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Housing - Renovation: Fixing OSB airtight layer

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    • CommentAuthorpmagowan
    • CommentTimeJan 29th 2012
     
    Why tyvek at the counterbattons VH? Is it better than cheaper roofing felt?

    The problem is that every solution is greater expense than the last. I think I might go with polythene at the OSB and ordinary roofing felt at the counterbattons. I will take the risk that enough of the insulation is outbound of the OSB to avoid condensation. I will monitor and if I am wrong I can either increase ventilation of the attic through MVHR or fix another polythene VCL on the inside of the rafters.

    Next thing I'll be giving this roof a coat of gold leaf!:surprised:
  2.  
    Posted By: pmagowanWhy tyvek at the counterbattons VH? Is it better than cheaper roofing felt?
    The outer most layer is the one that needs to be breathable to minimise condensation risk. Tyvek is breathable; bituminous roofing felt is not.

    I believe a well sealed wind barrier outside the insulation is almost as important as a well sealed air barrier inside (or part way through in this case) the insulation. So I would recommend Tyvek Supro Plus (or similar) with taped laps for the wind barrier/sarking membrane.

    David
    • CommentAuthorborpin
    • CommentTimeJan 30th 2012 edited
     
    Posted By: davidfreeborough
    Posted By: pmagowanWhy tyvek at the counterbattons VH? Is it better than cheaper roofing felt?
    The outer most layer is the one that needs to be breathable to minimise condensation risk. Tyvek is breathable; bituminous roofing felt is not.
    I wonder about this. If you have a sarking (OSB) layer, then a very vapour impervious insulation (PIR/PUR) on top, then why do you need a breatheable layer on top of the insulation and under the tiles? Equally, why would you need it on top of the OSB? the vapour has no where to breathe (out) to. Of course this assumes that you are allowing the structure to breathe in.
    Posted By: davidfreeboroughI believe a well sealed wind barrier outside the insulation is almost as important as a well sealed air barrier inside (or part way through in this case) the insulation. So I would recommend Tyvek Supro Plus (or similar) with taped laps for the wind barrier/sarking membrane.
    This is a different issue. However, again, if using a foil backed insulation board such as Celotex, if that was taped it would provide a pretty effective waterproof layer. But in any case, the idea is for the air barrier to be the OSB.

    Personally, for a build-up that has a foil backed, vapour and liquid impermeable insulation layer on top of a well sealed OSB layer, I see no reason for any additional layer except for VH's reason of temporary (and more resiliant) waterproofing.
    • CommentAuthorpmagowan
    • CommentTimeJan 30th 2012
     
    My plan was to sag the roofing felt over the counterbattons and this would not provide a complete barrier. My impression was that its job was to divert water that was blown under the slates off the roof. The sagging over counterbattons was so that a valley is formed between each counterbatton and thus the water is directed away from the wood and perforations by fixings. In doing this it would be very difficult to seal one layer to the next with tape. I have 400m2 of roof and with the price of tyvek, 2 layers is going to come to about £1000! Polythene is £100 a layer.
  5.  
    Posted By: borpinPersonally, for a build-up that has a foil backed, vapour and liquid impermeable insulation layer on top of a well sealed OSB layer, I see no reason for any additional layer except for VH's reason of temporary (and more resiliant) waterproofing.
    I agree that a breathable layer on top of non-vapour open insulation does seem of limited value. However, assuming a membrane is required for water & wind-proofing, then better to make it breathable than non-vapour open to reduce risk of trapping water in insulation/OSB. Have you checked with building control on what they will allow?

    David
  6.  
    Posted By: pmagowanMy plan was to sag the roofing felt over the counterbattons and this would not provide a complete barrier. My impression was that its job was to divert water that was blown under the slates off the roof. The sagging over counterbattons was so that a valley is formed between each counterbatton and thus the water is directed away from the wood and perforations by fixings. In doing this it would be very difficult to seal one layer to the next with tape.
    I understand the reasoning &, assuming no one puts a boot through the membrane, this is probably the most resilient approach from a waterproofing point of view. However, you still end up with an unprotected membrane penetration at each tile batten fixing, there are usually far more tile batten fixings than counter batten fixings & I think this misses the opportunity to add a really good wind barrier on top of the insulation.

    Any tape you use to seal the joints in the insulation will need to withstand enormous & repeated variations in temperature & humidity & is unlikely to stay fixed long term. The comparison with polyethylene is not fair because no building control officer is going to allow you to use polyethylene over the insulation. A fairer comparison would be with decent vapour permeable airtightness tape which is not cheap either.

    There shouldn't be any reason to use two layers of membrane. One layer fitted tight to the insulation with butyl tape to the laps should be sufficient. If its a low pitch roof & you're concerned about the seal around counter batten fixings then you can fit butyl tape to the underside of the counter battens before fixing them.

    David
    • CommentAuthorpmagowan
    • CommentTimeJan 30th 2012
     
    I get what you are saying David and I will think long and hard about it. The two layers refers to the VCL and the waterproofing layer. As I see it the layer under the slates is the waterproofing layer for any rain that is driven in by wind. Its other charicteristics are bonuses, such as wind proofing insulation, air tightness etc. The VCL (which will also be my airtightness layer as well as the OSB) has to be under the insulation otherwise it will get condensation on it.

    I suppose I could use a layer of Tyvek under the counterbattons, directly on the insulation as both a VCL and watertight layer. If breathable then the whole roof would be breathable. It could help air tightness if I took it under the insulation at the eaves but then it would direct water to the wrong place, unless I used a double layer taped down, one layer going onto the outside and one onto the inside of the insulation (my airtight layer on the EWI is the parge coat).

    One question. If the membrane is directly on top of the insulation how does it direct water away properly. Surely it would drain onto the wall? If over the counterbattons it drains into the gutter.
    •  
      CommentAuthorjoe90
    • CommentTimeJan 30th 2012
     
    This is the material I have used for many jobs originally recomended by a roofer friend of mine:-

    http://www.screwfix.com/p/protect-vp400-breathable-roofing-underlay-1-5m-x-50m/98160

    This is the feedback from a trade roofer regarding this stuff:-

    "After much research I found this to be the best breather felt that I could get, very tough, doesnt tear easily like others, great properties, this really is much better than tyvek etc.. screwfix were way under other prices and its 1.5 metres so less joins !"

    Whats peoples fedback on this?.
    • CommentAuthorborpin
    • CommentTimeJan 30th 2012
     
    Protect VP400 BRE Certificate http://www.redbooklive.com/pdf/KN2255_Protect_VP400_(web).pdf
  10.  
    Posted By: pmagowanOne question. If the membrane is directly on top of the insulation how does it direct water away properly. Surely it would drain onto the wall? If over the counterbattons it drains into the gutter.
    The counter batten terminates at the last tile batten & an underlay support tray or tilting fillet brings the membrane over the fascia such that it drains into the gutter.

    David
    • CommentAuthorpmagowan
    • CommentTimeJan 30th 2012
     
    Unfortunately due to my detail of cutting off the rafter ends to allow easier air tightness detailing I will have to extend my couterbatton right to the facia. In essence my counterbatton will become my rafter end and will be sized accordingly. This means that if I was to do your plan the tyvek would end at the wall. This may be OK as surely there is supposed to be very little water infiltration and a few drops running down the wall during a windy downpour will be insignificant, I would have thought!
  12.  
    Yes, this is a common detail. You just terminate the Tyvek at a drip flashing.

    David
    • CommentAuthorTimSmall
    • CommentTimeJan 31st 2012
     
    I think that a good design is one which is tolerant of minor defects and/or degrades gracefully over time. Whilst using vapour-impermeable layers outboard is likely to work if perfectly installed and maintained (in the long term), I think it's worth considering that most buildings are neither... Even those which started out well.

    So, if a small amount of water ends up where it shouldn't:

    1. Will anyone have any way of noticing before it's wreaked structural havoc?

    2. Will the surrounding structure be able to get rid of it before it does much/any damage?

    If the answer is "no" to both of those, then perhaps you are storing up trouble for the future?

    My intuition is that whilst some designs with outboard vapour-impermeable layers are probably fine with respect to those two tests, many probably aren't.

    Tim.
  14.  
    Posted By: Viking HouseI had a few issues with water getting into houses because the roofs weren't properly sealed over Christmas so I think we will start putting a layer of Polythene over the OSB before we put on the 200mm of EPS externally.


    Hi Seamus,

    Is this addition of a layer of polythene likely to be added into your standard detailing with OSB & 200mm EPS or are these special cases/old houses not new builds? I'm thinking of using your batten in the bottom 100mm of EPS method for my build, any further pictures you can show detailing the build up would be great to see.

    Thanks
    • CommentAuthorwookey
    • CommentTimeJan 31st 2012
     
    borpin, does builddesk do anything fancier than BS-whatever vapour calcs? Those are pretty unsophisticated and generally not at all realistic in practice (although AIUI they are fairly conervative so may well suffice). The ones that WUFI does are a lot more likely to be right.
    • CommentAuthorborpin
    • CommentTimeFeb 1st 2012
     
    Posted By: wookeyborpin, does builddesk do anything fancier than BS-whatever vapour calcs? Those are pretty unsophisticated and generally not at all realistic in practice (although AIUI they are fairly conervative so may well suffice). The ones that WUFI does are a lot more likely to be right.
    I have no idea how they compare to be honest. It does the condensation Risk Analysis to BS-whatever plus of course U-Values for a build-up and includes climate data. Not sure what WUFI does? A new thread required perhaps?
    • CommentAuthorborpin
    • CommentTimeFeb 2nd 2012
     
    Downloaded WUFI and my initial impression is that it is far less intuative than BuildDesk. Have not played for long and I am not sure I have the time to right now!
  18.  
    Posted By: willie.macleodIs this addition of a layer of polythene likely to be added into your standard detailing with OSB & 200mm EPS or are these special cases/old houses not new builds? I'm thinking of using your batten in the bottom 100mm of EPS method for my build, any further pictures you can show detailing the build up would be great to see.
    We will put a sheet of polythene on the OSB before we start putting on the EPS. I'll have a look at updating the detail.
  19.  
    Do you know how the water got through the roof layers Seamus?

    I was trying to work that out - the only thing I was planning on doing different was instead of laying the membrane to sag between battens I was going to have it tight against the EPS. Was also considering taping the joints, though I thought that may be overkill.
  20.  
    We didn't get enough time to finish it off before Christmas but its fine now!
  21.  
    Posted By: Viking HouseWe didn't get enough time to finish it off before Christmas but its fine now!


    Breathing a sign of relief that was all it was and not some failure elsewhere!!! Thanks for the update. Will hopefully be doing the same roof build up sometime this summer :bigsmile:
    • CommentAuthoridp
    • CommentTimeFeb 4th 2012
     
    Have you thought how dangerous it will be to put polythene on the outside of the roof. If it gets wet it will be lethal for the roofers. I'd put it on the inside of all the insulation where it belongs, with Tyvek on the outside not felt. Get rid of the OSB altogether as it's not needed and will trap moisture in the middle of the roof fabric.
    • CommentAuthorborpin
    • CommentTimeFeb 5th 2012
     
    Posted By: idpHave you thought how dangerous it will be to put polythene on the outside of the roof. If it gets wet it will be lethal for the roofers. I'd put it on the inside of all the insulation where it belongs, with Tyvek on the outside not felt. Get rid of the OSB altogether as it's not needed and will trap moisture in the middle of the roof fabric.
    I think you have missed the whole point of VH's build up with those suggestions!

    The polythene does not 'belong' inside the insulation on a breathing warm roof (or wall). The OSB acts as the air tight layer (and the structural integrity) and it also does not 'trap' moisture.
    • CommentAuthoridp
    • CommentTimeFeb 5th 2012
     
    I don't think i've 'missed the point', maybe you have.
    I didn't mention breathing roof construction -you did. The original proposed construction is not breathing with either a layer of OSB or polythene. OSB does not let moisture through at a rate that would define it as vapour permeable. It is effectively a vapour barrier.
    In standard construction the vapour barrier is on the warm side of the insulation -with more vapour permeable materials on the outside of that. Nothing wrong with that.
  25.  
    Posted By: idpGet rid of the OSB altogether as it's not needed
    Not sure it is safe to say this! as Borpin pointed out it provides both a structural function/helps prevent racking and provides air tightness layer. Sarking is expected in Scotland as well to keep building inspectors happy, with that much insulation it should be kept warm enough to avoid condensation.
    • CommentAuthorborpin
    • CommentTimeFeb 6th 2012
     
    Posted By: idpI don't think i've 'missed the point', maybe you have.
    I didn't mention breathing roof construction -you did. The original proposed construction is not breathing with either a layer of OSB or polythene. OSB does not let moisture through at a rate that would define it as vapour permeable. It is effectively a vapour barrier.
    In standard construction the vapour barrier is on the warm side of the insulation -with more vapour permeable materials on the outside of that. Nothing wrong with that.
    As you had not quoted who you were responding to, I assumed (sic) that you were answering VH's comment on the use of polythene, which is on a breatheable roof, so moving it inside is not an option.

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