Just a narrow air cavity between two exterior walls that captures solar heat and circulates it through the house naturally.
This design is known as the double-envelope house, and it was heavily studied during the 1970s energy crisis when fuel shortages forced engineers to rethink how buildings used energy. Researchers at universities and national laboratories discovered that a simple air space between two walls could act as a solar air collector, warming air to over 120°F (50°C) on sunny winter days.
The heated air rises through vents into the house, while cooler indoor air is pulled back into the wall cavity below. This natural airflow cycle — called thermosiphon circulation — moves heat through the home without fans, electricity, or mechanical systems.
Combined with thermal mass like concrete floors or brick walls that store heat during the day and release it at night, these homes were able to reduce heating demand by 40–60% in many climates.
Credit to : The Survival Lab