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    • CommentAuthorgoodevans
    • CommentTimeMar 2nd 2017
     
    If you wish to use EPS (graphite or plain white) above your suspended floor what calculation would you use to calculate the U value.

    Looking at the Celotex website (https://www.celotex.co.uk/download/aae9c39a-feb1-41d5-8476-570f52e52a6d) for a 0.4 P/A ratio floor it seems that an R value of 1.5 to 1.7 is gained from surface resistances, concrete blocks and beams, ventilated air void and (presumably because they use a P/A ratio for the U value) the soil.

    I would imagine this published U value does not include the psi value for the floor/wall that SAP adds on.

    All of the manufacturers use similar calcs - how do they do get to these U/R values if the space underneath is ventilated - or, as I wish to use eps, how can I benefit from the same calculations that they use for credit on my SAP rating.

    (I know SAP isn't good or accurate - but If I'm going to exceed building regs I want credit for that in my SAP assessment).
    • CommentAuthorringi
    • CommentTimeMar 2nd 2017
     
    How thick does the EPS need to be before the issues have no real effect on the U value?

    I find it odd using P/A radio where there is a vented void under the floor, as I would expect the number of vents to be increased based on the area of the floor.
    • CommentAuthorneilu
    • CommentTimeMar 2nd 2017
     
    Go onto the Jablite website.
    They have a U-value calculator and their Jabfloor 70 is a fairly standard expanded polystyrene insulation.
    • CommentAuthorgoodevans
    • CommentTimeMar 2nd 2017
     
    ringi - yes - I thought that was odd too and if given the choice I was going to run underfloor walls lengthways to reduce the number of airbricks.

    neilu - just after I posted I located the jabfloor suspended floor data - again they have a similar analysis but for a 0.4 p/a ratio an R value of more than 2 is assumed for screed, surfaces, beams, void and soil (equivalent to another 75mm of insulation). I see BR443 say disregard anything beyond a ventilated void but for suspended floors this seems not to be the case.
    •  
      CommentAuthordjh
    • CommentTimeMar 2nd 2017
     
    Posted By: goodevansI know SAP isn't good or accurate - but If I'm going to exceed building regs I want credit for that in my SAP assessment

    Why do you care what is in your SAP assessment?
    • CommentAuthorSigaldry
    • CommentTimeMar 3rd 2017
     
    I disagree that SAP isn't good or accurate. It works fine for what it's meant to do, provided correct information is put into it.

    I would say though that Garbage in = Garbage out.

    U-values don't include Psi values. Psi values include Psi values.

    - A U-value covers the planar element (the roof, wall, floor etc);

    - Psi values cover the junction and are determined via a totally different means;

    - SAP Calculations utilize U-values of roofs, walls, floors, windows, doors + air tightness, air infiltration, heating, hot water, controls, lighting, solar gains etc

    Either way, you want someone competent to undertake any of them, who will follow the appropriate conventions and methodologies.

    On the floor side of things, a suspended floor calculation takes account of P/A, but it doesn't have anywhere near as much of an effect on the overall U-value. The ventilated void should be handled by the procedures in BS EN ISO 13370 (alongside conventions from BR443). The conventions cover mean wind speed, wind shielding factor, area of ventilation openings.

    From BR443 See section 9.2 : "The calculation of the overall U-value of the floor involves combining U-values representing the floor (from the inside environment to the underfloor void), and the heat transfer from the underfloor space to the outside (by conduction through the ground and the walls of the underfloor void, and by ventilation of the underfloor void)".

    There's a lot more to it and you also need to reference BS EN ISO 6946 for the floor deck.

    Alternatively you can look on a manufacturers website, especially if they have BBA/TIMSA competent calculations and the calculations should already have taken all of these things into account. The conventions and standards are available, you just need a bit of competence and experience to ensure that they are applied and followed correctly.

    Competently produced Psi value calculation for the wall floor junctions will have separately calculated losses for perpendicular and parallel junctions (using 2D and 3D software as appropriate) and on top of the above, also will have followed BR497 conventions and calculated the underfloor solum temperature as it effects the heat losses.
    •  
      CommentAuthordjh
    • CommentTimeMar 3rd 2017
     
    Posted By: SigaldryIt works fine for what it's meant to do, provided correct information is put into it.

    What is its purpose?
    What proportion of its use is fed with correct information, do you suppose?

    What I think we're aiming at here is a good tool or tools to design low energy houses (or perhaps very low energy houses). As I understand it, SAP fails in that goal at the very start of its methodology by calculating target values using 'a notional dwelling of the same size and shape as the actual dwelling' and its figure of merit is based on comparisons against that. So it takes no account of energy-guzzling architectural shapes.
    • CommentAuthorringi
    • CommentTimeMar 3rd 2017
     
    SAP also allows green washing with PV panels......
    •  
      CommentAuthorfostertom
    • CommentTimeMar 5th 2017 edited
     
    Posted By: ringiI find it odd using P/A radio where there is a vented void under the floor, as I would expect the number of vents to be increased based on the area of the floor.
    I'd say P/A completely inapplicable with even slightly vented void.

    Posted By: SigaldryFrom BR443 See section 9.2 : "The calculation of the overall U-value of the floor involves combining U-values representing the floor (from the inside environment to the underfloor void), and the heat transfer from the underfloor space to the outside (by conduction through the ground and the walls of the underfloor void, and by ventilation of the underfloor void)".
    seems bananas, pursuing a factor that is negligible (at best). P/A is about netting the multi routes taken by conduction through subsoil, both downward and outward via foundation walls. If the whole underfloor is at or near outside temp due to ventilation, then the outgoing heat is vented to outside before it significantly reaches the subsoil.
    • CommentAuthorgyrogear
    • CommentTimeMar 5th 2017 edited
     
    As for the layman community, viz. those with concrete joists and floors, I think we are still waiting to hear *WHY* an underfloor void needs ventilating in the first place... If it's for radon, then somebody should perhaps say, "it is for radon".

    (But in that case, folks in non-radon areas would still need another explanation...).

    gg
    •  
      CommentAuthorfostertom
    • CommentTimeMar 5th 2017
     
    Posted By: gyrogear*WHY* an underfloor void needs ventilating
    No one can say - lost in unattributable history - folklore that needs to be put to bed - like that 3:1 inboard:outboard vapour resistance 'rule'.
    • CommentAuthortony
    • CommentTimeMar 5th 2017
     
    No, it is now simply because building regulations say so
    •  
      CommentAuthorfostertom
    • CommentTimeMar 5th 2017
     
    I think we're saying the same thing.
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