ASHRAE 90.1 Wall Construction

old forum
1

Hi,

I got a question relate to ASHRAE 90.1-2010 Wall Construction from the EP Construction Library. I found out that the U-value from the EP Library does not match with ASHRAE 90.1-2010 exactly (for example, for Climate zone 1 Extmass Wall, EP Library offers U=0.649, whereas ASHRAE requires max 0.580 using IP Units).

The other question is that is the library updated to ASHRAE 20.1-2013?

Thanks!

Best,

Rufei

2

Hi Rufei,

Thanks for reporting this. The libraries are from OpenStudio standards.

https://github.com/NREL/openstudio-standards/blob/master/openstudio…

You can check the details in GoogleSheets. Let us know if we’re making any mistakes.

Do you mean ASHRAE 90.1-2013? Here are the standards that are available. We can update the libraries for the next release.

Mostapha

3

Hi Rufei,

The reason why the OpenStudio wall constructions do not have the exact values as code is because the code insulation requirements refer to R-values without thermal bridges. In the real world, you usually have countless structural members that will cross your insulation like wood or steel studs, steel bolts, etc. These small members will compromise the R-value of your wall construction but, for the last few decades, it has been difficult to quantify exactly how much they compromise it (now we have LBNL THERM to help us with this). Accordingly, the codes have been written to refer to only the R-Value of the insulation and not the overall R-Value of the construction. Of course, when you run an energy simulation, you want to account for these thermal bridges somehow so that you get an accurate energy use forecast and this is why OpenStudio has made a library of constructions organized by climate zone but all a little but worse than the code requirements for those zones. I would assume that the R-values of these constructions come from constructions that were physically tested or ones that were modeled in a very detailed way in THERM. I would check the OpenStudio documentation if you are looking for the exact source.

-Chris

4

Hi Chris,

Thanks for your detailed explanation, that makes a lot of sense. Now I understand why the EP Construction has a higher U-value than the code. But I got one question for ASHRAE 90.1-2010 Extwall Mass Climate zone 7-8, the U-value is 0.054, but the code is 0.071, should I change R-16.84 to R-11.2 in order to make a U-value of 0.075?

Best,
Rufei