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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorBBLongman
    • CommentTimeDec 14th 2018
     
    Hi All,

    Just in early stages of planning Passivhaus build and starting to put together a budget.

    Roof requires 80off 4.3m @ 400mm deep timber I-joists and I was a bit taken aback at the price.

    Just over £8000 plus VAT on my first quote which is £100 per joist. I work out material costs to be somewhere around the £35 mark even if I were to be buy from a standard merchant.

    I realise that the I joist is accredited/tested/approved etc but I was expecting something around say £75 per joist.

    Can anyone point me in the direction of supplier with favourable pricing!

    Thanks

    Bennet
  1.  
    400mm deep seems a bit deep for a roof. What sort of roof is it?

    (My tables gives a 51mm x 203mm timber for a span of 4.39m at 406 centres for a load of 48 - 73 kg/m2 and that's for a flat roof).
    •  
      CommentAuthordjh
    • CommentTimeDec 14th 2018
     
    Posted By: Peter_in_Hungary400mm deep seems a bit deep for a roof. What sort of roof is it?

    An insulated passivhaus roof with insulation between the joists presumably. Blown in cellulose for example.
    An alternative construction with plain rafters as you suggest, plus purlins(? name) laid at right angles on top to give the required depth might work out cheaper and at least as thermal bridge free.
  2.  
    We used Steico joists, same depth, from a merchant called Elliotts who were fairly nearby to us in the South West.

    Be aware they have to order them in specially at that depth and are reluctant to sell them in smaller quantities than a 'pack' (I forget the size of these) since they are then harder to shift.

    It took a long time to get the order delivered but it was great when they finally turned up. No real problems other than the usual pitfalls of timber elements being not quite true and straight along their length. Nothing that a bit of persuasion from a carpenter couldn't fix.

    Watch for the different web materials too. They are apparently made from either hardboard or a type of OSB and you don't always know what you'll get. I think the hardboard has better mechanical properties and is what we were delivered but we had to run the structural calcs assuming the worst case just to be safe.

    Happy to share more details if you need them.
    • CommentAuthornick1c
    • CommentTimeDec 14th 2018
     
    You could try Severn valley sawmills. They do the cutting for Touchwood -a TF company that builds in I-beams. They estimate that the cost for a 3-4 bed house is £30-40k, dependent on design.
    • CommentAuthorphiledge
    • CommentTimeDec 14th 2018
     
    If Peters tables are correct and a 200x50 joist is structurally all that is required, Im sure it would be cheaper to make up your own out of 200x50 plus a 75x50 separated with a 125x6 plywood web. Run a circular saw or router down the edge of the wood to make slots for the web, 2 beads of glue and its job done. 200x50 meets structural needs and the rest just builds the depth. You'd need to make up a level bed to dry them on but for 80 probably worth the time to build a rack for 10??
    •  
      CommentAuthordjh
    • CommentTimeDec 14th 2018
     
    Posted By: philedgeRun a circular saw or router down the edge of the wood to make slots for the web

    No need to cut the main rafters, just fasten the pieces of web to the side with nails/screws/glue (whatever SE says). That way keeps the full strength of the rafters.
    •  
      CommentAuthorfostertom
    • CommentTimeDec 17th 2018
     
    I thought that for joint-strength-critical purposes, factory built and certified units were obligatory - yet the suggestion here is DIY I-beams?
    • CommentAuthorgoodevans
    • CommentTimeDec 17th 2018 edited
     
    My guess is that if you can get an engineer to sign off on it the BCO will say thankyou and away you go.

    Of course you have to find an engineer to sign off on it. The material constants are easily available screw/glue strengths are available. I think bending and shear and deflection an engineer would be comfortable with but I think an engineer would probably wish to over design the number of web stiffeners required (esp if the web was not symmetrical to the flanges) and also over design the lateral stability bracing of the beams.

    A simple method statement from the engineer and away you go

    Then its just the connectors.....
  3.  
    Posted By: fostertomI thought that for joint-strength-critical purposes, factory built and certified units were obligatory - yet the suggestion here is DIY I-beams?

    The suggestion was to replace the 400mm I beams with 51mm x 203mm beams which would be cheaper and would span the required 4.3M. If the need was for 400mm to give the insulation depth then this could be done DIY as it would not need to be structural

    Ermm - where is the OP
    •  
      CommentAuthordjh
    • CommentTimeDec 17th 2018
     
    Posted By: fostertomI thought that for joint-strength-critical purposes, factory built and certified units were obligatory - yet the suggestion here is DIY I-beams?

    AFAIK, on-site glueing doesn't count for structural purposes and mechanical fasteners will need to be both shear-rated and not pull out, so if an I-beam were wanted then I would suggest buying them from a factory. But what I think was suggested was effectively a Larsen truss, since as Peter says, the structural loads would be carried by solid timber, not by the truss as a whole. But the design of the whole thing would definitely need certifying by an engineer.

    I know it is possible to use crossed timbers as I originally suggested, because that is how my roof is done.
    • CommentAuthorBBLongman
    • CommentTimeDec 18th 2018 edited
     
    Hi All,

    Thanks for the comments and suggestions. Not signed in for a few days so missed the developments.

    Yes that is correct, the depth is for insulation. We are using AECB roof detail that specifies 400mm of insulation: mineral wool, blown cellulose or my current preference which is sprayed icynene.

    In the meantime I have actually had a far more reasonable quote back for £3.5k plus VAT which is close to the material costs. Can't believe the difference in price!

    Structurally we haven't had it looked at by an engineer yet but from what has been said I am guessing we won't have an issue with 400mm deep rafters at 600mm centres.

    Thanks

    Bennet
  4.  
    Bennet,

    To echo djh above, the thing you need to ascertain from your engineer is the fixings.

    As you say, you can be fairly confident in the trusses themselves, but we found that once our engineer had run the numbers we ended up with quite a lot of different fixing sizes and spacings depending on the various wind loads, cladding weight etc. for each area (we used them on the roof and the walls, as below) and what was being fixed to what - we clad outside of the trusses with a layer of breathable board and then membrane and battens over this.

    I'll let others comment on the Icynene but from memory it wasn't as great as we all first thought in terms of performance. Might be worth doing a search on here.
      Larsen Trusses.JPG
    •  
      CommentAuthorfostertom
    • CommentTimeDec 18th 2018
     
    Neat job. Do you fill the webs with insulation?
    •  
      CommentAuthordjh
    • CommentTimeDec 18th 2018
     
    Posted By: fostertomNeat job. Do you fill the webs with insulation?

    Looks like mineral wool pushed in, which will expand to fill the webs if cut right. See the exposed bit at the bottom of the photo.
    • CommentAuthorowlman
    • CommentTimeDec 18th 2018
     
    djh, I've been using semi rigid Rockwool type batts for sometime now for precisely that reason. I prefer it to the rigid types of insulation especially on refurb. jobs where joists etc. are less than perfect or with overlapping joins.
  5.  
    Posted By: djh
    Posted By: fostertomNeat job. Do you fill the webs with insulation?

    Looks like mineral wool pushed in, which will expand to fill the webs if cut right. See the exposed bit at the bottom of the photo.


    Thank you both.

    Yes, I divided the layers up so that they matched the thickness of the outer flanges with the hope that it would allow the middle section to expand out to the webs - rather than using fewer, thicker layers which might not sit tight up to the timber if made to span across different widths.

    Fiddly job but it seems to have worked!
    • CommentAuthorBBLongman
    • CommentTimeDec 18th 2018
     
    Yes that does look extremely neat. I'll be putting the insulation in from the inside as plan is to build water tight shell, cladding on wall, roof covering etc.

    Walls, which will be larsen trusses over block work, I was going to use blown cellulose.

    Icynene seemed attractive as it looks like it gets into all the little nooks and crannies.

    I'll take a look on the forum to see what has been said about it now you have alerted me to possible short comings

    Thanks

    Bennet
    • CommentAuthorBBLongman
    • CommentTimeDec 18th 2018
     
    Well, I have read all the threads on the Icynene. It seems a lot of the issues are around it's performance as an insulator compared to PU foam boards. In the case of roof build up with substantial thickness this shouldn't be an issue as by calcs still give a U-value at around 0.10.

    Did see some worrying stuff in the threads about warmcell or other blown cellulose materials slumping with time.

    Seems from reading this that the safest option is possibly mineral wool bats. Only issue for me there is they have to be installed as the building goes up. (Well in the walls at least) and having already battled the elements through one self-build I know how disheartening it can be to see materials soaked through after and unexpected shower!
    •  
      CommentAuthordjh
    • CommentTimeDec 18th 2018
     
    Posted By: BBLongmanDid see some worrying stuff in the threads about warmcell or other blown cellulose materials slumping with time.

    Doesn't happen when blown in by a reputable installer. Talk to them early about the best means of providing access to the space for them.
  6.  
    We have a portal timber I-beam frame which is 350mm thick and fitted with 15mm OSB3 racking on the outside. It is filled with Icynene and has 50mm RWA6 Rockwool on the outside of the OSB. This has a U value of 0.095 and without any airtightness membranes or tapes achieved 0.47ACH. The only problem with using Icynene was having to trim off the excess which was considerable. This was partly because the installer had never sprayed a thickness greater than 150mm. We decided against Warmcel because a friend had it installed in her house and over time it slumped slightly allowing a narrow strip at the top to be uninsulated.
      I-beam Frame.jpg
    • CommentAuthorBBLongman
    • CommentTimeDec 19th 2018
     
    I thought the installer would be responsible for trimming the Icynene afterwards?

    As for access for the warmcell I think this will be straight forward as where the roof I joists meet the external larsen trusses they will have clear access. Just need to provide access holes in the bottom of the window reveals. Only a single storey appartment so no complicated detail.
    •  
      CommentAuthordjh
    • CommentTimeDec 19th 2018
     
    Posted By: BBLongmanAs for access for the warmcell I think this will be straight forward as where the roof I joists meet the external larsen trusses they will have clear access.

    I recommend you talk to the installers as I suggested, because I think they will have a different view.
    •  
      CommentAuthorfostertom
    • CommentTimeDec 19th 2018
     
    This is obligatory watching for anyone thinking of blown cellulose or worrying about slump:
    https://www.greenbuildingadvisor.com/green-basics/video-dense-packed-cellulose
  7.  
    Posted By: BBLongmanI thought the installer would be responsible for trimming the Icynene afterwards?


    They would have trimmed the excess if we had asked them to, but there would have been a cost which we preferred to save by doing it ourselves.
    • CommentAuthorBBLongman
    • CommentTimeDec 20th 2018
     
    Thanks for posting the video and the warning about access.

    I'll take your advice and discuss with the installers but if the installation method is as shown in the video I can see that it will be rather difficult.

    Really starting to think that mineral wool might be the best option. Going to need a lot of it!

    Thanks

    Bennet
    •  
      CommentAuthordjh
    • CommentTimeDec 20th 2018 edited
     
    Posted By: BBLongmanif the installation method is as shown in the video I can see that it will be rather difficult.

    I can't see the video. Tom, or anybody else, do you have a link to a video on youtube or somewhere else that will play? In the meantime, what did it show? The point is they need access at several places so they can build up the cellulose without gaps.

    Mineral wool needs even more access, so I'm not sure why that would be preferred? Not that I've got anything against it.
    • CommentAuthortony
    • CommentTimeDec 20th 2018
     
    Cheaper may be
  8.  
    We had mineral wool fibres blown into our timber frame bungalow walls and eight years later when the bungalow was dismantled we could clearly see that the walls were fully filled.
    •  
      CommentAuthorfostertom
    • CommentTimeDec 21st 2018 edited
     
    Posted By: djhI can't see the video. Tom, or anybody else, do you have a link to a video on youtube or somewhere else that will play?
    It works for me, clicking the above link. Try again? Anyone else have problem?

    Googling "dense packed cellulose video national fiber" brings up plenty but looks like National Fiber website is gone - found no direct link to the video, which really is the definitive instruction, so needs to be preserved.

    Posted By: djhIn the meantime, what did it show?
    That would be quite an essay - a lot there. See if you can view it, first.
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