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    • CommentAuthorMarkyP
    • CommentTimeJun 29th 2016 edited
     
    I'm getting some conflicting views from manufacturer, architect and BC on my roof build up. It's a room in the roof design for a loft conversion and new build section as follows, from inside to out:

    PB
    VCL
    75mm PIR under rafter right down into eaves, ties in with EWI to walls
    175mm Crown Rafter Roll (0.032 W/mK) between rafter full depth (no ventilation void)
    OSB 18mm Smart Ply sarking over rafter T&G glued, nailed (air tightness layer for wool, bracing for roof as removing other internal bracing)
    batten/breather
    slate tile

    BC signed off the build up per submitted drawings but Smart Ply data sheet recommends a ventilation layer behind OSB, eaves to eaves. Architect now back tracking slightly saying rather ambiguously we should have a "small" venitlation void under the sarking. Site joiner says he fully expects BC on site to insist on ventilation behind the OSB. Who to believe?

    My goal with OSB was to keep it air tight, and especially to avoid wind washing the wool. It also allows me to remove some awkward bracing timbers on the inside of the loft spce to be converted. Using VCL on the inside of the build up, and the system being relatively vapour open from the wool out, I thought this would be OK.
    • CommentAuthorDarylP
    • CommentTimeJun 29th 2016
     
    ...get a Condensation Risk Analysis (CRA) that shows the risk to be negligible? if at all?
    Good luck :wink:
    • CommentAuthorEd Davies
    • CommentTimeJun 29th 2016
     
    My roof design is similar (90 mm PUR, 235 mm I-beam rafters with mineral wool fill). IIRC, I couldn't get my simplistic condensation risk analysis to work with OSB even with a VCL but that might be because I was taking worst case OSB permeability values, about which I saw some confusion.

    https://edavies.me.uk/2014/04/osb_vapour/

    Previous closely related discussion:

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993
    • CommentAuthorringi
    • CommentTimeJun 29th 2016
     
    How much better would "normal" ply do, or thinner OSB?
    •  
      CommentAuthordjh
    • CommentTimeJun 29th 2016
     
    Is it really difficult to add some more thin OSB between the rafters to provide a ventilation space?
    • CommentAuthorMarkyP
    • CommentTimeJun 30th 2016 edited
     
    Posted By: djhIs it really difficult to add some more thin OSB between the rafters to provide a ventilation space?


    I guess not hard technically but it's a very large roof so would be a fiddle of a job and time consuming and also I would imagine BC would push me to a 50mm air gap so lost insulation is another factor.

    BC come for a site visit on Monday, I'll ask him what he thinks, perhaps I'm worrying about nothing and he will be fine so long as a decent VCL. Otherwise DarylIP's suggestion of a CRA.

    Posted By: Ed DaviesMy roof design is similar (90 mm PUR, 235 mm I-beam rafters with mineral wool fill). IIRC, I couldn't get my simplistic condensation risk analysis to work with OSB even with a VCL but that might be because I was taking worst case OSB permeability values, about which I saw some confusion.

    https://edavies.me.uk/2014/04/osb_vapour/" rel="nofollow" >https://edavies.me.uk/2014/04/osb_vapour/

    Previous closely related discussion:

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993" rel="nofollow" >http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11993


    what did you do in the end?
    • CommentAuthorMarkyP
    • CommentTimeJun 30th 2016
     
    Posted By: ringiHow much better would "normal" ply do, or thinner OSB?


    I think I'm right that T&G OSB is only available in 18mm. We designed with T&G specifically for the air tightness. Hadnt looked at ply, is it considered to be more vapour permeable?
    • CommentAuthorEd Davies
    • CommentTimeJun 30th 2016
     
    It'll have 150x18 mm treated softwood sarking boards with full-fill mineral wool so no ventilation under. On top will be breather membrane and ventilation, of course. BCO was happy with my argument that the mineral wool underneath is vapour open enough that with all points near the surface within 75 mm of the edge of a board any condensation would dry out quickly enough.

    Before I push the mineral wool in I'll staple across a membrane to protect it from windwashing. Shown orange in the drawing. Blue is the breather membrane and yellow the VCL.
      overview.png
    •  
      CommentAuthordjh
    • CommentTimeJun 30th 2016
     
    FWIW, we used 2 no 9 mm ply layers, with offset joints and glued together for our 'sarking'. We did that because we needed to bend the sheets but it gave us an airtight layer. On top of that we put a 'metal' breather membrane and then the metal roof (i.e. no battens or anything and no need for an 'orange' membrane). We insulated with blown-in cellulose, which presumably helped airtightness. The 'metal' breather membranes have a sort of pan scourer material fixed on top of the membrane itself, which provides some ventilation and drainage under the metal.
    • CommentAuthorringi
    • CommentTimeJun 30th 2016
     
    Posted By: Ed DaviesBefore I push the mineral wool in I'll staple across a membrane to protect it from windwashing. Shown orange in the drawing. Blue is the breather membrane and yellow the VCL.


    Why do you need the orange membrane as the blue breather membrane should protect from windwashing?
    • CommentAuthorEd Davies
    • CommentTimeJun 30th 2016
     
    Breather could be made reasonably airtight but I have more confidence of managing a reasonable job working inside.
    •  
      CommentAuthordjh
    • CommentTimeJun 30th 2016
     
    Am I interpreting your drawing correctly? It looks to me like you're intending using separate pieces of orange membrane between each pair of rafters. You'd need to tape or glue the edges of the membrane to the rafters if you wanted airtightness, which would be quite time consuming and expensive, I suspect.
    • CommentAuthorEd Davies
    • CommentTimeJun 30th 2016 edited
     
    Sorry, getting a bit off-topic from the OP's question but, yes, one lot of “orange” for each rafter bay. It comes in 1 metre wide rolls so should overlap reasonably well with 600 mm centre rafters. It's not the airtightness layer (the VCL should do that job) but is to stop wind gusts. With the mineral wool packed up against it and fitting it over the inside corners of the rafter flanges I think it should be airtight enough to prevent significant air movement.
    •  
      CommentAuthordjh
    • CommentTimeJun 30th 2016
     
    Ah, right. Then I think I'm with Ian that the blue membrane and the sarking boards will likely provide enough protection.
    • CommentAuthorMarkyP
    • CommentTimeAug 30th 2017 edited
     
    to resurrect an old thread as am approaching the point where I need to insulate the upper floor roof. BC will accept a CRA showing that with VCL on the inside, condensation risk is eliminated in my proposed rafter zone build up (from inside to out):

    PB
    VCL
    75mm PIR under rafter right down into eaves
    175mm Crown Rafter Roll (0.032 W/mK) between rafter full depth (no ventilation void). meets with EWI to walls
    OSB 11mm sarking (was planned 18mm) air tightness layer.
    counter batten/breather membrane draped to create air space over OSB
    batten
    slate tile

    However, and I think he's right, BC points out that detailing the VCL at the eaves, around uprights for dwarf walls and so on will be hard or impossible. Also accepts that OSB is somewhat breathable. Not sure we've yet fully agreed I can proceed with the original build up yet.

    However, some other options I’m considering..

    1. use a rigid insulation between rafter and create a ventilated void afterall. Could be done drilling a couple of 30mm holes in the OSB top and bottom of each rafter bay. I could use enhanced/graphite EPS between rafter, foamed in neatly. I have 175mm rafter depth so a reasonably decent (for retrofit) u-value attainable with EPS between and PIR under rafter. But this makes my OSB air tightness layer a complete waste of time and money other than for the structural rigidity which did allow some bracing to be removed (there would have been other ways around this, though). I’d aim to get away with a 25mm ventilation space to maximize insulation depth.

    2. create “breather holes” in the OSB maybe drilling 50mm diam holes at 600 centres the length of each rafter bay. Cover these from the inside with breather membrane, taped to OSB, or perhaps taped and silicone/clamped down with screwed lathes, to maintain air tightness of OSB skin overall. The idea here being that the fully mineral wool filled rafter bays will each be regaularly punctuated with air tight holes of very low vapour resistance. The OSB alone may itself have been breathable enough, so this breather hole method would add some insurance/peace of mind.

    I have no idea if BC would go for option 2. seems like a slightly oddball DIY idea. but I think it could work.
    •  
      CommentAuthordjh
    • CommentTimeAug 30th 2017
     
    Posted By: MarkyPOSB 11mm sarking (was planned 18mm) air tightness layer.

    Is 11 mm OSB airtight?

    BC points out that detailing the VCL at the eaves, around uprights for dwarf walls and so on will be hard or impossible.

    Have you done detail drawings for these features so you know how you will do them? If not then the BCI is correct, but if you have then all you need to do is show him the drawings.

    In my earlier posting I didn't explain the interior side of our construction. It was, from the bottom:

    - skimmed 9.5 mm plasterboard (the roof is curved hence thin sheets)
    - Intello membrane
    - 8.5 mm Panelvent board
    - blown-in cellulose
    and above as previously stated
    - 2 no 9 mm ply
    - Solitex UM Connect 'metal' breather membrane
    - standing seam aluminium

    Notice, no ventilation gaps anywhere. Ecological Building Systems did a CRA and provided a warranty and my BCI was happy with that.
    • CommentAuthorMarkyP
    • CommentTimeAug 31st 2017 edited
     
    11mm OSB I believe will be adequate for its purpose. It was designed to create a skin over the fully filled rafters, prevent air movement through the insulation, wind washing I think it gets called on here. The boards are taped (by me, as carefully as I possibly could, and am considering some other belt and braces methods to further improve the air tighntess of the joints in the long term. I think the OSB will serve well enough as installed.

    No detail drawings (budget meant a basic BC drawings service, much of the detail specified by me and changed by me as we've gone along), but my BCO is as happy to talk method on site. He's been helpful, pragmatic and flexbile on every point of detail so far.

    only way I could see to address the VCL was to take foil backed PIR right down into the eaves, as far as I could go. Foamed around the uprights for the dwarf walls and the joists, then alu taped joints to make a VCL. But struggle to see it being vapour tight being detailed around all that timber. Issue is living space below the eaves, plasterboard here meets the wall plate, not air tight and sections of ceiling below the eaves voids are vapour open.

    I do wonder if it's worry about nothing. If I can make the PIR underskin air tight as far as possible, then small amounts of diffusion through foamed joints, or even very small air leaks, probably wont matter. I think the buildup will be moderately vapour open outbound, via the OSB.

    encouraged to read there is no ventilation space whatsoever in your build up, even over the ply skin behind the aluminium. In my case, there's a counterbatten space over the sarking, and windy void behind the breather, OSB will be subject to drying air movement which I thought would help.

    am planning MVHR also, so interior humidity will be controlled.

    still half considering my breather holes idea. Had further thought I could limit use of them to the at risk rafter areas where VCL detailing is tricky, near the eaves and again near the ridge where I have some collar ties.
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