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Incorrect manufacturer declared U Values

  • 18-06-2009 11:32AM
    #1
    Jimbo
    Closed Accounts Posts: 2,379 ✭✭✭


    I was just calculating a u value for a cavity wall and noticed that the u values declared by some manufactureres on their brochures seem significantly exagerated.

    Take for example a standard rendered cavity wall (medium weight block) with 60 mm rigid phenolic insulation in cavity:

    Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
    External resistance - - 0.0400
    External Render 0.017 0.57 0.0298
    Block 0.103 0.57 0.1807
    Cavity - - 0.1800
    Insulation 0.06 0.021 2.8571
    Block 0.103 0.57 0.1807
    Internal Plaster 0.0125 0.43 0.0291
    Internal resistance - - 0.1300
    ∑ Thermal Reistances = 3.6274

    U-value of wall = 0.276
    W/m2/K

    (Not taking wall ties into account)
    Compare that to say: http://www.insulation.kingspan.com/uk/pdf/k8.pdf (Page 4)

    Their tables state that using dense blocks, which would increase the u value, 60mm insulation would give a U Value of 0.25.

    Are people being misled, or are my calculation incorrect?


Comments

  • #2
    d'Oracle
    Registered Users, Registered Users 2 Posts: 7,024 ✭✭✭d'Oracle


    They state the assumptions made for their calculation.

    The assumed conductivity of the block and the width of the block (probably a lot less significant in this case) are different to those in your calculation.


  • #3
    Jimbo
    Closed Accounts Posts: 2,379 ✭✭✭Jimbo


    There's only slight differences though.
    They shouldn't effect it that mcuh


  • #4
    nmg-lky
    Closed Accounts Posts: 42 ✭nmg-lky


    They take into account alot more factors than what you have shown and work it out over a large area. You should not use there values, always go by your calculations. This is the advice i got from SEI anyway


  • #5
    d'Oracle
    Registered Users, Registered Users 2 Posts: 7,024 ✭✭✭d'Oracle


    Jimbo wrote: »
    There's only slight differences though.
    They shouldn't effect it that mcuh

    I must be missing something.

    Your calcs has a lambda value of 0.57.
    Theirs has an assumed Lambda of 1.13.

    Thats huge.


  • #6
    Jimbo
    Closed Accounts Posts: 2,379 ✭✭✭Jimbo


    I only gave an example calc.
    A higher Lamda value for the blockwork would increase my u-value even more.


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  • #7
    Pkiernan
    Registered Users, Registered Users 2 Posts: 4,236 ✭✭✭Pkiernan


    Insulation 0.06 0.021 2.8571

    Quick question, where are you getting the 0.021 value above from?
    Is this a Kingspan figure, or a standard from the Part L manual?

    It may be that Kingspan have a better value than 0.021 for some of their products.


  • #8
    Jimbo
    Closed Accounts Posts: 2,379 ✭✭✭Jimbo


    I got that from the Kingspan docs.

    It's generally about that for that type of insulation.


  • #9
    recedite
    Closed Accounts Posts: 13,989 ✭✭✭✭recedite


    They have a new IAB cert out for Kooltherm products which gives a TC of 0.02 for the 60mm thick board.
    But that still only brings the overall U-value down to 0.26.
    I notice they are claiming 0.24 for standard density block with brick.


  • #10
    Jimbo
    Closed Accounts Posts: 2,379 ✭✭✭Jimbo


    recedite wrote: »
    They have a new IAB cert out for Kooltherm products which gives a TC of 0.02 for the 60mm thick board.
    But that still only brings the overall U-value down to 0.26.
    I notice they are claiming 0.24 for standard density block with brick.

    Im getting 0.27:
    Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
    External resistance - - 0.0400
    Brick 0.103 0.77 0.1338
    Cavity - - 0.1800
    Insulation 0.06 0.02 3.0000
    Block 0.103 0.57 0.1807
    Internal Plaster 0.0125 0.43 0.0291
    Internal insulation 0 0.021 0.0000
    Internal resistance - - 0.1300
    ∑ Thermal Reistances = 3.6935

    U-value of wall = 0.271
    W/m2/K


    Something is wrong there. Big difference between 0.27 and 0.24


  • #11
    buzymc
    Closed Accounts Posts: 6 ✭buzymc


    I contacted them before about this, the silver foil on the insulation creates a low-e cavity and gives the cavity a resistance of 0.644
    I think if you update your calculations this may be the difference.


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  • #12
    Jimbo
    Closed Accounts Posts: 2,379 ✭✭✭Jimbo


    buzymc wrote: »
    I contacted them before about this, the silver foil on the insulation creates a low-e cavity and gives the cavity a resistance of 0.644
    I think if you update your calculations this may be the difference.

    Never heard that before, it changes things fairly substantially:
    Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
    External resistance - - 0.0400
    Brick 0.103 0.77 0.1338
    Cavity - - 0.6640
    Insulation 0.06 0.021 2.8571
    Block 0.103 0.57 0.1807
    Internal Plaster 0.0125 0.43 0.0291
    Internal insulation 0 0.021 0.0000
    Internal resistance - - 0.1300
    ∑ Thermal Reistances = 4.0347

    U-value of wall = 0.248
    W/m2/K

    Not quite the 0.24, but considerably better.

    If you're right, Kingspan should really make it clearer.


  • #13
    recedite
    Closed Accounts Posts: 13,989 ✭✭✭✭recedite


    That was interesting. By the way, why use 103mm for the width of a block instead of 100? If anything materials are smaller than their nominal size.


  • #14
    Jimbo
    Closed Accounts Posts: 2,379 ✭✭✭Jimbo


    recedite wrote: »
    That was interesting. By the way, why use 103mm for the width of a block instead of 100? If anything materials are smaller than their nominal size.

    It's a standard value I have in my spreadsheet.

    4 inches is slightly more than 100mm, but your probably right - block manufacturers have proably rounded their dimensions to metric by now.
    I was probably trying to squeeze what I could out of a wall construction when I set the spreadsheet up.


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