To find out if you have sufficient insulation in the attic, measure the thickness of the insulation. If it's less than an equivalent of the R-30 (about 10 to 13 inches), you could probably benefit from adding more. Before insulating, seal any air leaks and make necessary roof repairs and other necessary repairs. If you are in a conditioned part of the house, remember to also insulate and air-seal the access to the attic.
Properly insulating cathedral-type ceilings will allow the roof temperature to be kept closer to room temperature, providing a uniform temperature distribution throughout the house. Cathedral-like roofs must provide space between the roof cover and the roof of the house for adequate insulation and ventilation. This can be achieved by using lattice beams, scissor truss frames or sufficiently large beams. For example, cathedral-type ceilings constructed with 2x12 beams have space for standard 10-inch slats (R-30) and ventilation.
Unventilated cathedral-type ceilings (warm roof design) are also an option. The thermal roof design makes it possible to install more insulation in the roof cavity, since the need for a ventilation space is eliminated. It is important that the roof cavity is fully sealed with respect to the lower conditioned space to prevent moisture ingress and roof degradation. Exterior walls need insulation for the simple reason that they are the barrier between the inside and outside of your home.
In winter, the heat seeps through the walls and, during the summer, the heat returns to the walls. Adequate wall insulation helps prevent this cycle of heat loss and gain. Options include battered fiberglass and blowing options, both effective if installed correctly without leaving gaps. If you have vertical walls with the attic behind them, you should also insulate them.
In addition, if you are remodeling or building a new home, make sure that the attic deck is raised above the roof beams to provide sufficient insulation space. A common way to insulate low, unventilated spaces is to seal the area and insulate the foundation walls rather than the floor between the house and the mezzanine. If you have or will have an unventilated mezzanine, it's best to seal and insulate the foundation walls rather than the floor between the mezzanine and the house. Even in a house with an unconditioned basement, the basement is more connected to other living spaces than to the outside, making basement wall insulation preferable to roof insulation.