every morning.)
  14.  
    Posted By: Ed Davies
    (Sort of like how the single glazed windows in the static caravan I'm staying in act as a dehumidifier so long as I wipe them dry every morning.)


    Karcher window vac - great for this, gets rid of the water without having to worry about drying towels. Been there done that! Depending on the time of year we are talking about 1.5 tank fulls on a static - 150ml water held on the windows.
    • CommentAuthorEd Davies
    • CommentTimeApr 21st 2014
     
    I use a car windscreen towel which, in the worst cases, gets completely saturated. I put it in the well ventilated broom cupboard with the instant gas heater and it's dry, or very nearly so, by the next morning. If I was here permanently then, yes, a gadget like that might be good.
    • CommentAuthorTimber
    • CommentTimeApr 21st 2014
     
    BS5250 states (correctly or not) that if using a solid board as the sarking, it needs to be considered as a high resistance underlay situation and so vented below the sarking board.

    If using planks with gaps, it is considered a low resistance membrane and ventillation can be from above.

    I don't fully agree with this (if it works for a sheathed timber frame wall, why not a roof), but that is what 5250 says.
  17.  
    I was going to say, the white wood sarking boards = no vent gaps needed
    OSB = ventilation gap needed!

    I prefer the idea of the sarking boards over the OSB.
    • CommentAuthorEd Davies
    • CommentTimeApr 22nd 2014 edited
     
    Posted By: TimberIf using planks with gaps, it is considered a low resistance membrane and ventillation can be from above.
    Thanks - yes, would have gaps (few mm) between the planks and would be ventilated above. Do you know if they say whether the insulation can be against the board or if a gap under the board, above the insulation, is needed?

    Posted By: bot de pailleOSB = ventilation gap needed!
    That's surprising - I thought various forms of vapour open insulation (sheep's wool, mineral wool, perhaps cellulose fibre) packed in OSB boxes was common.

    Just to be clear - you mean a ventilation gap between the OSB and the insulation? Open to the outside world?

    With the sarking boards with few mm gap between them I can see the point of a ventilation gap between the board and the insulation which is not open to the outside world so that the insulation under the middle of the board is not as cold as the board but 75 mm (half a 150 mm board width) away from a ventilated gap. With OSB I can't see any equivalent reason for a not-open-to-the-outside-world gap.

    Does the thickness of the OSB make a difference?


    Edit: spurious and confusing “not” crossed out.
    • CommentAuthordickster
    • CommentTimeApr 22nd 2014
     
    Don't forget a stainless mesh (plasterer's edge stuff) to stop the wee mices.
  20.  
    I would go with 18mm T&G OSB sarking board with the only gap being the ventilated one above the breather membrane & sarking board.

    Tell Building Control that its a hybrid warm roof & if they question it then tell them its no different to a timber framed wall with a ventilated gap on the outside. If you need a reference then choose a breather membrane where the BBA certificate specifically states it can be used "fully supported" over sarking boards.

    Looking at the Tyvek Supro BBA there's some interesting wording in Section 15:

    http://construction.tyvek.co.uk/Tyvek_Construction/en_GB/assets/downloads/certificates/roof_bba_tyvek_supro_in_warm_non_ventilated_cold_ventilated_roofs.pdf

    According to Section 15.1, it is OK to fully support it with softwood sarking boards or insulation. However, it defines the former as a "cold ventilated roof" & the latter as a "warm non-ventilated roof". Does this imply that the softwood sarking boards require ventilation from below? At what thermal resistance does softwood become insulation? Would using 22mm T&G woodfibre sarking boards get around the problem as these are recognised as having useful thermal resistance?

    David
    • CommentAuthorEd Davies
    • CommentTimeApr 22nd 2014 edited
     
    Posted By: davidfreeborough…tell them its no different to a timber framed wall with a ventilated gap on the outside.
    Exactly - it's what I'll have on my gables. That'll only be 9 mm though … hmm, may need some little noggin things to seal behind the joins.

    Woodfibre is an interesting idea which I hadn't previously considered because of cost but if it sorts out any residual condensation risk while providing structure and some insulation it might make sense.

    And, yes, vermin resistance is needed. The steel roof people have a scheme for filling in the ends of the profile while allowing ventilation but that's mostly to stop rain getting blown in - will consider if it's likely to stop animals, too.
    • CommentAuthorTimber
    • CommentTimeApr 22nd 2014
     
    If gaps between boards, then BS 5250 says that insulation can be fully packed.

    HOWEVER BS 5250 does state that if you can model and prove anther construction method, then that is also fine.

    So for me, it makes sense to go for T&G OSB, rafters fully filled, vented above breather membrane with a high resistance VCL on the inside. Model it and use that to satisfy BCO. T&G OSB is still sufficiently breathable with a high resistance membrane (well fitted) on the inside, and being T&G will act as a good air tightness membrane on the outside to prevent against wind washing.

    For the record - I was stating what the standard says. I actually dislike some of BS 5250 as it confuses things like warm and cold roofs as well as requirements for VCLs on SIPs (to name a few issues with it).
    • CommentAuthorEd Davies
    • CommentTimeApr 22nd 2014
     
    Posted By: TimberIf gaps between boards, then BS 5250 says that insulation can be fully packed.
    That's a useful fallback to know about, thanks.

    Still prefer T&G but a bit concerned about condensation. A lurker on this forum has just sent me an analysis with software which was a couple of years ago approved for this sort of thing which shows a risk but he's a bit unsure about the resistance of the OSB and also didn't include a vapour barrier, just the higher resistance of my PUR.
    • CommentAuthorEd Davies
    • CommentTimeApr 23rd 2014
     
    Some thoughts on the vapour resistance of OSB (with speculation on why there might be so much confusion and such a wide range of stated values):

    http://edavies.me.uk/2014/04/osb_vapour/
    • CommentAuthorTimber
    • CommentTimeApr 23rd 2014
     
    Ed - no VCL then no wonder it showed a problem. I am confident that if modelled with a high resistance VCL on the warm side, the model would show no condensation risk with T&G OSB sarking. It works for timber frame walls!
    • CommentAuthorShevek
    • CommentTimeMar 15th 2021
     
    Most people in this thread seem to prefer 18 mm T&G OSB for breathable sarking. But then you go to other threads and people are suggesting 18 mm OSB internally as a vapour control layer! Can it really perform both tasks?

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