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Thermal properties on Construction Data, Envelop Summary(output)

asked 2017-04-06 11:20:30 -0600

Jéssicamarasantos's avatar

updated 2020-03-08 12:58:41 -0600

I’ve been using Design Builder to simulate my Project. On construction model data tab, I set the number of layers, the materials and the material thickness for each layer. On the calculate tab, I got 2,95 for U-value (wich is in bold). However, when I run the simulation, I got for U-value two differents values : 4,23( U-factor with film) and 11, 59 (U-factor no Film). I got confused because my maximum wall U-value allowed is 3W/m2 and the output values are much higher. Wich one should I base on?


I've attached two shoot screens about my question. The components wall are cement sand render, concrete block and cement/plaster/mortar - cement screed.

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where are you seeing the U-values reported? Some screenshots of the material input and thermal properties reporting would help the forum understand the issue.

mdahlhausen's avatar mdahlhausen  ( 2017-04-06 18:44:40 -0600 )edit

I've attached some screenshots. Thank you!

Jéssicamarasantos's avatar Jéssicamarasantos  ( 2017-04-06 22:02:24 -0600 )edit

@Jéssicamarasantos please only use the "Answer" post option if you are posting a solution to your own question. If you are adding more details to your question (adding new images, etc.), then edit your original post instead.

Aaron Boranian's avatar Aaron Boranian  ( 2018-07-06 14:42:01 -0600 )edit

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answered 2018-07-06 16:13:41 -0600

There's a bunch of different values between your screen shots, so I'll clarify what each of them represent.

  • 11.6 $W / m^2K$ for U Factor no film: U-value for only wall material layers defined in the Layers tab inDesignBuilder's Constructions Data window.
  • 4.24 $W / m^2K$ for U Factor with film: U-value for combination of wall material layers and air films on interior and exterior surface. More resistance from air films leads to smaller U-value than the 11.6 value above. This is the value EnergyPlus uses in it's calculations for surface heat loss over the simulation. I would have expected this to be equal to the upper bold value shown in the Calculated tab in DesignBuilder's Constructions Data window (3.90 $W / m^2K$)
  • 2.93 $W / m^2K$ for U Factor that you circled at the bottom of the Calculated tab in DesignBuilder's Constructions Data window: this is accounting for thermal bridging impacts, such as heat leaving walls through studs easier than insulation. I would have expected this to be HIGHER than the 4.24 value above, since there is less resistance and more heat transfer.

The Envelope Summary Table Report documentation isn't too helpful, but for more discussion on envelope outputs from EnergyPlus, you can read posts here and here.

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Asked: 2017-04-06 11:20:30 -0600

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Last updated: Jul 06 '18