[Trace-users] Metal framing in concrete block wall (how to model correctly for LEED)

Tue Oct 21 12:30:56 PDT 2014

Catalina,

My interpretation is that the correct ASHRAE 90.1 table for your wall is Table A3.1C. An 8” CB wall, density 115 lb/ft3, with insulated, unreinforced cells (none are grouted) gives U-0.21. The R-7.5 that is highlighted in Table A3.1A below is for continuous insulation (per A3.1.2.2), which you do not have. Per A3.1.1, the U-factors in the tables already include R-0.17 and R-0.68 for exterior and interior wall films. Potentially, you could add R-0.56 for the gyp wall to Ru-4.78 for the Table A3.1C block wall assembly, and get Reff-5.34, for an assembly U-0.187. Your TRACE wall construction shown below needs to be modified by changing R-8 CMU to R-3.672 (subtracting your surface resistances = 4.78-.333-.685) and you would have to justify the air gap value or delete it as well.

Regards,
Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

[cid:image001.jpg at 01CFED44.013A7100]  [cid:image002.jpg at 01CFED44.013A7100]

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From: Trace-users [mailto:trace-users-bounces at lists.onebuilding.org] On Behalf Of Caballero, Catalina
Sent: Tuesday, October 21, 2014 11:32 AM
To: 'trace-users at lists.onebuilding.org'
Subject: [Trace-users] Metal framing in concrete block wall (how to model correctly for LEED)

Good morning experts,

I have a question, we have received a comment from a LEED reviewer requesting for the following information:

Table 1.4 of the template appears to indicate that all the assembly U-value for the exterior walls in the Proposed model may not account for the thermal breaks due to the steel-framing portion of the construction assembly. Each construction assembly U-value is not a direct inverse of the insulation R-value for that construction assembly. The R-value of the insulation located between the steel framing must be de-rated when determining the assembly U-value for the exterior wall assembly to account for the reduced thermal resistance of the metal framing components. Revise the Proposed model, as needed, so that all components of the exterior wall construction assembly are accounted for when calculating the assembly U-value for assembly type in the actual design. In addition, update Table 1.4 reflecting the changes. Refer to Table A3.3 in ASHRAE Standard 90.1-2004 for additional guidance regarding how to de-rate the R-value for insulation located between steel framing assemblies.

We have the following wall composition in the project:
[cid:image014.jpg at 01CFED40.CA9891C0]

And we are using the following assembly value: U-0.125 (= R8 Assembly) based on table A3.1A .
[cid:image016.jpg at 01CFED40.CA9891C0]
[cid:image017.jpg at 01CFED40.CA9891C0]

However since this table in my understanding is only accounting for the concrete block, the insulation and the metal framing, I have added the 5/8” gypsum board (layer 1) and the 5/8” hardie board (layer 7) to the modeled assembly as below.

[cid:image018.jpg at 01CFED40.CA9891C0]

Do you think the reviewer would object in any of this process? Or do you consider it should be done differently?

Thank you for your answers, greatly appreciated.

Catalina Caballero.  AIA. Assoc., LEED GA.
Sustainability Coordinator

Johnson, Avedano, Lopez, Rodriguez & Walewski Engineering Group, Inc.
Engineering for High Performance Buildings.
MEPF - BIM - LEED - Cx

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