EPWindowMat U Value

Dear All,

I'd appreciate it if you could reply the following questions:

1- A building code requires: Thermal Transmittance (Summer U value) U= 1.9 W/m2K (max) for glazing elements performance.

I'm wondering what number to assign to the "_U_Value" input of the EPWindowMat component. The Honeybee hint tells that it should represent the conductivity of the window in W/mK, and it refers to NFRC's U Factor under winter conditions.

On the other hand here NFRC defines: "U-factor indicates how much energy will be lost from a building through its windows by specifying how many BTUs can pass through one square foot of material in an hour."

I'm a bit confused what unit and what number to assign.

2- By knowing a Summer U Value, is it possible to estimate the Winter one?

Best,

Attachments:

UValue.jpg, 98 KB

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Replies to This Discussion

Permalink Reply by Aryan Shahabian on August 7, 2017 at 11:56am

Well.. I guess I found the answer for my first question:
See here : The U factor Field.. the unit is W/m2K.

So if I'm not wrong, it would be great if Mostapha or other developers of Honeybee could correct the component hint for next versions..

(Also, it seems that there is a small mistake in the last part of the mouse hovering hint of the other component, "EPNoMassMat" where it says: "if you want to create a material..." )

Permalink Reply by Chris Mackey on August 12, 2017 at 4:53am

Aryan,

Thanks for finding this issue. It looks like one of us wrote that description a long time ago and forgot to fix it. I'll update it now and you are correct that the input is supposed to be SI U-Value (W/m2K). I should also add that the input U-Value is supposed to include the resistance of air films on either side of the window as defined by NFRC.

Generally speaking, when people refer generically to "U-Value" they mean the winter U-Value because the U-Value typically has a much larger impact on heating energy use than cooling energy use. So I would use the winter U-Value if you have it but, otherwise, the summer U-Value is really not that different from the winter one (typically a 3% difference in my experience) and you can just use the summer one.

Finally, if you need a really accurate simulation and have the exact manufacturer spec for your glazing in LBNL WINDOW, you can use the "Honeybee_Import WINDOW IDF report component to use this in your simulations.

-Chris

Permalink Reply by Chris Mackey on August 12, 2017 at 5:00am

Component description fixed:

https://github.com/mostaphaRoudsari/honeybee/commit/8d42cb545a69f7a...

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