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U-values - what are they and how do you calculate them

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  • 24-09-2006 3:30pm
    #1
    Keeks
    Registered Users Posts: 2,199 ✭✭✭


    As the title says U-values - what are they and how do you calculate them?


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    #2
    mad m
    Registered Users Posts: 9,005 ✭✭✭mad m


    I think its how much your house can hold the heat in,like good insulation etc.Heating your house for less and keeping it warm for less.

    Havent a clue how you calculate it though.Im sure they have a meter for it.


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    #3
    AJL
    Registered Users Posts: 124 ✭✭AJL


    Find link below to an explanation. Also gives some values.
    www.sei.ie/uploadedfiles/InfoCentre/whatisauvalue.pdf
    Most companies can supply you wit the U values of their products. Obviously you may in some cases have to veirfy the info.


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    #4
    Capt'n Midnight
    Moderators, Recreation & Hobbies Moderators, Science, Health & Environment Moderators, Technology & Internet Moderators Posts: 91,836 Mod ✭✭✭✭Capt'n Midnight


    Q = UA dT

    Q = Heat flow = how many watts you need per second in heat
    U = the value from the walls/windows/floors/ceilings of each building materials
    dT = "delta T" the temperature difference between inside and what's on the other side of the wall

    you add up them for each area of material to give you the amount of heat loss,

    the lower the value of U the better

    values you see quoted are generic actual ones vary due to the way each building is constructed


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    #5
    ardara1
    Closed Accounts Posts: 495 ✭✭ardara1


    Keeks wrote:
    As the title says U-values - what are they and how do you calculate them?
    Hi Keeks -

    3 steps to calculate a u-value
    Each material in a build-up will have a THERMAL CONDUCTIVITY (Lambda)
    Eg Brick - block - timber - insulation etc, this should be taken from product literature.

    Dividing the thickness of the material (in metres) by the thermal conductivity gives you a RESISTANCE for the material (Eg Aeroboard 50mm = 0.050/0.037 = 1.35 of a RESISTANCE

    Add all the resistances up - for all your material and divide that figure into 1 (the inverse) - THATS the U-value.
    Difficulties arise when one of the layers is BRIDGED by another material - eg timber framing through fibre glass - this needs to be proportioned and accounted for.

    Give me an idea of your build up and I'll have a look at it for you - I use a BRE program that can be bought for £25 strg.


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