Designing a house that meets Passivhaus standards is made more difficult when the house is small. A small house has more surface area per cubic foot of living space than a large one. Thick concrete walls aren't the most efficient way to achieve thermal mass in a house of less than 130 square feet.
Thomas Primault, the designer of Mizu, a Japanese-style timber structure in Bretagne, France, chose another option: 6 mm of Enerciel wall coating impregnated with phase change material. The biobased PCM has a melt point of 23° Celsius, absorbing heat when the temperature inside the tiny office structure climbs above 23, and releasing heat when the temperature falls below 23. The building envelope also includes vacuum insulated panels in the floor and mineral fiber insulation in the ceiling and walls.
TreeHugger's Lloyd Alter describes the result, a tiny house that meets Passivhaus standards, as "a stunning little gem." Blogger Elrond Burrell writes that monitoring data and experience indicate that the building is performing very well, but "it isn't obvious how much the PCM is playing a part in the performance."