HPXML generated surface constructions using <AssemblyEffectiveRValue> appear to be handling interior and exterior air films incorrectly

[chrisbalbach]

HPXML to OS Workflow Version: v1.9.1

Issue Description

When specifying Wall, Roof, Rim Joist and Floor/Ceiling surfaces of a home, we are using the HPXML AssemblyEffectiveRValue Element. We believe the "HPXML to OS workflow" methods are incorrectly applying values for surface interior and exterior film coefficients for user entered AssemblyEffectiveRValue data.

We believe it is not appropriate for the "HPXML to OS" workflow to apply assumptions for interior and exterior air films in the transformation of an Assembly R value into EnergyPlus material and construction elements, since EnergyPlus handles these air films separately.

The impacts of 'double counting" these relatively small interior and exterior air films are small (for fully insulated assemblies). For completely 'uninsulated' assemblies such as an uninsulated 2x4 stud exterior above grade wall (we still have lots of those in New York State!) we believe the relative impact is not negligible. Perhaps code elsewhere in the "HPXML to OS workflow" is accounting for this "double counting" elsewhere, but we were unable to determine where.

Expected Behavior

We believe that constructions defined within EnergyPlus should not have interior and exterior film coefficients consider as part of their overall thermal performance descriptions. EnergyPlus documentation describes this expectation here, under "Field: Outside Layer":
https://bigladdersoftware.com/epx/docs/24-1/input-output-reference/group-surface-construction-elements.html#construction-000

The behavior of surface interior film coefficients are set by EnergyPlus, by defining this object:
https://bigladdersoftware.com/epx/docs/24-1/input-output-reference/group-simulation-parameters.html#surfaceconvectionalgorithminside

The behavior of surface exterior film coefficients are set by defining this object:
https://bigladdersoftware.com/epx/docs/24-1/input-output-reference/group-simulation-parameters.html#surfaceconvectionalgorithmoutside

Both of these OpenStudio/EnergyPlus objects have defaults, but they do not appear to be able to be disabled.

Observed Behavior

Reviewing of how these constructions are created (in an attempt to 'back put' a cavity R-value but not the R-value of the full cavity heat loss path), we notice several methods that apply assumptions for both interior and exterior film coefficients when transforming AssemblyEffectiveRValue into EnergyPlus materials and constructions.

The value of these air film assumptions and are how they are used are well documented in the "materials.rb" resource file and called many times from both the "constructions.rb" and "measure.rb" files.

Recommended Changes

Update code to remove references to interior and exterior air films contained within "Materials.rb" from related methods in "measure.rb" and "constructions.rb".
Update "HPXML to OS" and "OpenStudio-ERI" workflow documentation to reflect the changes.
NOTE: This recommended change may impact ASHRAE 140 test comparison results.

[shorowit]

@chrisbalbach As you point out, EnergyPlus constructions should not have interior/exterior air films. Since the HPXML AssemblyEffectiveRValue does include air films, we have to subtract them before creating the EnergyPlus construction. Which part of this do you object to?