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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthoran02ew
    • CommentTimeApr 28th 2021
     
    can anyone help to explain these figures atrributed to the internl and external surface resistance of walls,roofs etc. i have used verious numbers seemily plunked from the internet for a Therm wall to floor junction and they have vastily changed the Ufactor results from 0.1 to 0.01 externally and 0.3 to 0.05 internally
      Capture floor to wall.PNG
  1.  
    There's a reasonable explanation here:

    https://www.htflux.com/en/documentation/boundary-conditions/surface-resistance-heat-transfer-coefficient/

    Like all models it's a simplification. In this case a representation of heat transfer, down to a single-value coefficient which changes depending on the orientation of the surface. I think the values in the link are the 'standard' ones to use.

    The reason for the single value is to 'normalise' the results because different factors are at play - as it says indoors heat transfer is mainly through radiation, whereas outdoors it's driven more by convection.
    • CommentAuthorEd Davies
    • CommentTimeApr 28th 2021
     
    Perhaps telling us about the numbers you've “plunked” from the Internet might give a bit of a clue?

    Can you explain what is meant by internal and external U-factor results?
    • CommentAuthoran02ew
    • CommentTimeApr 28th 2021
     
    DT thanks that is very helpfull

    however the figures im actuall looking for is the film coeficients not the surface resistance, this is something ive discovered in the last hour. which is aparently the inverse of the surface resistance.

    ED
    i thi nk some of the problem is im imputting the surface resistance figures into the film coeficient box, hense the very low Ufactors. not to mention the rushed post on GBF complete with spelling errors.



    can any one help with the calculation between surface resistance and film coeficients
    •  
      CommentAuthorfostertom
    • CommentTimeApr 28th 2021
     
    I can see you've entered the labyrinth! Let us know if you ever get out.
    • CommentAuthoran02ew
    • CommentTimeApr 28th 2021
     
    Posted By: fostertomI can see you've entered the labyrinth! Let us know if you ever get out.


    My head hurts! but i am making some headway.
  2.  
    Posted By: an02ewcan any one help with the calculation between surface resistance and film coeficients


    This might not help much, but there's a bit more explanation towards the end of the thread here:

    https://discourse.ladybug.tools/t/therm/1174/3

    "This filmCoefficient in W/m2K represents the “U-Value” of the air film between the edge of the THERM materials and the surrounding environment that is at the specified _temperature. The extra resistance from this air film is why the full construction U-Value that you are getting out of THERM is a lower than just the (conductivity of material) / (depth of the material). Accounting for air films is particularly important when you get constructions that have a high overall conductivity (like a single pane window), since almost all of the resistance of such a construction is due to the air films."
    •  
      CommentAuthordjh
    • CommentTimeApr 28th 2021
     
    Posted By: an02ewfilm coeficients

    What is a film coefficient in your context? Some quick googling makes it seem like something imported into THERM from WINDOW and may represent the surface films used on glazing. But what does it have to do with your calculation?

    The page DT linked to gives the normal values for surface resistances as others have said. Those are what should be used in the appropriate place. It sounds like you're trying to input values into the wrong part of whatever interface you're using.
    • CommentAuthorRobL
    • CommentTimeApr 28th 2021
     
    Pilkington spectrum uses a "heat transfer coefficient" from a vertical window surface to inside and to outside. I think it's a higher number outside due to some kind of average wind speed, and inside it's assumed to be still air.

    Inside they use 7.7W/(m^2.degC)
    Outside 25W/(m^2.degC)

    As DT said, it gets more important as the wall buildup is worse, so a single glaze window could be made of copper and be just as insulating, but not give as good a view!
    •  
      CommentAuthordjh
    • CommentTimeApr 28th 2021
     
    Posted By: RobLInside they use 7.7W/(m^2.degC)
    Outside 25W/(m^2.degC)

    1 / 7.7 = 0.13
    1 / 25 = 0.04

    i.e. the same ISO 6946 standard figures quoted in the link DT linked to.
    • CommentAuthoran02ew
    • CommentTimeApr 28th 2021
     
    Posted By: djhWhat is a film coefficient in your context? Some quick googling makes it seem like something imported into THERM

    Yes, because it’s American it uses film coefficient not surface resistance, one being the inverse of the other, again as you pointed out above.
    And all of it made much easier by the add-on supplied by Warm which have prepared boundary condition that take this calculation into account.
  3.  
    The 'film' in this case is not a real physical film, it is often referred to as the 'boundary layer'. The bulk air is idealised as being turbulent and well mixed, so the same temperature (and same average velocity) everywhere. No temperature or velocity gradient, so there could be no heat transfer through it, or friction forces.

    To cover up this gap in the theory, there is idealised to be a thin 'boundary layer' or film of air, usually laminar (un-mixed), bridging the gap inbetween the bulk air and the solid surface, which has a temperature and velocity gradient, and so has heat transfer resistance and friction.

    The thickness of the supposed boundary layer depends on the speed of air flow (so the wind speed, and natural convection speeds, which depend on the height and orientation and temperature) and the surface roughness, plus the physical properties of the air which depend on weather etc. The idealised thickness for heat loss calcs may be different for friction calcs.

    All this gets way too complicated for building design, so people just use standard values for the resistances, which are always wrong but usually are not very significant, and are comparable across different buildings.

    If the circumstances are that the surface resistances make very much difference to your design, then best to stop and think again, check the assumptions, because the standard values are unlikely to be accurate.

    Edit to add: the standard coefficients also include an allowance for radiation, so implying that the radiant heat is going off to somewhere that is the same temperature as the bulk air. This is obviously a simplified version, eg if the radiant heat is going into the clear night sky then much more heat will be lost than the standard coefficients predict, so then you get frost or dew on the outside of windows. If the wall surface is in view of the ground (or solid floor), that might be warmer or cooler than the air at different times of day, or year. Etc
    • CommentAuthoran02ew
    • CommentTimeApr 29th 2021
     
    Thanks will for the understandable explaination. My problem that started the post, i discovered i was using the inverse figures and youtube came to the rescue.



    However i was interested in this

    Posted By: WillInAberdeenThe thickness of the supposed boundary layer depends on the speed of air flow (so the wind speed, and natural convection speeds, which depend on the height and orientation and temperature) and the surface roughness, plus the physical properties of the air which depend on weather etc. The idealised thickness for heat loss calcs may be different for friction calcs.


    How long before building elements are checked in wind tunnels to eeak the last grain of thermal efficiency from there design? or are they already?
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