Highly conductive materials are the best way to store heat for use in thermal energy. 'Thermal mass' describes a material's capacity to absorb, store and release heat. Dense materials like water, concrete, stone, or tile have a high capacity to store heat and are referred to as 'high thermal mass' materials.
The materials that make up the floors, walls, and other parts of your greenhouse will determine how well it absorbs, stores, and transfers heat. Choose high quality materials with high insulating values, use double walls with an air gap between, and perhaps utilize a greenhouse cover in order to further trap heat within.
