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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorTullich
    • CommentTimeApr 13th 2019
     
    Hi all,

    Attached is an extract from a document called "Calculating potential freedom from structural damage of thermal insulation structures in timber built systems", produved by Pro clima. Catchy title I know!

    The text is specifically for timber frame details in the Scottish Highlands.

    The document goes on to say that the only other acceptable way to avoid the issue, is to use the equivalent quantity of insulation outboard of the timber frame as is contained within it.

    Grateful to hear what anyone thinks about this.
      IMG_20190413_103902.png
    •  
      CommentAuthorfostertom
    • CommentTimeApr 13th 2019
     
    Posted By: Tullichthe only other acceptable way to avoid the issue, is to use the equivalent quantity of insulation outboard of the timber frame as is contained within it
    Absolutely - that's the answer.

    Otherwise, the outer board, be it OSB or soft board, is inevitably exactly where the zone of condensation begins, and maybe spreads inwards. That may happen at least momentarily in any cool climate - the key question is how long it will stay wet before re-drying. 'Next summer' seems to be good enough. But in some climates/conditions it may never fully re-dry before next winter, then the board gets steadily wetter and wetter year by year. This can be seen graphically in WUFI simulations. Do we trust that bituminous softboard outer sheathing, recommended as the answer, is really immune forever to decay in that case?

    The cure is to put substantial non-organic insulation outboard of the studwork and its outer sheathing, then any condensation happens in non-organic non-structural zone, and OSB outer sheathing is fine.

    Anyway, studwork with insulation between is prime thermal bridge - really poor practice. A substantial layer of insulation across either the inner or outer face of the sheathed studwork easily cures the bridging. Best on the outboard face, for reason given above.
    • CommentAuthorTullich
    • CommentTimeApr 13th 2019
     
    Thanks Tom.

    If using an airtightness layer on the internal side of the timber frame (OSB or equivalent) would this not also serve to provide racking restraint? Could the exterior structural sheathing board then be substituted for an insulation board (woodfibre from Pavatex, Steico, etc) instead?
    • CommentAuthorTullich
    • CommentTimeApr 13th 2019 edited
     
    I'm guessing not as it doesn't meet the "inorganic" criteria?

    In Swedish TF's do they not just use an external membrane in place of external racking resistant boards, and have an internal air tightness / racking make up as described above?
    •  
      CommentAuthorfostertom
    • CommentTimeApr 13th 2019 edited
     
    Trouble with airtightness layer on the inside is ... trouble! It's inevitably multi-punctured by everything from joist ends to services, all having to be sealed with 100% precision even where inaccessible. Sticky tapes - last forever? Plastic membrane - never ever punctured?

    To me, better to put the airtightness layer on the outside of the studwork - an uninterupted tea-cosy dropped over the whole. Make it of OSB gapfilling glue-and-screwed to the studwork - foolproof, easy for chippies, durable, easily repaired. OSB doesn't have declared airtightness yet according to PH Certifier Peter Warm, I've heard him say that he's never seen a PH fail airtightness due to using OSB. If OSB airtightness is slightly suspect, it can be backed up with blown-in cellulose insulation between the studs, which has quite good airtightness, as well as other valuable characteristics. The two together offer robust in-depth airtightness, worlds away from plastic sheet and sticky tape fragility and exquisite worknanship requirement.

    Having sheathed the exterior with OSB, is v easy to EWI over it externally. Note, no internal vapour or airtightness layer, no need for battened service void, just plasterboard straight onto the studwork, electricians free to make holes in it as convenient.
    •  
      CommentAuthordjh
    • CommentTimeApr 13th 2019
     
    Posted By: TullichIf using an airtightness layer on the internal side of the timber frame (OSB or equivalent) would this not also serve to provide racking restraint? Could the exterior structural sheathing board then be substituted for an insulation board (woodfibre from Pavatex, Steico, etc) instead?

    I don't see why not, but I would ask Pro Clima (via Ecological Building Systems) to confirm it.

    PS. Here's the rest of the text of that section that you quoted above:

    "If the OSB boards are to be installed
    externally they have alternatively to be
    insulated additionally on the external
    side. In this way the dew point is
    abated. The thickness of insulation apart
    from the OSB boards should be as high
    as the insulation between the posts.

    "Concrete masonry and quarrystone
    masonry should be insulated externally.
    An internal insulation should only be an
    additional insulation in connection with
    an external insulation."

    The documents are available from https://proclima.com/service/downloads
    Thanks for drawing attention to them.
    •  
      CommentAuthordjh
    • CommentTimeApr 13th 2019
     
    Posted By: fostertomTrouble with airtightness layer on the inside is ... trouble! It's inevitably multi-punctured by everything from joist ends to services, all having to be sealed with 100% precision even where inaccessible. Sticky tapes - last forever? Plastic membrane - never ever punctured?

    Our airtightness layer is on the inside, because of all the trouble I heard of people building with it on the outside!

    I don't think there's trouble likely with ours, or I wouldn't have done it, but it's true it influenced some decisions. There are no ceiling-mounted light fittings on the first floor, for example, and no switches or sockets on the exterior walls (although I've now discovered wire and battery-free switches that just stick to the wall so that could change a bit if I wanted).
    • CommentAuthorTullich
    • CommentTimeApr 13th 2019
     
    Thanks Tom and djh.

    To give a little more background to the build....

    I'm determined to only use materials available from local builders merchants. I'm in the Scottish Highlands and don't want to risk delays in supply or to pay extortionate delivery costs. I've settled then on stick timber and Rockwool, which are both self build friendly materials for a hands on construction.

    I costed blown cellulose at the outset, but per 100mm thickness it's £19/m2 @ 60kg/m3 density. Rockwool is less than £8 for the equivalent thickness.

    Aside from materials the other constraint is the need to get the build to wind and watertight as quickly and cost effectively as possible. I'd hoped to avoid any genuine external insulation and to rely instead on the stud/counterstud frame approach to increase available depth for insulation.
    •  
      CommentAuthordjh
    • CommentTimeApr 13th 2019
     
    There's a lot more information in the New Zealand version of that study. I haven't read it all but it might be worth looking in there for alternative possibilities. Canadian building practice is also worth looking at.
    • CommentAuthorGreenPaddy
    • CommentTimeApr 16th 2019
     
    Hi Tullich,

    my first thoughts are that wanting to get weather tight quickly, will probably mean the OSB skin on the on the outside, as we traditionally do with timber frames here in the highlands.

    That lets you insulate with non-waterproof insulation in the dry from the inside, but of course doesn't stop you adding a layer of rigid board over the outside, which I've done on some occasions, including over the roof, for the very good reasons pointed out above.

    Consider how you'll demonstrate to building control as regards vapour control, for your building warrant. I'm not giving any direction on this, by making the comment, but if you're not using a proprietary wall build up, which has been tested, and it sounds like you're going for stick build, then the responsibility will be on you to demonstrate no (or virtually no) interstitial condensation. Good design, and use of cond modelling software would do this.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 16th 2019
     
    Spot on
    • CommentAuthorborpin
    • CommentTimeApr 16th 2019 edited
     
    Totally agree with Tom.

    If you put enough insulation outboard, then the timer is always on the warm side of any condensation. If it did by some remote chance get wet, it would then dry.

    Also agree that the easiest way is to make a tea cosy of OSB and then insulate over. I would though use Icynene insulation between the studs as that helps with the air tightness and yet lets vapour out.

    I built with a metal frame from U-Roof http://www.u-roof.com, created a tea cosy of Insulation and then sprayed Icynene inside. Worked perfectly! The metal frame has it's own challenges when finishing inside, but the roof is self racking so less requirement for purlins etc.

    No wood, nothing to rot!

    https://www.borpin.co.uk/2012/09/07/the-u-roof-frame-is-up/
    •  
      CommentAuthordjh
    • CommentTimeApr 17th 2019
     
    • CommentAuthorTullich
    • CommentTime5 days ago
     
    Really appreciate your input so far, and the links suggested. Thanks.

    I'm familiar with the REMOTE wall concept, and the excellent video series produced by the Cold Climate Housing Research Centre. I've often wondered why nobody produces a t & g profiled sheet (as seen on external woodfibre boards etc) which could be installed in one thickness to save layering sheets and the additional labour costs?

    With EPS100 being the default material for EWI the issue is availability of materials locally again. Any number of retailers will sell me EPS70, but EPS100 is a special order item and the delivery costs are eyewatering. Is EPS70 a safe (albeit lower density) substitute?

    Icynene's not a self install option in much the same way as cellulose isn't, your steel frame looks amazimg and terrifying in equal measures though borpin!

    If I were to bite the bullet on an externally air tightened timber frame option, the next step would be to figure out how this can be integrated with a suspended timber floor. I'm already feeling a little bit like i'm selling out by considering EPS options, I really don't want to pour a slab....
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Posted By: TullichWith EPS100 being the default material for EWI the issue is availability of materials locally again. Any number of retailers will sell me EPS70, but EPS100 is a special order item and the delivery costs are eyewatering. Is EPS70 a safe (albeit lower density) substitute?

    AFAIK, the difference is the load-carrying ability of the material, which is pretty much irrelevant for EWI. But then I've never investigated EWI in any detail, so I may be ignoring something.

    If I were to bite the bullet on an externally air tightened timber frame option, the next step would be to figure out how this can be integrated with a suspended timber floor. I'm already feeling a little bit like i'm selling out by considering EPS options, I really don't want to pour a slab....

    The way I thought was that investing oil-derived materials in the fabric of a building is one of the best uses for oil. Far better than burning it; although I do admit that leaving it in the ground is better. I think the same goes for heat-processed materials like lime or cement. So I'm relatively content that I have EPS and concrete under my straw bales. A suspended-timber floor with piles underneath and PH performance proved too difficult to organize for me.

    The Murricans and Canucks have a lot of experience with suspended timber floors in a lot of different climates, so I'd look there for good practice.
    • CommentAuthorborpin
    • CommentTime5 days ago edited
     
    EPS under a raft

    http://www.isoquick.co.uk
    • CommentAuthorborpin
    • CommentTime5 days ago edited
     
    Posted By: TullichIcynene's not a self install option in much the same way as cellulose isn't
    True, but it is quick and improves air tightness to boot.
    • CommentAuthorgoodevans
    • CommentTime2 days ago
     
    Posted By: TullichIs EPS70 a safe (albeit lower density) substitute?
    I think EPS70 is ok - I'm using it at the moment for my EWI. The data sheets for my adhesive and render system states EPS70 is also OK.

    If you do use EPS as the rot-proof outer insulation layer of the timber frame ensure you have a higher density sound attenuating insulation layer inboard - wool batts, cellulose or spray foam.
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