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U-Value question.

  • 10-06-2008 11:31pm
    #1
    Dr.Millah
    Registered Users, Registered Users 2 Posts: 582 ✭✭✭


    Hey guys.
    Im working on 2004 Q5 part A.

    Im a bit tired at the moment and im not sure if im making a very stupid mistake or if i am right, if im wrong, i sure cant find the mistake.

    Anyway il explain my problem.

    The original Total resistance of the house is 2.8571 this is obtained by 1 % .35 (u value of house)

    Now it tells you that there is 100mm of glass fiber in the attic, this has a conductivity of 0.04, so when you divide 0.100 by 0.04 you get Resistance of 2.5000.
    So if you take this insulation away (2.5000R) from the resistance of the house (2.8571) you get a value of 0.3571 R. This is the roof without any insulation.

    Now the required u value is 6.2500R so with the roof already having a value of 0.3571 R without any insulation only 5.8929R is required by the insulation alone.

    Now when you Work out how think the Fibre glass will have to be to have such a resistance by (Thickness = conductivity x Resistance) you get a value of 235.7mm of fiber glass insulation.

    In the solution papers it says 135.7mm, but this does not add on the 100mm previously in the roof space, and the question does not state that this insluation is removed or left in place.

    When the same formula as above is done to the Urethane Board insulation you get a value of 135.5 mm. Which is also far from the solution papers which says 78mm.

    I think they intend leaving the exsisting Insluation in the roof, but they do not make it one bit clear in the question.

    I hope something like this doesnt happen next week, because you know what you are doing but you will get a totally wrong answer due to there lack of clarity.

    I hope what i explained is right and im not missing out on something stupid haha, thanks.

    "Edit" Yea in the papers they leave the original insulation in the roof and add the other onto it, such a long post about not much sorry about that! I just found the question deceiving.


Comments

  • #2
    Dr.Millah
    Registered Users, Registered Users 2 Posts: 582 ✭✭✭Dr.Millah


    Also, i have one other question (sorry i would have asked my teacher but we never did this chapter and im learning it myself)

    When calculating the U-Values in the 2007 paper, for the resistance i put up to 5 numbers down from the calculator after the decimal point. This leads to a tiny fraction of a difference in my Total Resistance in comparison to the books. Mine is 1.823 and theres is 1.828. Doesnt sound like much but when you multiply these figures out by the area and temp difference this small difference becomes more noticable,
    I get a figure of 3430.7watts and they have 3445.78.
    And so on through the question untill you work out the total cost of heat lost and my figures get 661.0 euro and theres get 663.98 euro.
    Technically mine is clocer to the actual figure but it doesnt matter does it?
    Is there a certen % of "error" allowed as long as you show the techniques and so on?
    If not how are you ment to know how many digits after the point to put in because nearly every year they have different amounts


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