to all.
im creating a few wall types for our office standards, going into the Wall assembly i found that i was able to add in the Heat Transfer Coefficient (U), however when revit takes into account all of the materials in the wall and adds up all of the U values it doesnt match the numbers we get when we do the math manually, is there a way for me to locate the formula that revit is using to ensure that its adding things up correctly? or am i not doing something right?
thanks for you reply
James C.
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Hello @Anonymous
You are most probably doing it right. This is an old observation and the discrepency in calculated values appears to be due to the way cavities are treated by Revit in the wall layers. Example: Add the default "Thermal/Air layer" with a typical thickness 40mm and material set as "Air" is calculated in Revit as having a conductivity of 0.0250 W/(m·K) ie: the air cavity appears to have the same Thermal Conductivity/Resistance as a solid insulating material of the same thickness... If you are doing your manual calculation based on BRE standards; then you will find some discrepancies. (PS: I assume that if that discrepancy applies to air cavity it most probably applies to other material)
To solve that you will need to edit the xml file and enter your own U and R values as per the standard you need to follow. THIS POST HERE discusses this in detail and it might help you solve that issue.
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oh wow.. ok thanks for the tip ill dig my heels in and see if i can get this thing to work
regards
James C.
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@Anonymous
Just a follow up in reply to your msg:
<Material id="mat-ADD UNIQUE NUMBER HERE">
<Name>TYPE NAME HERE ex: 6 in metal studding</Name>
<Description>ex: 6 in (150 mm) metal studding</Description>
<Thickness unit="Meters">0.140</Thickness>
<Conductivity unit="WPerMeterK">0.14</Conductivity>
<Density unit="KgPerCubicM">650</Density>
<SpecificHeat unit="JPerKgK">1200</SpecificHeat>
</Material>
NOTE: Once done
Good Luck
YOUTUBE | BIM | COMPUTATIONAL DESIGN | PARAMETRIC DESIGN | GENERATIVE DESIGN | VISUAL PROGRAMMING
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Hi, thanks for the explanation, but I have two doubts about the calculations:
1 - In your example, (25mm gypsum+ 100mm air + 25mm gypsum), In my opinion the calculations should be:
Wall R-value = Gypsum R-value + Air R-value + Gypsum R-value = 0.11030 + 0.19019 + 0.11030 = 0.32979
So, U-value is = 1/R-value = 3.03223, but in your example is 1.3289, ¿is that correct?
2 - ¿Which value uses Revit, the calulated value (2.43326) or the manually defined value (1.3289)?
Thanks for your help!
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