Calculating U-factors the DOB-Friendly Way

Calculating U-factors the DOB-Friendly Way

By Alex Rippere

As a building code consultant, the most maddening energy code objection I’ve encountered in the past two years pertains to establishing a proposed exterior wall’s maximum thermal conductivity, aka the U-factor (lower=better).

For example, an above-grade mass wall needs to meet a U-factor of no more than 0.104 for commercial buildings and 0.090 for multifamily residential buildings over three stories. The code also provides a simpler method of establishing compliance, based on the minimum resistivity (R-value, where higher is better) of continuous insulation—in addition to any between-stud cavity insulation.

Why does this matter?

Calculating DOB U-factors is actually crucial and miscalculation of U-factors will cause massive headache and severe delay. Few things are worse for architects than learning that they need to thicken the exterior walls and redraw all sheets right before full New Building approval simply because they never provided enough insulation in the first place. Doubly so if the project depends on “zone green” exterior wall thickness zoning floor area deductions, where zoning and foundation approval can hinge upon energy code approval. So what is the most painless way to calculate a wall’s U-factor?

Option 1: COMcheck

For the most part, nearly every required U-factor can be calculated in COMcheck for common wall types. COMchecks only get complicated when DOB examiners make you de-rate continuous insulation values to account for things like panel clips. COMcheck jobs no longer need separate thermal wall details to establish R and U values, only clear details on the R-value of provided insulation and identification of insulation as cavity (between studs) or continuous. For new buildings and major alterations, COMcheck requirements generally can be limited to exterior wall details and elevation diagrams showing wall area.

But what if our job is an energy model and the modeling software doesn’t include U-factor provisions for generic wall types?

Option 2: ASHRAE Appendix A3

Nearly every U-factor that COMcheck provides has an equivalent in Appendices A1-A9 of ASHRAE Standard 90.1. Appendix A3 is the go-to resource for calculating above-grade wall U-factors. Even better, simple tweaks will bring nearly any proposed block, concrete, or stud wall in line with the corresponding wall types in Appendix A3. For example, a typical 8” thick, fully grouted solid block wall with 2” of rigid foam insulation and resistivity of R-10, would have a U-factor of 0.085, per table A3.1A. What about 12” or 6” block, as designed? Most DOB energy examiners will allow the 8” block as an equivalent in our analysis.

Option 3: THERM Analysis

What happens if our proposed wall type just doesn’t have an equivalent in COMcheck or ASHRAE Appendix A3? Per the letter of the law, we could deploy something like fantastically complicated “2D zone calculations” for mass wall assemblies containing metal. For simplicity’s sake, however, DOB prefers THERM software computer outputs for any analysis that accounts for heat flow across a wall assembly, not simply through the wall assembly, such as those calculated under the parallel path method. This makes THERM analysis practically mandatory for projects with complex curtain wall assemblies that have insulated spandrel panels or “shadow boxes.” THERM is a free program provided by Lawrence Berkeley National Laboratory, and Metropolis Group can perform THERM analysis of a wall assembly in accordance with DOB and National Fenestration Rating Council-approved guidelines.

Option 4: Hybrid Calculation

One can also combine THERM analysis with other methods. For example, our 8” block wall with 2” rigid foam insulation gets much more complex if we break up our foam with continuous steel channels to support a curtain wall system. In this case, we can use THERM analysis on the curtain wall, channels, and insulation to grab an R-value for that part of the assembly. Then, we treat that THERM-derived R-value as a continuous R-value in COMcheck for an 8” block mass wall. If we clearly show examiners how we obtain the numbers and provide all supporting documents, they should approve us.

OK (to Approve) Computer

The law (ASHRAE 90.1 sections A9.2 and A1.2, which also can apply under NYCECC) gives DOB examiners great latitude in judging a wall’s thermal performance. The industry can rely too heavily on the “sum-of-R” parallel path calculations, which offer many granular items to bog down DOB examiners. An examiner can even invoke section A9.2(b), which forbids “sum-of-R” calculations in all but a handful of instances, as ASHRAE frowns on over-simplified analysis when using highly conductive metal in a wall assembly.

To make the examiner’s evaluation easier, we can have a computer calculate our U-value (COMcheck, THERM) or choose tabular values from ASHRAE Appendix A3, consulting Appendix A9 only when required. Doing the U-factor math ourselves invites too much complexity and confusion for all involved.