The cabin is designed to let in as much natural sunlight as possible in winter to heat up the concrete mass. In summertime, warm outdoor air is kept out, but the cabin remains visually connected to the natural elements through the generously proportioned windows.
The interior walls made of solid concrete create a large, effective thermal mass. Excess heat from the fireplace can be pumped into the pipes in the concrete to store this heat. The walls then slowly release this heat over time.