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Housing - New Build: Joist cold bridge.

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  1.  
    Hi, I am about to fit my joists and have opted for the easi joist system for the lower level of sound transfer and also the speed of plumbing and electrics. I assumed that I would fit the joists into the internal block wall through to the cavity. I am now wondering if I could use joist hangers and nail the joists onto the internal wall and so would not break into the cold cavity at all? If I can do this then it would reduce my thermal bridging. Any opinions would be appreciated.The concern I have is the strengh of the hangers compared to the wall holding the joists. What is the common method of fixing and what do the experts think?
    Many thanks.
    • CommentAuthortony
    • CommentTimeApr 5th 2010
     
    Don't use nail in joist hangers -- they will move!

    Use a "tony tray" to stop air leakages -- cold bridging is not a problem tho I hate aerated blocks as they crack so much so hope you are using better blocks.

    Normal joists are cheaper than fancy ones too by the way
  3.  
    Posted By: tonyDon't use nail in joist hangers -- they will move!
    No they won't! If you use proper joist hangers and following the correct fixing schedule, they will not move and will be as strong as the wall that supports them.

    Posted By: tonyNormal joists are cheaper than fancy ones too by the way
    Engineered floor trusses are cheaper than joists, are stronger and use less material to boot as well as making routing plumbing, electrics and HVAC ductwork much easier.

    Paul in Montreal.
  4.  
    Posted By: gustyturbineI am now wondering if I could use joist hangers and nail the joists onto the internal wall and so would not break into the cold cavity at all? If I can do this then it would reduce my thermal bridging. Any opinions would be appreciated.The concern I have is the strengh of the hangers compared to the wall holding the joists. What is the common method of fixing and what do the experts think?

    The main issue with building the joists into the wall is not cold bridging, but air leakage. It is difficult to seal the joists to the masonry & there are bound to be air leaks where they meet. There are several ways around this, but the most common are:

    1. Rubber joist boots fitted around joist ends before they are built into inner leaf - not my preferred technique;
    2. Masonry joist hangers built into inner leaf as work proceeds - very strong, but you need to build a few courses of blockwork on top before loading up the joists;
    3. Timber joist hangers nail fixed to a rim board which is bolt fixed to inner face of inner leaf - only as strong as your bolt fixings.
    4. Tony Tray - roll out 600mm DPC to top of inner leaf before placing the joists on top, lay blockwork between the joists finishing flush with top face & fold the DPC over the top.

    My preference would be 2.

    David
    • CommentAuthortony
    • CommentTimeApr 6th 2010
     
    sadly using built in joist hangers do not stop air leakage -- the air leakage is initially the same as built in joists though as the built in joists shrink air leakage increases.

    Parging of the blocks helps but a 'tony tray' sorts it good n proper -- mine was breathable not poly or plastic though.
    •  
      CommentAuthorjoe90
    • CommentTimeApr 6th 2010
     
    On a garage conversion I am just doing (turning it into another bedroom) the BCO has just stipulated bolting a timber to the wall with rawlbolts at 500mm centres and fixing floor joists to this with joist hangers. We have concrete blocks for our wall but I would not like to do the same for Thermalite or similar.
  7.  
    Thanks for the advice. My way of thinking was that the Easi joist system would not shrink to any great degree as it has a metal web design and so the air leakage should not increase to same degree as the timber shrinks (if I build it into the wall) or if I can fix it to the inner face of the inner wall then this would not effect air leakage at all as the cavity will be broken into and obviously this will reduce any possible draughts. My concern is the strength of the different fixing methods and avoiding a squeaky floor!
    Any additional costs due to the purchase of the Easi joist system will be clawed back from the plumber and electrician, so if it a better sound insulating method it is a win win situation as I see it?
    Thanks again.
    • CommentAuthortony
    • CommentTimeApr 6th 2010
     
    using joist hangers is generally a more noisy way to go.
  9.  
    Posted By: tonysadly using built in joist hangers do not stop air leakage -- the air leakage is initially the same as built in joists though as the built in joists shrink air leakage increases.

    With timber joist hangers there is no penetration of the blockwork, so there should be no difference to the rest of the wall. The Denby Dale Passivhaus uses this technique with resin fixed bolts.

    Masonry joist hangers they can be seated on silicone sealant &, after the next course of blocks has been laid, the joint can be raked-out to a depth of 5-10mm & filled with silicone sealant.

    In either case, if parging or wet plastering is used on the rest of the wall then it will also be required in the floor void.

    I think the Tony Tray works well for pre-cast concrete floor units, but not for timber joists where you need to fill the gaps between the joists with blockwork. These sections of blockwork will have DPC/membrane above & below & joists to both sides, so they are prone to working loose.

    The only way to truly sort it is to use pre-cast concrete formwork (lattice girder panels) on a mortar bed with a cast in-situ concrete slab to the top...but then that's another subject.

    David
  10.  
    Posted By: tonyusing joist hangers is generally a more noisy way to go.
    How so?

    Posted By: tonysadly using built in joist hangers do not stop air leakage
    Don't understand why this is the case. Anyway, all our engineered trusses were sprayfoamed around the hangers and rim joist post installation and prior to the ceilings being installed.

    Paul in Montreal.
    • CommentAuthortony
    • CommentTimeApr 6th 2010
     
    Fixing to a rim joist is very different to fixing to masonry which is what I thought we were discussing.
  12.  
    Posted By: tonyFixing to a rim joist is very different to fixing to masonry which is what I thought we were discussing.
    It's the same technique if joist hangers are used - though one will use fastenings rated for use in masonry and the other fastenings rated for use in wood. I still don't understand your comment about joist hangers being noisier.

    Paul in Montreal.
  13.  
    Posted By: tonyFixing to a rim joist is very different to fixing to masonry which is what I thought we were discussing.

    The Denby Dale Passivhaus combines the two by fixing a rim joist to the blockwork & using timber joist hangers. See the photo to the right of the following article:

    http://www.building.co.uk/story.asp?storycode=3148204

    David
  14.  
    Many thanks David. This is the sort of system I had in mind. Have you had any experience of it and would you be happy to use it in your self build? I hope you do not mind me asking but what is your background? I was thinking of hangers directly onto the inner wall so can you see any problems with this? I assumed this would also improve the air tightness of the property as I would not break into the cavity. I do not really want to be the first to try any system as I want to ensure that my floor remains squeak free for many years (I hope). Many thanks,
    Steve.
  15.  
    Sorry I forgot to mention that my inner skin is 100mm concrete block and will then be finished with wet plaster.
    Thanks.
  16.  
    Steve

    My interest is in researching, specifying & planning my own build. I have a set of well thought through details worked on by various building experts, but no plot of land yet.

    My build will use a concrete 1st floor & so I can't vouch for this technique. There is the existence proof of the Denby Dale build, but because the whole first floor is being held up by resin fixed bolts in 100mm blockwork, it's essential to get a structural engineer to sign-off every aspect of the detail.

    That said, it offers good potential for airtightness & it should brace the walls well. If you use engineered timber I joists & rim joists you will get a very rigid floor & little risk of shrink induced squeaks.

    Timber joist hangers have small holes for nail fixing. They are not suitable for fixing direct to the inner leaf. Instead use the industry standard masonry joist hangers. These are built into the wall as the work proceeds, a steel tongue lapping over the blockwork & into the cavity. The joist remains entirely inside the inner leaf, so the blockwork is continuous apart from the steel tongue passing through the mortar joint.

    As Tony says, there remains a risk of air leakage, but I've heard of people sitting the steel tongue on a bed of silicone sealant, raking the mortar bed above the tongue & filling that also with silicone sealant.

    David
  17.  
    Thanks for the advice. So if I use the masonary joist hangers and do not seal perfectly I should still have a much better structure compared to joists going through into the cavity? Would you agree? Do you also agree that engineered joists have better sound insulating properties? It is hard when you do not much building knowledge to get through the sales pitch! Thanks again David.
    • CommentAuthortony
    • CommentTimeApr 7th 2010
     
    I dont agree -- built in is a better structure -- direct loading rather than indirect via hanger.

    I would say that normal wood joists had better sound insulating properties simply because they are heavier and would cut out more noise as a result though the effect would be small as most of the sound would go through the gaps.

    Re siliconing the joist hangers in --- problems will also arise with masonry airleakage at corners, bed joints and perpendicular joints, cracked blocks etc -- parge coat would help but i use a different philosophy involving belts and braces.
  19.  
    Tony, the information available tends to show less noise transfer with engineered joists. I know what you mean about the weight of the wooden joist and how the density of solid timber may help reduce sound transfer through the joist but this does not seem to be the case. Maybe part of the reason for this is that the webs joining the upper and lower surface is smaller in cross sectional area and so less vibration etc travels through it compared to a solid timber joist? I am also insulating between the joists so I hope this will also reduce sound transfer. Some of my joist spans are fairly wide so I am certain that the engineered joist is the one for me. I have priced both and they cost the same for my property after a little negotiation. I will also save on labour for electrical work and plumbing and at least the joists will not have holes drilled through them for pipe runs.
    I would just like to minimise the draughts but I guess that all the information and advice given is just an opinion so some will be wrong? Decisions decisions. Thanks, Steve. :bigsmile:
    • CommentAuthortony
    • CommentTimeApr 8th 2010
     
    I would hate for you to have any draughts between the ceiling and floor, design them out if you can.
  21.  
    Posted By: gustyturbineSo if I use the masonary joist hangers and do not seal perfectly I should still have a much better structure compared to joists going through into the cavity? Would you agree?
    From a load bearing point of view, it is better to build in the joists, but it is now the industry norm to use masonry joist hangers as these are far better from an airtightness point of view. I am not sure of how they perform acoustically, but building in the joists maximises flanking noise transmission & hangers can only be better.

    Posted By: gustyturbineDo you also agree that engineered joists have better sound insulating properties?
    We have engineered I joists in our current house & the floor is much more rigid than any other timber floor I have had. There is much less impact noise & board squeaking. However, the transmission of airborne noise will be determined by the level of insulation in the joist cavity, the grade of ceiling/floor boards & the use of resilient layers/bars.

    Resilient bars can be used to support the ceiling. Instead of nailing the ceiling board, screw steel resilient bars to the joists & screw the ceiling board to these. This prevents nail pops & reduces sound transmission from joists to ceiling. To upgrade further you can use one of the heavier grades of plasterboard (e.g.SoundBloc) or Fermacell (gypsum woodfibre board). For best performance you can fit a 19mm cement particle board floating floor on top of a 22mm chipboard subfloor with a layer of resilient rubber in between. This is what's done in separating floors for flats & is what I would do if forced to use timber.

    David
    • CommentAuthorgooday
    • CommentTimeMay 29th 2012
     
    We are building an extension with Plasmor Fibolite blocks internal and external with a 200mm fully fille cavity and ancon wall ties. We are having timber joists, so Im not keen on the 'Tony Tray' due to increased risk of movement of blocks siting on a poly layer.

    We are now getting to joist level and I was wondering if using 'coving adhesive' would be ok for a parge coat behind the joist hangers to join to the wet plaster above and below. I have a few bags left over from a previous project.

    Any comments/suggestions welcome.

    Thanks
    • CommentAuthortony
    • CommentTimeMay 29th 2012
     
    It is better than nothing but will crack eventually, impossible to do behind the hangers? And above rhe hangers may need mastic repeatedly?
    • CommentAuthorgooday
    • CommentTimeMay 29th 2012
     
    Thanks Tony, I will do behind the hangers before they are put in.

    What else would you suggest?
  25.  
    Check the hanger manufacturers notes. They normally insist the hanger is mounted tight to the masonry to avoid rotation.

    David
    • CommentAuthorgooday
    • CommentTimeMay 29th 2012
     
    so that means no parge coat behind, and therefore air leakage,how can I avoid that?
  27.  
    You can use masonry joist hangers with a piece of airtightness tape between the hanger & the masonry inner face. Once the joists are fitted this can be incorporated into the parge coat or plaster between the joists. And you can seal around the inner leaf penetration with flexible silicone sealant.

    Or you can do as the they did at the Denby Dale Passivhaus & hang all the joists off face mounted timber joist hangers on a rim board fixed to the inner face of the inner leaf with hex head resin bonded anchors.

    David
    • CommentAuthorgooday
    • CommentTimeMay 29th 2012
     
    Thanks David, Could I put a piece of tyvek air guard the whole way along and then tape the top joint to the hanger and then expamet the lower piece into the plaster below.
    • CommentAuthortony
    • CommentTimeMay 29th 2012
     
    You are getting dangerously close to my tray there now.

    I prefer not to use bolts or hangers but, old fashioned as I am, I like to sit the joists on the walls, no creaking or nasty noises or possibilities of future creeping movements etc
    • CommentAuthorgooday
    • CommentTimeMay 30th 2012
     
    Yes, I will be very similar except it wont go through the mortar joint but stay inside the block work behind the hangers.

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