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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Fundamentals: Defining thermal insulation

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  1.  
    • CommentAuthortony
    • CommentTimeMay 8th 2009
     
    How much insulation needs to done to call something thermal insulation?

    Everything insulates to a greater or lesser extent --- to imply that a material is thermally insulating there must be a minimum job that it does.

    We need numbers -- I would say U values should be those numbers but that would always imply a specific thickness --
    • CommentAuthorBowman
    • CommentTimeMay 8th 2009
     
    Surely for a material to be termed insulation it needs a low heat capacity as well?
    •  
      CommentAuthorfostertom
    • CommentTimeMay 8th 2009
     
    Why? Isn't 20ft of solid ground a good insulator? Certainly has low U-value.
    • CommentAuthormike7
    • CommentTimeMay 8th 2009
     
    Greetings.

    Posted By: fostertomWhy? Isn't 20ft of solid ground a good insulator? Certainly has low U-value.

    Depends on the ground. I have figures for conductivity of granite or sandy soil with 8% water from GS2000 of 2.8W/mK, which makes for a u-value of .47, ie. not good . At the other extreme, dry silty clay figure is .4, so six metres of that gets you a u-value of .07.

    Posted By: BowmanSurely for a material to be termed insulation it needs a low heat capacity as well?


    Depends on the heating regime. If you heat 24/7, then the thermal capacity is not relevant, since although there will be heat flowing through, the temps within the material will not be changing. If heating is not continuous, then we're into what I think is called increment/decrement delay. Oh joy. This sounds like a fancy way of saying it will take time for things to warm up/ cool down.
    • CommentAuthortony
    • CommentTimeMay 9th 2009
     
    Any more for any more?
    • CommentAuthortrule
    • CommentTimeMay 9th 2009
     
    Thermal resistively K.m/W or its inverse thermal conductivity W/m.K are used to derive U and R values based on the thickness of material.

    The amount depends on the application, in the case of building the minimum is specified in the building regs.

    So, for housing, if a material thickness (in combination with other building elements) can meet the building regs within the space available for the task of insulation, then I presume said material could be considered "thermal insulation".

    But all materials offer some thermal insulation, so it really does depend on the application. Personally I think there is enough regulation in this area already, having read through quite a bit of building regulation.
    • CommentAuthortrule
    • CommentTimeMay 9th 2009
     
    <blockquote><cite>Posted By: fostertom</cite>Why? Isn't 20ft of solid ground a good insulator? Certainly has low U-value.</blockquote>

    It is a good insulator however it also has high thermal conductivity and I suspect this far outweighs insulation properties for a building in a temperate climate.
    • CommentAuthorralphd
    • CommentTimeMay 10th 2009 edited
     
    Anything >R2=insulation, <R1 is not an insulator. I'm talking imperial units here (hr/BTU/sf)

    The initial comment about everything insulating is technically true but belies the fact that adding material can increase heat flow; i.e. look at heat sinks in electronics.

    -Ralph
    • CommentAuthorbrig001
    • CommentTimeMay 10th 2009
     
    My old boss once asked me to pass him a 10M ohm insulator. He meant a 10M ohm resistor, but argued that anything that good an (electrical) insulator should be called an insulator. Same thing here, I don't think you can draw the line, you end up with a grey area which could be either.
    •  
      CommentAuthordjh
    • CommentTimeMay 10th 2009
     
    Posted By: ralphdthe fact that adding material can increase heat flow; i.e. look at heat sinks in electronics.

    For circular pipes there is a 'critical thickness' of insulation. Add less than that and it increases the conductance because the increase in surface area outweighs the added resistance.

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