The un-sexy but important part of timber framing - How do you enclose  the frame ? I've worked enough with structural insulated panels to understand both their pluses ( speed, great insulation ) and their  minuses ( cost and utility installation issues ). This is an effort to  develop a site built panel system that is all on the plus side of the  ledger. We've wrapped the frame in a very lightly built 2x4 stud wall - this gives us space to install the electrical, water, waste and vent pipes without interrupting the insulation. The 2x4 frame has been furred out from the timber frame 5/8" so that we can slip Sheetrock between  the timber frame and stud wall after all the utilities have been  installed. The next photo shows the outside of this system - The walls have been wrapped with a continuous layer of 5 1/2" thick rigid  insulation, 11" thick on the ceiling. This 600 square foot cottage  should have a heating and cooling bill about as close to zero as you can get in our northern Vermont climate for not much more money  then a traditional (and not even half as efficient ) 2x6 stud wall with fiberglass insulation.