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Different uValues from different suppliers of Fiberglass with same w/mk value of .034

  • 04-11-2015 09:35PM
    #1
    05Mav
    Registered Users, Registered Users 2 Posts: 30 ✭


    Hi Guys,

    I was looking at fiberglass insulation in a sloping roof from two different suppliers. The w/mk is .034 and is the same for both. The roof build up is:

    Natural Slate
    2x2 batten
    Solitex Membrane
    300mm rafter (250mm insulation)
    Airtight membrane
    2x2 batten (service area with 50mm insulation)
    12.5mm plasterboard.

    One supplier gives me a uValue of .13 and the other .14.

    Confused by this I asked them how they had different values, one said they were using different software than the others. I have asked someone else to work out the uValue but haven't heard anything form last week.

    In my simple mind i thought with the same w/mk the uValue would be the same.
    Would anyone be able to tell me which is right.
    Many Thanks


Comments

  • #2
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,147 Mod ✭✭✭✭BryanF


    Look up tgd part L 2011 and do your own sums


  • #3
    05Mav
    Registered Users, Registered Users 2 Posts: 30 ✭05Mav


    Thanks for that Bryan.
    Tried it didn't like it. I got a result way lower and left with way to many more questions on how it is calculated.
    Thanks


  • #4
    JonathonS
    Registered Users, Registered Users 2 Posts: 701 ✭✭✭JonathonS


    Possibly rounding?

    The actual difference might only be .001, ie .134 vs .135


  • #5
    sydthebeat
    Subscribers, Paid Member Posts: 44,924 ✭✭✭✭sydthebeat


    could be any number of things

    look at BRE 443


  • #6
    Metric Tensor
    Registered Users, Registered Users 2 Posts: 3,732 ✭✭✭Metric Tensor


    They may have taken a different spacing or thickness for your ceiling joists.

    As differences go it's not a huge one but I realise how it could be affecting your DEAP results!


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  • #7
    05Mav
    Registered Users, Registered Users 2 Posts: 30 ✭05Mav


    Thanks Syd.

    One is using BRE software and one is using JMP. I though as both insulation had .034w/mk they would at least give the same over all uValue.
    Does that mean the system for calculating is slightly flawed and open to interpretation.
    Why is there not one industry standard. Should DEAP or PHPP not be used instead.


  • #8
    Metric Tensor
    Registered Users, Registered Users 2 Posts: 3,732 ✭✭✭Metric Tensor


    Neither DEAP nor PHPP calculate u values. They just use them as part of their process to calculate asset ratings/behaviours.

    I suspect in this case it's the input to the calculation packages that us varying rather than the packages themselves. Computer software is only as accurate as the person using it!


  • #9
    sydthebeat
    Subscribers, Paid Member Posts: 44,924 ✭✭✭✭sydthebeat


    05Mav wrote: »
    Thanks Syd.

    One is using BRE software and one is using JMP. I though as both insulation had .034w/mk they would at least give the same over all uValue.
    Does that mean the system for calculating is slightly flawed and open to interpretation.
    Why is there not one industry standard. Should DEAP or PHPP not be used instead.

    Hi 05MAV

    what i linked to above is the convention for calculating U Values, and their use in DEAP

    BRE 443: 2006

    look at section 4.9 in the document above and you can see where there is room for opinion in these calculations.

    there can be many reasons for differences.... different mechanical fasteners input, different emissivity surface inputs, different air gaps inputs, different thermal bridging ratio inputs

    see page 21 for different conventions

    all these can effect u values, while at the same time all thermal conductivity values being the same.


    people think we have it easy ;)


  • #10
    MOTM
    Registered Users, Registered Users 2 Posts: 412 ✭✭MOTM


    With the bre calculator it'll give you a detailed report telling you exactly what the inputs were. If the other calculator outputs the same detail you could compare the differences


  • #11
    galwaytt
    Registered Users, Registered Users 2 Posts: 12,734 ✭✭✭✭galwaytt


    ....assuming that's a sloping rafter-as-ceiling, then BRE calculator is giving 0.153 as near as I can get.

    Bridging factor is one question I'd ask - I've assumed std TF detail as 12%.

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  • #12
    05Mav
    Registered Users, Registered Users 2 Posts: 30 ✭05Mav


    Thanks Guys
    Your right Syd it's far from f****** easy.
    Head wrecked now.


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