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?



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…” )


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.


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Component description fixed:


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@chris: just for clarification, the 90.1 window maximum assembly U-values published already include interior and exterior surface films?

I’m confused a bit because when decomposing the window construction “ASHRAE 90.1-2010 EXTWINDOW METAL CLIMATEZONE 4-6” (for example) built into the EPConstructions Honeybee library, the IP U-value shows 0.55 (which aligns with 90.1-2010), however if the air films are included in this U-value, shouldn’t this decomposed U-value be higher as EP is adding in the air films? If I pass this 0.55 decomposed U-value into the AirFilm component with Exterior windows surface selected, I get a 0.359 U-value output with airfilms…is this double counting window air films when using this 90.1-2010 window library or did I miss a step along the way? any insight appreciated! thanks!

Your confusion is well-merited @josh.greenfield and it is entirely the fault of the “Honeybee_Decompose EP Construction” component. The R-values and U-values that come out of that component are inaccurate for any window construction or material.

I think the full history of it is that @mostapha originally got that component to work well for all opaque constructions but never implemented it correctly for window constructions. I tried to implement it for window constructions at one point but calculating U-values for air gaps between window panes is complex and it was going to be a nightmare to implement it given how messy our code is organized in honeybee-legacy. I have code that calculates the window U-value correctly in the honeybee_energy library now for the upcoming [+] release and I think getting it to work correctly in legacy might be a lost cause at this point. Several times, I have wanted to just remove the R-value and U-value inputs but I know it’s tough to remove functionality that is working correctly for opaque constructions. If you have an opinion on the matter, I am all ears.

But the simple answer is don’t trust the U-values out of the Decompose components whenever it’s a window construction or material. The truth is that the U-value of 0.55 is actually inclusive of some standardized air films that E+ strips out when it interprets simple glazing system materials.

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