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Title 24 vs 90.1 U-factors for metal frame walls

asked 2017-10-03 12:37:17 -0500

updated 2017-10-30 12:22:24 -0500

So this has been driving me nuts for a long time, and I thought I'd ping this community for feedback.

Title 24 (part 6), Reference Appendices, JA4 provides lookup tables for the performance properties of envelope construction assemblies. They are supposedly based on ASHRAE calculation methodologies. However, they tend to be significantly different from the values provided in ASHRAE 90.1's Appendix A.

For a specific example that comes up all the time, consider a 2"x6" metal stud wall, 16" on center with R-19 batt insulation in the cavity. Per 2016 T24 Appendix JA4, Table 4.3.3 - U-factor = 0.183 Per Standard 90.1-2016, Table A3.3.3.1 - U-factor = 0.109

That's a huge difference! What makes it worse is that this is the go-to wall type for 90% of the nonresidential projects I see in Southern California. However, Title 24-2016 has mandatory requirements for metal frame walls that say U-0.151 max, which means the construction assembly is "illegal." And even worse, when I'm providing energy modeling services for LEED and tell the architect the U-factor of their wall is U-0.109, and their mechanical engineer providing Title 24 modeling says "nope, U-0.183." (Although honestly, I feel like it's not being mentioned as continuous insulation is still not a thing here, even though Title 24 has essentially required it since 2014. But I digress...)

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answered 2017-10-03 12:57:20 -0500

Looks like different assumptions went into the assembly analysis. From the Title 24 Reference Appendices:

Assumptions: Values in this table were calculated using the zone calculation method. The construction assembly assumes an exterior air film of R-0.17, a 7/8 inch layer of stucco of R-0.18, building paper of R- 0.06 (BP01), continuous insulation (if any), the insulation / framing layer, 1/2 inch gypsum of R-0.45 gypsum board (GP01), and an interior air film 0.68. The steel framing is assumed to be 0.0747 inch thick with a 15 percent knock out. The framing factor is assumed to be 25 percent for 16 inch stud spacing and 22 percent for 24 inch spacing. The EZFrame internal default framing percentages are 15 percent for 16 inch stud spacing and 12 percent for 24 inch spacing. To account for the increased wall framing percentage the frame spacing input to the EZ Frame program is reduced to 13.218 inches for 16 inch stud spacing and 15.231 inches for 24 inch stud spacing. Actual cavity depth is 3.5 inch for 2x4, 5.5 inch for 2x6, 7.25 inch for 2x8, 9.25 inch for 2x10, and 11.25 inch for 2x12. High density R-30 insulation is assumed to be 8.5 inch thick batt and R-38 is assumed to be 10.5 inch thick. The thickness of the stucco is assumed to be reduced to 3/8 inch when continuous insulation is applied.

And from ASHRAE 2016 Appendix A:

For the purpose of Section A1.2, the base assembly is a wall where the insulation is installed within the cavity of the steel stud framing but where there is not a metal exterior surface-spanning member. The steel stud framing is a minimum uncoated thickness of 0.043 in. for 18 gage or 0.054 in. for 16 gage. The U-factors include R-0.17 for exterior air film, R-0.08 for stucco, R-0.56 for 0.625 in. gypsum board on the exterior, R0.56 for 0.625 in. gypsum board on the interior, and R-0.68 for interior vertical surfaces air film. The performance of the insulation/framing layer is calculated using the values in Table A9.2-2. Additional assemblies include continuous insulation uncompressed and uninterrupted by framing. U-factors are provided for the following configurations: a. Standard framing: Steel stud framing at 16 in. on center with cavities filled with 16 in. wide insulation for both 3.5 in. deep and 6.0 in. deep wall cavities. b. Advanced framing: Steel stud framing at 24 in. on center with cavities filled with 24 in. wide insulation for both 3.5 in.deep and 6.0 in. deep wall cavities.

I would review the assumptions above against your design assembly, and determine which is more appropriate. Or, perform the calculation for your assembly yourself (following ASHRAE section A9.4 or the Title 24 equivalent) to justify the value used to your code official/architect/engineer.

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Great insight, Eric. I should have read the fine print..

I'd love to get my hands on this "EZ Frame" tool. I tried calling CEC's energy hotline phone number, but they said the tool was integrated into the CBECC tools.. If you look at CBECC-Com, you can create a "composite" layer that includes the framing and cavity insulation, but the configuration options are very limited. For a 2x6 metal framed wall, you get to pick between 16 and 24" on center, and have to choose a 5.5 inch web depth, when a 6.0 inch web is typical in speaking with a couple architects. Just one assumption of many...

Greg Collins gravatar imageGreg Collins ( 2017-10-03 15:32:58 -0500 )edit
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answered 2017-10-03 14:58:03 -0500

ASHRAE 90.1 uses the series/parallel calculation method to determine the insulation value of wall assemblies which takes the weighted area average of heat flux through the frame and insulation sections. This method works fine for most wall assemblies but in regards to calculating walls with steel framing, this is not the recommended way. This is because the metal frame has a significantly different thermal resistance compared the cavity insulation so the frame section will also reduce the effectiveness of the cavity insulation touching the frame.

The California Energy Commission (CEC) funded their own research that lead to the development of Joint Appendix 4. I unfortunately couldn't find the results of this research but from what I've heard, they did actual testing of various wall assemblies as well as THERM models. Based the research, JA4 uses the modified zone method of calculation which divides the insulation section to multiple sections: the insulation in between the frames and insulation adjacent to metal frame. Otherwise, the calculation is similar to the parallel path method, just with more parallel paths. This same method is used to calculate the U-value of windows which is why NFRC rated windows have a assembly U-value and a center of glass U-value.

ASHRAE needs to update Appendix A since it is using an inappropriate calculation method for metal framed constructions. Title 24 JA4 is much more realistic in terms of actual performance of the wall, so it is recommended to use those values instead of the ASHRAE values.

If you want to learn more about the differences in parallel path method and zone method, I'd recommend reading about it in the ASHRAE Handbook of Fundamentals Chapter 27 which goes over the different methods as well as why parallel path method is wrong for metal frames.

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Nice background on the T24 methods. I'll note that ASHRAE 90.1-2016 Section A9.2 'Require Procedures' states that for steel-framed walls, the parallel-path method is appropriate when using the insulation/framing layer adjustment factors from Table A9.2-2 (which uses significantly derated 'effective' R-values for insulation installed between steel framing). While the HOF does state that the modified zone method is recommended, it also cites the use of the 90.1 adjustment factors for calculating U-value for constructions containing metal.

Eric Ringold gravatar imageEric Ringold ( 2017-10-03 15:27:04 -0500 )edit

This makes a lot more sense. Thanks, Ken.

I have reviewed chapter 27 in the past, and actually had a handy MZM calculator on ORNL's website, but my bookmark no longer works and I can't find it searching their website.

I'd definitely be interested to find that CEC-funded research you mention, if you happen to come across it.

Greg Collins gravatar imageGreg Collins ( 2017-10-03 15:37:24 -0500 )edit

Also (from 2013 HOF, pg 27.5) for a given steel-framed construction, the assembly R-values determined from measurement (R-6.61), modified zone method (R-6.73), THERM simulation (R-5.63) and ASHRAE parallel-path with adjustment factors (R-6.40) are all pretty similar (with THERM being the most notable outlier). Based on this, I would need to see more evidence before deeming the ASHRAE method inappropriate.

Eric Ringold gravatar imageEric Ringold ( 2017-10-03 15:40:00 -0500 )edit

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Asked: 2017-10-03 12:37:17 -0500

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Last updated: Oct 03 '17