Comparing U-Value in Ashrae 2019 (SI) and LBT 1.3.0

Hi dear community

I was adapting the U-value factor in the same Ashrae zone for non-residential buildings for walls, roof, floor, and the window between Ashrae 90.1 2019 and what LBT 1.3.0 with climate zone, vintage, and mass inputs give us.
The results were not completely the same, I hope @chris could help me with this.

Best,
Nariman

Hi @Nariman.Rafati ,

The values of the Honeybee constructions look good to me. ASHRAE 90.1 and IECC just say that the U-value of the assembly can’t be more than those specified values and all of the Honeybee constructions look like they are just under.

I should also say that hitting the building code U-value exactly can come with a big risk of not complying with the code since there can be different ways to compute the resistance of air films and other properties that could change the U-value slightly. So you want at least a little tolerance between the value specified in the code and the value you are using in your design.

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Thanks @chris. You clarified it very well.

Hi!

Thanks @chris for clarifying that. I had the same question @Nariman.Rafati had and the answer makes absolute sense.

However, I noticed that in the case of fenestration the values for “operable” and “window” are above the ASHRAE 90.1-2019 and also 2016 (didn’t check previous versions) maximum values. I’m sure there’s something I’m missing, but would be really grateful if anyone could shed some light on this.

Thanks,
Ines

Hi @ines.idpe ,

Thank you for catching this. It seems like the committees at ASHRAE made a relatively big change in ASHRAE 90.1 2019 where they did away with the distinction between fixed windows and curtain walls and this was causing us to pull the incorrect window construction from the openstudio-standards package (from which we derive all of the construction sets that ship with Ladybug Tools).

I just pushed an updated version of the honeybee-energy-standards package, which includes the corrected window constructions:

You should be able to get the updated standards within an hour or so by using the LB Versioner. Then, you should see that the U-values for windows are closer to what you expect:

I’ll also note that the U-values being a hair higher than the ASHRAE value is ok. The way that we calculate U-Values in the honeybee library tries to mimic what EnergyPlus does and this is different than the NFRC standard that’s typically meant to be used for code compliance. But you were right to point out the relatively big difference that I just fixed. Thanks again for reporting it.

From what I can tell, this was the only change like this that was made in 2019 and all constructions other than fixed + operable windows should be correct in the last stable release of LBT.

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Oh, I see now the distinction between glazed surfaces in ASHRAE.
It now reads the right values, thanks so much for fixing and clarifying @chris!!!