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Re: [EnergyPlus_Support] U-factor with film / no film

U-factors in the HTML reports are "rated" values reflecting energy transmission at "rated test conditions". EnergyPlus uses the rating conditions as per ASHRAE 90.1 (see this standard for more information as well as the energplus documentation...there is a document called something like "OutputDetails", though I don't remember the name).

The "without film" is the "conduction" U-Value without the additional convective air film resistances added to the total R-Value (U-value=1/R-value). A wall surface for example, has different film coefficients depending if it is interior facing or outdoors facing...with wind would have higher convection rates....convection rates for horizontal surfaces are different to those of vertical surfaces, so again wall surfaces will have different convection coefficients than ceilings, or also different for floors.

Although energyplus dynamically calculates convection coefficients based on air and surface temps (amongst other parameters depending on other user inputs) to more accurately calculate instantanious U-values for heat transfer, it is still nice to see the ratings at standard conditions of componants so that you can compare like for like.

Sometimes a componant is rated with a value that has no film coefficient on the outer surface, such as the below ground C-values from Standard 90.1, then the without airfilm values can allow you to still compare your wall's performance to the wall minimum requirements of the standard, or if you use European standards, material conduction resistances are fixed but the film coefficients may differ slightly. You could thus easily just add the needed film R-values from the new standard to calculate the U-values as per the new/other standard.

It is a good experience to calculate some U-values by hand or in a spreadsheet. ASHRAE 90.1 is a good starting place and introduces the concept of how thermal bridging degrades the R-values of layers (example the metal stud framing in the metal/insulation layer of steel-framed walls). It also references more detailed calculation examples from the ASHRAE Handbook of Fundimentals.  

Mit freundlichen GrüÃ?en- Sent from my iPhone (excuse the brevity)

i. A.
Jean Marais
b.i.g. bechtold
Tel.   +49 30 6706662-23

On 30.09.2013, at 10:59, <alexmac_00@xxxxxxxx> wrote:


Dear all,

could you tell me the difference between U-factor with film and U-factor no film in the html report? 
Thank you,



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