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On the Envelope tab/Attic sub-tab, use the “|| Path” button (located under the Cavity Insulation control) to open the Parallel Path pop-up. There is a checkbox under both the Base and Improved Layers section. Check the checkbox to indicate a radiant barrier is present as part of the roofing layers.
Are you assuming that the radiant barrier is not continuous? If it is continuous, it would not be in parallel with other insulation but would be in series with other insulation. I guess if you are building a model of parallel paths and you have a continuous radiant barrier you would need to add the radiant barrier to each of the parallel paths.
The location of the radiant barrier control on the || Path popup, does not indicate that it is modeled in parallel with the insulation. Most of the components specified on that popup are components that are part of each path in the parallel path calculator and thus run in series. The only things that vary across the three parallel paths are the cavity insulation and joists.
We model attic using a method published in the ASHRAE Heating and Cooling Load Calculation Manual. This method generates an effective thermal resistance of the attic space and roof assembly, which is then modeled along with other ceiling components and the insulation as a single envelope component. This contrasts to the method of modeling the attic as a separate zone. The ASHREAE method relies on resistance numbers generated by ASHRAE for different combinations of ceiling R-value, presence of reflective surface, ventilation rate, and roof absorptance. The method does not specify a particular installation method for the radiant barrier.
I read your response and still need to understand how one models radiant barrier. Weather integrated on the roof sheathing or stapled across the rafters.
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