Hire an insulation company to install closed cell foam in all the exterior stud and rafter bays. With 2×4 studs and 2×6 rafters, the R-values of the walls and roof will be 24 and 38, respectively, compared to the recommended values illustrated in Figure 11. In 2021, the author received a quote of $6,000 for this service on the house described in Austin, Texas.
Figure 11. Insulation Guidelines
If you insist on shaving every dime possible off the cost of this house, you can install your own fiberglass insulation (covering your skin and face of course). If you take this alternative, make sure to install 2 inches of rigid foam tightly underneath the roof deck to avoid condensation in the winter.
Install rock wool insulation for sound deadening in the interior walls if desired. There is no need to fasten this material to the studs, just make sure not to leave any voids in the stud bays.