Validating THERM results with ASHRAE 90.1

First, I’d like to say great job with the Therm/honeybee interface - and thanks to LBNL for sharing. I watched all the tutorials online and I’m pretty sure I’m doing everything right, but I’m still not able to match ASHRAE 90.1’s U-values for steel frame walls (Table A3.3) and Wood-Frame Walls (Table A3.4). Has anyone tried this before and been successful? Here’s my process:

A few disclaimers first: I work exclusively in SI units (feet, BTU, etc) and I use R-values… even for the whole assembly. It’s easier for my brain to compute.

From the ASHRAE 90.1 User Manual (2007), I found that the “base wall assembly” referenced in these charts consists of (from exterior to interior):

Exterior air film: R-17
Stucco: R-.08
Exterior gyp: R-.56
Frame/cavity: per chart
Interior gyp: R-.56
Interior air film: R-.68

Here’s the ASHRAE chart I tried to replicate

This brought up several questions – I read somewhere that the ASHRAE values come from actual hotbox testing, not math. Was there insulation within the steel stud? What was the conductivity of the steel? How was resistance added? As we know, there are two ways to increase the resistance in the wall:
a. Increase thickness (maintain same R-value/inch)
b. Increase resistance (maintain thickness)

So I tried both options and found that it made a difference in overall R value of the wall, particularly with the metal studs. I left the conductivity of the studs alone at 50 W/mK but did test the two resistance increase methods (mentioned above) and tested scenarios where the metal stud was filled with air instead of insulation. Here’s how I labeled that:

I know all my conversions are right, because I’m able to get the rated R-value that I expected. I found that the empty stud where I added thickness (r-1/in) got closest to ASHRAE values, but it was still R-1 to R-2 ft2hrF/BTU higher than the ASHRAE values. Has anyone had similar experiences? Any wisdom to share? I know that ASHRAE physically tested their assemblies, so there might be some variation there…?

matching ASHRAE materials:

Matching ASHRAE air films:

ASHRAE (686.2 KB)
ASHRAE chart.3dm (490.4 KB)

@Emccormi ,
This is an interesting validation that you have done here. I don’t know too much about the ASHRAE table or the hot box testing but there are a range of things that can cause a difference of R 1-2 (in IP). I assume that you meant to say that all of your values are in Imperial (IP) at the top and not metric (SI). Some of the things that can cause a difference of R-1 include:

  • Small intermittent screws and fasteners that are not in the therm model (there are a number of studies showing that these fasteners typically degrade the R-Value by ~5%).

  • Studs on the bottom and top parts of the cavity that aren’t captured in the 2D plan representation of the THERM model.

  • Differences in the conductivity of the studs. Steel can vary in its conductivity. For example stainless steel has about 1/3 the conductivity of typical carbon steel. The same is particularly true for wood, which can have differences in density and conductivity as a result of the environment in which the tree was grown (even when the same species of tree is used for the wood).