@SaeranVasanthakumar - please accept my apologies for the delay in responding.
Some 2 years ago we were commissioned to examine the % of a wall that was actually, rather than theoretically, timber. We were also interested in whether this was driven by on-site fixes to installation or standard construction practice. The general guide was that timber was 20% in the wall. Our analysis was for the buildings that we could get access to was some walls were up to 45% timber! This was due to internal wall to external wall connections; lintels over doors and windows; and other normal construction techniques in timber frame walls. We also looked at what software was available to model these kinds of construction issues. What concerned us is that many of these are not well dealt with by simulation programs that model buildings room by room, rather than one single space. What worries me is the idea of smearing an equivalent R value across a timber + insulation construction. We are aware that modelling the heat capacity of the timber in the wall is important. Smearing that across the wall concerns me immensely. I would love to be able to model temperatures on the inside of these structural components compared to the adjacent insulated parts of the wall.
I can make the report available. What was most intriguing from this study was that we demonstrated that simplistic heat loss calculations are insufficient: we found, counter intuitively, that if you operated the windows and heating and cooling of a house in the manner that we do typically in New Zealand then a wall with lower R value because of a greater than expected proportion of timber (40% instead of 20%) actually performed better! Actual performance contradicting the expected performance based on the simplistic physics of a Heat Loss calculation.