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Housing - Renovation: Thermal bridge pads on base of roof trusses

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1 to 18 of 18

  1.  
    • CommentAuthorukproptop
    • CommentTimeJun 5th 2017 edited
     
    Hi would love a view on this. The architect has specified Armatherm thermal pads underneath the steel shoes holding the posts of some new roof trusses. I wanted to see if anyone thought this is really necessary. The build up under the posts from ground level is 1) new concrete pads 400mm deep 2) new slab 150mm thick 3) armatherm pads 4) 6mm steel shoe 5) post of truss (which is internal to building) upright into shoe. The new 130mm of celotex floor insulation will be on top of the new slab and fitted around the shoes / trusses.
    The idea is to stop the cold bridge between the warm truss and the cold slab and any risk of condensation at this point. With glulam members and 500mm + of slab is this really necessary? Is there a cheaper alternative? Armatherm comes out at over £1k for 24 pads which I would rather not spend unless absolutely necessary. Thanks in advance.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 5th 2017 edited
     
    The fixing bolts bridge right though? Pity the insulation can't go under the slab, on top of the pads - https://www.kore-system.com/kore-products/floor-insulation/kore-passive-slab/technical can provide hi-compressive-strength insulation for such point loads
    • CommentAuthorukproptop
    • CommentTimeJun 5th 2017
     
    Good point, the bolts will be thermal bridging anyway. Is it worthwhile at all? Is there really risk of less condensation?
    •  
      CommentAuthordjh
    • CommentTimeJun 5th 2017
     
    As Tom says, it would be better if the insulation were under the slab.

    I'm not clear whether the slab is ground-bearing or suspended. You say that from ground level there are 400 mm deep pads. Do you mean they are short columns lifting the slab off the ground, or do you not really mean from ground level at all? I also don't understand where your '500mm + of slab' comes from?

    The pads will reduce the area in contact very significantly and so reduce heat transfer and the risk of condensation. According to http://www.armadillonv.com/index.php/armatherm-gf there are also washers and bushes available and if they were used with stainless bolts that would reduce heat transfer even more.

    As to whether the pads are necessary I would ask your architect. What modelling has he done? Point out the cost and ask whether there's a cheaper way to do the job. What are you trying to achieve, insulation-wise?
    • CommentAuthorukproptop
    • CommentTimeJun 5th 2017 edited
     
    Sorry if not clear. This is a barn conversion, there is an existing slab. New pads have been dug through the existing slab and then a new structural topping will go on top of this, followed by insulation and screed. The insulation can't go under the slab without digging up the existing slab as the engineer doesn't want it on top of the existing slab. The posts will sit in steel shoes on top of the new structural topping with the new pads directly beneath them. Pads are already in place so no going back anyway. So with the pads (400mm) and new slab (100mm) the steel shoes will be sitting on approx. 500mm of concrete before you get to the bare earth beneath that- although with earth off to the side after 100mm down.
    I am trying to understand if this is a £1,000 I really need to spend on the thermal pads under the steel shoes or not. With the 4 x bolts through the shoes and thermal pad into the concrete below there is a thermal bridge even with the pads. It just feels to me that it could well be a waste of money. Thanks again for any thoughts.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 5th 2017 edited
     
    The pads are already in, topping awaited? I guess that's a 600x600x150dp ragged depression in the slab surface? Could the column be sat on say 100thk high-compression insulation within that depth, direct on the pad, with horizontal location being provided by more high-compression insulation 50thk shaped around the column to fit tight within the (trimmed) 'walls' of the depression? Then the 130 floor insulation over that?

    Pity they didn't line the base of the pad excavation with high compression insulation, and the 'walls' of the excavation incl slab thickness with ordinary insulation, before casting the pad - then you could bolt happily into the pad, which is insulated from ground, continuous with the slap-top insulation. You could still excavate more, around the pad, to slide insulation down around it - then you really would be 550mm of concrete away from the cold ground.
    • CommentAuthorgyrogear
    • CommentTimeJun 6th 2017 edited
     
    Posted By: ukproptopI am trying to understand if this is a £1,000 I really need to spend on the thermal pads under the steel shoes or not. With the 4 x bolts through the shoes and thermal pad into the concrete below there is a thermal bridge even with the pads. It just feels to me that it could well be a waste of money.


    I'd tend to agree with you - wood being a natural insulator...
    http://makeitwood.org/benefits-of-wood/insulator.cfm
    Maybe the architect has more experience working on steel structures ?

    gg
    •  
      CommentAuthordjh
    • CommentTimeJun 6th 2017
     
    The problem is also mentioned in that link. Wood is hygroscopic and will happily transport water to a cold place to rot it.
    • CommentAuthorgyrogear
    • CommentTimeJun 6th 2017
     
    But where would the water come from ?

    because apparently:

    Posted By: ukproptopThe posts will sit in steel shoes on top of the new structural topping


    gg
    •  
      CommentAuthordjh
    • CommentTimeJun 6th 2017
     
    The water source is vapour in the atmosphere inside the house.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 6th 2017
     
    That's it - the shoes will be permanently likely to be wet with condensation. The bolts, tho small in cross section, are highly conductive, effectively in contact with subsoil always colder than the interior, quite capable of cooling the equally conductive shoe.

    The 550mm 'thickness' of concrete provides no insulation at all, being surrounded by subsoil. Now, if the 4 sides of each pad (incl slab thickness) were encased in insulation, that would be a bit different.
    • CommentAuthorgyrogear
    • CommentTimeJun 7th 2017
     
    Thank you both, I am now the wiser !

    gg
    • CommentAuthorringi
    • CommentTimeJun 15th 2017
     
    Stainless steal bolts will conduct less heat and last longer.

    Could plastic insets be used in the holes so the bolts don't touch the steal, then put thermal pads under the washers?
  14.  
    Hi Julian, sounds like the steel posts punch through the insulation so the thermal pads beneath the posts are of no benefit, it may make more sense to wrap the posts in insulation to reduce their cold bridging effect.
    Water vapour diffusion has been mentioned here but there's been no mention of airtightness which is 100 times more important. If there's no moist air present where the posts are bolted to the pad there will be no condensation.
    There's never been a proven case of structural decay in a building as a result of water vapour diffusion, but there's been plenty of problems resulting from lack of airtightness.
    • CommentAuthorukproptop
    • CommentTimeJun 19th 2017
     
    Thanks everyone for your comments they were helpful.
    We pushed engineer further. So really I was just trying to make sure if I spend £1,000 on these thermal pads that they will make a difference (insulation) and are needed (condensation). Then end result has been that it was realised the posts would sit on the top of the bolts securing the shoe to the ground and would have a 5mm air gap around them with horizontal bolts also securing them. Some some thermal break but also the bridge points can't be avoided anyway.
    The engineer basically conceded the pads would be a total waste of time and instead we should just sit the shoes on some non-shrink thermal grout, which will provide some thermal break, with the air gap also ensuring condensation is very unlikely. The timber will also be treated so should repel any moisture if there is any.
    Thanks again and £1,000 saved.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 19th 2017
     
    •  
      CommentAuthordjh
    • CommentTimeJun 19th 2017
     
    Not really, one is steel columns, the other is timber.
    •  
      CommentAuthorfostertom
    • CommentTimeJun 19th 2017
     
    But both about column bases which don't bridge the insulation

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