Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
Haven't used TRNSYS in a long time, but I'd wager it's (still) very similar to other apps (e.g. EnergyPlus, DOE-2.2, ESPr) when it comes to multilayered constructions (holding one or more composite insulation layers).
When it comes to wood-framed constructions, ASHRAE Fundamentals does recognize the suitability of straightforward parallel path and isothermal plane methods to calculate the effective R-value of a composite insulation layer (wood framing + batt insulation). With e.g. 2x6 studs, that's a 140mm cavity thickness. Therefore, the effective composite insulation layer thermal conductivity: k = 0.140 / effective layer R. Determining framing factors (or weighing factors), between framing vs insulation, may be tricky depending if you're strictly interested in clear field or overall R-values. Check out related assumptions (including weighing factors) in ASHRAE 90.1 Appendix A for "Wood-Framed Walls". Finally, I'd simply do a volume-weighted average of (wood framing vs insulation) specific heat and density for the composite layer.
This will likely not do with more sophisticated assemblies, e.g. integrating more recent, thermally-broken cladding clips and outboard insulation. The suggested approach would be to rely on pre-simulated wood-framed assemblies, based on the ASHRAE RP-1365 methodologies. The simplest would be to consult thermalenvelope.ca, pick "wood-framed and timber" construction type, and select the assembly that is closest to the wall assembly you're looking into. Each assembly (here, Detail 8.1.19) holds both material data (input) and thermal data (output) sheets. Less straightforward, but one should be able to reasonably determine effective composite insulation layer thermal conductivities.
One important caveat is that the 2nd method above only generates effective clear-field R-values, which do not include the effects of sill plates, fenestration headers, etc. To this end, one should further derate a composite insulation layer conductivity by applying appropriate PSI-factors, such as along rimjoist/floor intersections.
