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Does Simple Glazing System include air films ?

asked 2021-04-07 11:27:21 -0500

mval gravatar image

Hi everyone,

I am using OpenStudio and the object WindowMaterial:SimpleGlazingSystem

I have calculated the overall U value with WINDOW 7.7. Do i need to enter the U value with air films in that object or should I enter the U value without the air films ?

According to the EngineeringReference the U value includes the film but when i look at the Report: Zone Component Load Summary at the section Fenestration Conduction in Estimated Heating Peak Load Components the value is lower than expected by around 20%, when the values for infiltration and exterior wall conduction are closed to what expected.

I have noticed that the value in Fenestration Conduction varies with the wind speed value in the design day so i guess the issue comes from the air films. Or is there something else ?

Thanks for your help

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answered 2021-04-07 16:42:51 -0500

Keigo gravatar image

updated 2021-04-07 22:37:49 -0500

The U-value in WindowMaterial:SimpleGlazingSystem includes air films, but it's just a rated value. In simulation, actual wind speed in the weather data is used. And according to Engineering Reference, the wind speed is also adjusted by the building height.

Please refer to WindowMaterial:SimpleGlazingSystem section in Input Output Reference. It says the U-Factor is the rated (NFRC) value for U-factor under winter heating conditions.

According to ANSI/NFRC 100, the rated (NFRC) value for U-factor under winter heating conditions are as follows.

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The wind speed is 5.5m/s, resulting in the exterior convective film coefficient of 26W/m2K. (The formula used here is h=4+4V. 4+4*5.5=26W/m2K.)

I think if your model is located at wind speeds greater than 5.5 m/s, or if the height of your model is quite high, the actual U-factor used in energy simulation can be higher than the U-factor you entered in WindowMaterial:SimpleGlazingSystem. Conversely, if the wind speed is lower than 5.5 m/s, then the actual U-factor can be lower than the entered U-factor.

Please note that Input Output Reference cautions against using WindowMaterial:SimpleGlazingSystem. "There may be significant differences in performance between the simple window system and the user more detailed model."

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Thanks Keigo for your quick and detailed answer. I will look into it, maybe the shape and orientation of my building impact the wind speed which improves the overall U of my fenestration.

mval gravatar image mval  ( 2021-04-09 08:36:12 -0500 )edit

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Asked: 2021-04-07 11:27:21 -0500

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Last updated: Apr 07