In short insulation in your walls or roof slow down the transfer of energy from one side of your home to the other.  High density, thick walls or floors made from stone, earth, concrete etc. have the ability to store heat and depending on the time of year either release it into the house where it is needed or to the outside on cool summer nights.

A properly built thermal mass building works best in climates with high differences in day and night temperatures. That being said, the earthbag rooms that I built at Reflections Camp in Kabak Valley had a wonderful balance of temperature. Much, much more than the uninsulated bungalows that I built on the cheap.

In the winter time the sun is low and your thick walls will collect its heat during the day and as the night cools that heat will emit into your house to keep a pleasant balance. As well, the heat you generate inside your house will be constantly storing its energy in the walls and even as your wood burning stove goes out you will have a pleasant temperature inside for quite some time.

Unfortunately though, materials such as stone, earth and concrete have extremely low insulation values. This means once they have been emptied of the heat that they have stored they will have little means of slowing down the transfer of the remaining heat from the inside to the outside and your house can soon get too cold. To slow that process down I would choose a heavily insulated roof vs. a thick earth roof. That way the remaining heat would have to fight its way out. A great combination would be insulation directly on top of your roof and covered by a thick earth roof. In this way, heat in the summer time will not only store itself in the earth roof instead of penetrating inside but will be further slowed down by the insulation material.

Insulation works in a different way.  In the summer time, thin, uninsulated walls allow heat from outside to transfer inside and causing high electric bills. In the winter time an uninsulated wall or roof will allow all your heat to transfer quickly out of the house again causing high energy bills.

Strawbale houses have high insulation value but low thermal mass. Cordwood houses built from hardwood have low insulation value and wood in itself is not great for thermal mass. Perhaps best when building a cordwood home softwood is a better choice since they are less dense and have a higher insulation value. Earthbag walls and stone walls have very little insulation value but high thermal mass. In the end, with a well insulated roof your choices may not have negative consequences.