We're working on a 20 year old timber frame/stress skin house. TF Built by world famous timber framer, panels by Winter, installed by excellent installer who is still a top installer. The issue was that we were seeing ridges under the shingles and heat loss, visible by snow melting, at the roof panel joists. We added two layers 2" Polyiso, 2x4 strapping and 2nd layer of sheathing. (a cold roof is recommened by most SS manufacturers but rarely done). We noticed from the interior that screwing the 2x4s to the oak frame through the polyiso and stress skin drew the stress skin much tighter to the beams. It is obvious to us that as the timber frame moves throughout the year (drying/expanding /contracting) it either loosens the nails attaching the stress skin to the frame - or they are loosened by the beams drying, cracks the foam installed at the joints - and opens up the panel joints. Again this is a long term issue; there were no observed problems with the stress skin or with the initial installation. Using screws to attach the skin may have helped. Installing a vented roof would have helped the breathability but not the panel joint issues. Wondering if this is an issue we'll hear more about in coming decades. Bob I