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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthoradam_w
    • CommentTimeJun 8th 2018
     
    Morning All,

    Im sure you'll all be please to hear that my build is progressing nicely and I'm not at the stage of applying my racking/sarking board for which i'll be using 11mm OSB3.

    My question is how to I incorporate expansion joints between the boards while maintaining an air tight barrier and what size joints should I leave?

    Thanks in advance,

    Adam
    • CommentAuthorjfb
    • CommentTimeJun 8th 2018
     
    i screwed and glued mine with an expansion joint (PU expanding glue). noggins for the horizontals.
    I guess you cut just screw/nail with expansion joint and then tape afterwards but might not be as failsafe in the long run.
    • CommentAuthorjfb
    • CommentTimeJun 8th 2018
     
    can't remember the joint size but it wasn't that much. should be on the tech spec for OSB3 somewhere.
    • CommentAuthoradam_w
    • CommentTimeJun 8th 2018 edited
     
    Thanks John, I have noggins for the horizontals and verticals but I guess my main area of concern was where the roof meets the walls and at the apex of the roof due to the angles, I was considering using Sika Multiseal flashing tape, any thoughts? Also I checked the manufacturers datasheet and 3mm all round is the recommended expansion gap.

    Thanks again,

    Adam
    • CommentAuthorMarkyP
    • CommentTimeJun 8th 2018 edited
     
    this has been covered here before and I think there are various schools of thought. I taped my OSB sarking with Tecson. It was a pain, mainly because it kept raining when I wanted to apply it and the roofers didnt want to wait around while I messed about with rolls of blue tape. There are some doubters as to the longevity of such tapes, but mine holds fast after 18months and some huge daily temperature swings under the slate roof. Others have used PU glue or silicone between joints - I think this effectively reduces the expansion gap to zero, which might be fine as I am somewhat doubtful it's anything more than an established trade practice that in most cases isn't actually necessary. But if you fully fill 3m gap with glue, then even with the somewhat spongy nature of the glue I struggle to see it providing much scope for movement. Personally I would only worry about joints at abutments with other structural elements. Here I think you'd want a method which allowed a functional movement/expansion joint. My joiners were fairly casual about the expansion gap between sheets, but the underside of the sarking is still visible as we have yet to line the loft rooms and there is no indication the boards have lifted away from the rafters due to expanding into each other in the 18months since it was fitted.
    • CommentAuthoradam_w
    • CommentTimeJun 8th 2018
     
    Hi Mark,

    I tend to agree with what you're saying about differential movement at the abutments and that was what lead me to think a tape / flashing would be most appropriate.

    I'm also thinking that applying the tape externally would be the best solution as far as weathering goes. I do have some Sika flashing tape laying around so I may give that a go this weekend. Is adhesion all I really need to worry about or are there other aspects I should be considering as well?

    Applying a flexible tape as these intersections would also help save a lot of time as I wouldn't need to be as accurate on my cuts and joints of the OSB which considering im not a master carpenter, could be a LOT of time saved!

    Thanks again,

    Adam
    • CommentAuthorMarkyP
    • CommentTimeJun 8th 2018
     
    I'm not familiar with the Sika tape so cant comment on suitability. Whatever you use, ensure it is explicitly for the purpose you intend and suitable for application over OSB. I would be looking at specialist air tightness tapes in this context and not general purpose trade stuff. Note also the application methods recommended by the manufacturer - some require a primer over OSB and a rubber roller is recommended for a really good bond.

    see this thread for related discussion: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14931&page=1
  1.  
    Some thoughts:

    Install boards reasonably dry so they don't shrink beyond installed dimensions in service.

    Consider increasing gap from 3mm to 6mm (why not?) as...

    Cheaper option may be silicone or *flexible* expanding foam in a 6mm gap (Soudal flexifoam?)

    Looks like the edges of runs may need a lot more than 3mm, mfr info should advise.

    *Don't* use the cheap ali tape.
    • CommentAuthoradam_w
    • CommentTimeJun 11th 2018
     
    Morning!

    Thank you both for your input. I've spent most of my weekend cutting and dry fitting the OSB on the roof of my extension and I think I should be able to get a pretty good seal all the way round with just the expanding woodglue while maintaining a 3mm all round. Ideally I think I would be best staying away from tapes where possible as I want to ensure the longevity of the extension is as high as it can be.

    As has been mentioned previously, its the angular intersections that will cause the most trouble but taking your suggestion Pete, could I just spray the flexifoam on the joints once the boards have been installed? In my experience of the product, it can be very sticky and I think I would be better relying on this over a tape, would you tend to agree?

    Oh and don't worry, ali tape, is not going anywhere near my build ;)

    Thanks again,

    Adam
    • CommentAuthorsmileypete
    • CommentTimeJun 12th 2018
     
    Ideally the foam should expand out a little either side so it's locked into place, but easier said than done as it can be bit unpredictable to say the least.

    A warm well shaken can of foam, decent gun (like the Yato - one handed) with a clean tip, plus misting the joint with a fine sprayer can all help, maybe best to practice with offcuts if there's time.

    Reading around it looks like both the foam and OSB itself may not be 100.000% airtight, if this is the aim then a decent coat or two of acrylic paint might help things here.

    Ali tape can be problematic as it won't take any stretch being metal, IME doesn't stick at all well to non porous surfaces either. Might help a little as a belt and braces type thing though.
    • CommentAuthorDarylP
    • CommentTimeJun 12th 2018
     
    ali tape DOES stick well to non-porous surfaces. That's what it is designed for, to stick to foiled insulation boards!
    • CommentAuthorsmileypete
    • CommentTimeJun 13th 2018
     
    Sorry, usual busy day at work, meant it doesn't stick well to /porous/ surfaces. Doh!
    • CommentAuthoradam_w
    • CommentTimeJun 13th 2018 edited
     
    Pete,

    Thanks again! I'm not going for absolute perfect airtightness as this is just an extension rather than a complete build but I would like to do my best where possible, specially if its just small changes here and there.

    Looking at the drawing attached my plan is to leave 3mm expansion gaps at point A and C with the flashing tape over these 'open' joints for weatherproofing. Expanding wood glue at point B and between points B and C, then once everything has settled, expanding foam from the inside applied to point B and C.

    Just to clarify, this structure is to then by covered by 100mm of EPS and then tiled roof and cladded on the walls.

    What are your thoughts?

    Kind regards,

    Adam
      sealing.jpg
    • CommentAuthortony
    • CommentTimeJun 13th 2018
     
    Where is the air tightness barrier and where is the vapour barrier?
    • CommentAuthoradam_w
    • CommentTimeJun 13th 2018
     
    Tony,

    The air tightness is to be formed by the OSB and the appropriate glue and foam method as described above. There is a lot more detail to my intended build-up which can be seen below but I just wanted to focus on the sheathing aspect so simplified the diagram. Your thoughts would be appreciated.

    Kind regards,

    Adam
      section-15.jpg
    • CommentAuthorsmileypete
    • CommentTimeJun 13th 2018
     
    My thoughts would be to do a trial with glue and/or foam using some offcuts and see what works best for you.

    Also worth reading manufacturers info where available and pick the brains of experienced trades. Where there's a BCO involved they may have their own ideas too...
  2.  
    Whilst it has nothing to do with OSB expansion joints........
    I notice that you have an insulation support tray between your graphite EPS insulation at DPC level. The general wisdom today is that this is not needed and you can EPS over the DPC without problems. A saving in both time and expense.
    • CommentAuthortony
    • CommentTimeJun 13th 2018
     
    I love the way the insulation joins up especially at the eaves

    I can’t see what the eps under the concrete slab is doing or the slab

    I can’t see the point of movement joints in the osb it will change dimensionally as it absorbs water the first time and not shrink back again.
    • CommentAuthoradam_w
    • CommentTimeJun 14th 2018
     
    Morning!

    Thank you all for your valuable comments, its greatly appreciated!

    SmileyPete - I think the trial box could definitely be worth a go, i'll try and mock one up later and see how I get on. The only problem with is that I will only get a 'success at time of fitting' rating as opposed to 'success over time' which I was hoping to gain from other peoples experiences but its better than nothing I guess :)

    P_I_H - Thank you for your comment, I'm still a bit away from that stage but I will certainly take your comment on board. From what I've read, general consensus seems to be XPS below DPC, would you tend to agree?

    Tony - Thank you for your praise, this sectional view has taken a lot of time and research in order to build the most cost effective and efficient extension that I can.

    The slab was poured about 6 weeks ago and you'll be pleased to know that no under-slab insulation was used.

    As for the expansion joints between the OSB, is the 3mm not to allow for the dimensional change in the presence of water? Surely if I build it butt to butt then when the dimensional increase occurs, my structure will be distorted?

    Thanks again for all your help.

    Kind regards,

    Adam
  3.  
    Posted By: adam_wP_I_H - Thank you for your comment, I'm still a bit away from that stage but I will certainly take your comment on board. From what I've read, general consensus seems to be XPS below DPC, would you tend to agree?

    The jury is still out on XPS or EPS below ground. I go for EPS below ground. There have been studies, mostly industry based where the EPS camp shows EPS out-performs XPS below ground whereas the XPS camp shows the reverse. Both will take up water/moisture given enough pressure but EPS will release the water fairly easily whereas XPS will not, also EPS insulation value is not much affected by a small amount of water uptake. I would go for EPS (or graphite EPS) below ground with the addition of a gravel backfill to the trench to provide a french drain to give drainage to what ever insulation you decide upon. (provide a drain away for the french drain otherwise you have a moat) This of course will depend upon soil type.

    Have you done the costings on graphite EPS vs. a thicker plain EPS to give the same u value. Plain EPS might work out cheaper.
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