The R factor, or resistance to the transfer of temperature factor, is approximately 1 for every 10 cm (3.93 inches) of soil. A typical Green Magic Home has an average of 60 cm (23.62 inches) between walls and cover, which gives an R factor of 6. But it can be increased up to an R factor of 50 by first applying a layer of 7.5 inches of closed-cell polyurethane foam, whose R factor is 6 per inch – see SPRAY FOAM – and then covering it with 20 cm (7.5 inches) of soil, grass, and natural gardens.