OpenStudio, U-Factor maximum value for simple glazing?

asked 2014-10-29 10:17:52 -0600

Julian Nino
274 ●1 ●10 http://www.linkedin.co...

updated 2014-10-29 15:16:16 -0600

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I'm trying to create on the "simple glazing systems window material" a material that have the same U-factor for the skylight with curb,glass,% of roof with the assembly of U-factor of 1.98 Btu/ft2.h.R for a Semiheated Space as specified on ASHRAE 90.1-2007 for Climate Zone 3A but for some reason doesn't let me pass 1.2 for I-P or 6.81 for SI.

Its there a maximum value for the U-Factor to create a simple glazing system or its bug a of my OS? Because every time that I try to change my U-factor it resets.

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answered 2014-10-29 11:14:37 -0600

Archmage

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It is not a bug in OS nor anything you are doing wrong. EnergyPlus itself cannot model a U-value higher than 5.8 W/m2-K. The reason for this is that there is some thermal resistance in the surface-to-air interfaces and with standard film coefficients the U maxes out at about 5.8. To go any higher there would need to be 3-D fins, a tall curb, or projecting glazing to augment heat transfer, but in EnergyPlus the windows are flat planes. I believe most reviewers understand this and you can just go with the highest value the software can use.

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This is something that I need to have in mind for my other projects because they have some semi heated spaces as well, Thanks for your answer :D!!!

Julian Nino ( 2014-10-29 11:21:41 -0600 )edit

Is there a UH "Answer of the day" or "Answer of the week" feature?

__AmirRoth__ ( 2014-10-29 14:15:09 -0600 )edit

I couldn't understand why it kept being reset... go to UH, find the answer, what a great feeling. Thanks for the explanation @Archmage. For for those who might want to know what's the actual maximum value accepted by OpenStudio, it's 1.232771 (as of OS 1.6.0)

Julien Marrec ( 2015-03-26 08:36:55 -0600 )edit

@Archmage, this has become an issue for the CBEC-Com team. We are considering using SurfaceProperty:ConvectionCoefficients:MultipleSurface to change the inside air film coefficients to allow for higher total U-values. Do you have any thoughts on that approach?

macumber ( 2016-01-07 10:58:09 -0600 )edit

That would change how the windows are actually performing during simulation timesteps but not the side calculation used for single-U-factor report. The fenestration rating standard prescribes the (fixed) film coefficients that are used for the U-factor report. When EnergyPlus simulates it dynamically varies the convection portion of the film coef and it doesn't even have the concept of the radiative portion of film coefficient because it directly couples to the zone's IR scene. If there was a timestep air-to-air U-factor report it would vary a lot and I suspect often go higher than 5.8.