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General: insulate steel from concrete base?

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    • CommentAuthorSprocket
    • CommentTimeMay 4th 2012
     
    The two things are connected.
    The steelwork won't get that cold if eg. it's only losing 6W at the pad.
  2.  
    Posted By: SprocketWell that's about 600kwh per year lost to the pads.
    So at 14p/unit that's about £84 a year.
    Still sounds pretty insignificant (for this building) to me.
    It obviously depends upon the building, but it would be a fair proportion of a Passivhaus' heating bill.

    Whatever the house, a few sheets of insulation would pay for themselves in a relatively short time.

    David
    • CommentAuthorSprocket
    • CommentTimeMay 4th 2012
     
    > a few sheets of insulation would pay for themselves

    I'm not convinced (and am enjoying playing Devil's advocate here)
    10mm of that TBG300 stuff is a U value of (0.18/0.01) = 18
    No better than 100mm of concrete.

    The 100mm concrete was just a big simplification and is fairly worst case. In reality the concrete next to the pad will warm up because it is surrounded by more insulation (ie. more concrete). The steel will stay at 20C but there will be a gradient in the concrete. As you go further away from the pad the concrete area is greater but so is the thickness.

    As an insulator 100mm of concrete is as good as 10mm of TBG300. But in reality you have much more than 100mm of concrete (and then soil etc etc).

    Unless you can very cheaply and easily (and safely) get some serious insulation into that gap I don't think it's worth the trouble. It's also very structurally important. I would not want to be guinea pig with something new and unproven. I love foamglas (this building has huge amounts of it) but I would not use it there.
    • CommentAuthorSprocket
    • CommentTimeMay 4th 2012 edited
     
    Forgot to say, the 14p/unit is pretty worst case also of course.
    It's perhaps not likely that a passivhaus would be paying 14p/kwh for heat.

    eg. This steel framed building is heated by GSHP powered by PV so operating cost is a lot less.
    (or alternatively, we've already sunk 100 years worth of heating bill into the boreholes :-/ )

    It's just not worth the trouble of making the strutural engineer unhappy.
  5.  
    £84 per year for x years pays for a lot of EPS insulation. It doesn't need to go under the steel, it can be wrapped around it.

    However, I don't see any reason why the steel couldn't be supported by the concrete slab with Foamglas between the slab & the pad foundation. That way the slab provides lateral support & spreads the load, the whole structure stays warm & there's no issue with heat loss. It shouldn't cost any more than the difference between the cost of EPS & the cost of Foamglas & so it would pay for itself in a couple of years (even with a more cost effective heating system).

    David
    • CommentAuthorNovice1
    • CommentTimeMay 4th 2012
     
    Thanks Guys
    Excellent debate. Think compromise probably in order. TBG300 + washers etc is probably the only thing the engineer will be happy with. The quote for 6 300x300 peices of TBG300 was ~£950. Will probably wrap the insulation around steel and make sure condensation not an issue. May consider a VH interseasonal store (UFH coils in sand) under the slab to restore eco-karma. Steel columns may act as interseasonal vertical rads. May be impossible to calculate or slow the disharge from the store!!

    In my structural engineers defence he has to balance new and old. Decades of EPS/Foamglass/TBG300 etc v. centuries of experience with concrete. If I was gambling with my professional indemnity insurance concrete would win.

    Nevermind will spend some exra on the glazing||
    • CommentAuthorEd Davies
    • CommentTimeMay 4th 2012
     
    6 * 0.3 * 0.3 = 0.54 m².

    TBG300 has a max loading pressure of 300 MPa so that'll bear 162 MN.

    For context, the thrust of a Saturn V was 34 MN. 10 N is, roughly, the weight of 1 kg so it'll hold up 16.2 million kilograms or 16'200 tonnes - a little less than a third of the Titanic. I'm not sure how big your house will be but I suspect your safety margins are admirable.

    Maybe TBN100 is worth a look?
    • CommentAuthorSprocket
    • CommentTimeMay 4th 2012
     
    But what is TBG actually made of? Is plastic flow (ie results in longer term movement) an issue?

    I don't know. I just got stuck in the devil's advocate part. Couldn't help myself.
    Anyway, flipping sides a bit, along the way I found this which was not directly related but was interesting:-

    http://www.nesea.org/uploads/universe/docs/wysiwyg/documents/R._Miller-Johnson_and_J._DAloisio_Concrete_Steel_and_Glass.pdf

    It's got some nice thermal images.
    •  
      CommentAuthordjh
    • CommentTimeMay 4th 2012
     
    Posted By: Ed DaviesMaybe TBN100 is worth a look?

    Or reduce the pad area. TBN100 is presumably cheaper but as well as 1/3 strength it also has poorer thermal performance 0.29 vs 0.18 according to the FAQ.

    Posted By: SprocketBut what is TBG actually made of?

    The data sheet says "Cured, glass-fibre-reinforced polyester resin". It's good to know they cure it :) It's still pretty expensive fibreglass at £150 for a dinner plate, but I suppose you get a certificate.

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