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How to model wood stud (or Wood frame) walls in TRNSYS?

asked Dec 11

a_aymmugan's avatar

updated Dec 16

Hi, I have been stuck for a while to get the proper methodology on modelling wood frame or wood stud (light timber frame) walls in TRNSYS. So these walls have dimensioned studs at specified spacing with cavity insulation within them. So the wood stud layer has two materials in it. How can we model such a layer/wall in TRNSYS. Because in TRNSYS, you only get to input one value for specific heat capacity, density and conductivity.... Hence what should I do to model this sort of a wood frame wall where i need to consider the thermal capacitance of wood, U value of wood and the U value of cavity insulation as well.

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answered Dec 17

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.


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Thank you for the in-detailed response.

a_aymmugan's avatar a_aymmugan  ( Dec 17 )

Let us know if you're eventually successful in applying the suggested approach with TRNSYS. If so, it's customary to mark the answer as correct (click on the checkmark). Helps future UMH users.

Denis Bourgeois's avatar Denis Bourgeois  ( Dec 18 )

I used the weighted volume average for the density, specific heat capacity and effective R method for resistance. I was able undertake the simulation using those two methods. Thank you very much!

a_aymmugan's avatar a_aymmugan  ( Dec 18 )

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Asked: Dec 11

Seen: 135 times

Last updated: Dec 18 '24