It’s Not Rocket Science #4: Humidity Control

The building you see above is the oldest timber building in the world.
It was built in the year 756AD.
Hats off!

It is the Shosoein Treasure House in Nara, Japan, built to house the treasures of the Emperor Shomu (701–756). Buildings like this are the only traditional Japanese buildings that don’t employ post and beam construction. This type of Japanese-style “log cabin” is called azekura construction and was popular around 300BC to 800AD. It was probably a Chinese import from Neolithic times (Late Stone Age: 10,000BC – 2,000BC) but all the examples that still exist are from the 8th century and all of these use the azerkura  construction on an elevated floor.

In Neolithic times, a similar type of construction had also been used in Europe for the storage of grain. The  Shosoein Treasure House is the oldest surviving example and was used to store the Emperor’s treasures that included furniture, games, musical instrument, art, books and things from as afar as Persia.

In Japan, storehouses such as the Shosoein Treasure House may be traditional buildings but they are not vernacular buildings. Firstly, their owners had to be wealthy enough to own things worth preserving, the owner also had to have sufficient land to separate them from other buildings for reasons of fire protection and, the timber was expensive and expensively worked. Much care was lavished on their construction so they could preserve their contents but only Shosoein has done so for 1,200 years.

It’s widely believed that the timber and the method of construction of these storehouses is responsible for regulating the internal humidity and so help preserve what they contain. The timbers are supposed to expand on rainy days to block the gaps between them and prevent further entry of moisture, and then contract on dry days to allow interior moisture to escape. However, this has never been proven – according to Terunobu FUJIMORI, all-round architectural historian, architect, and professor at the University of Tokyo’s Institute of Industrial Science. (Hmm, his name seems familiar.) I imagine him saying something like that would make him rather unpopular but whether or not it’s a hygromyth is easy enough to prove. I expect this paper, “Temperature and relative humidity environment of the North section of Shoso-in repository [in Japanese]”, does so but haven’t yet been able to access it and find out for myself.

Fujimori believes that simply elevating the building off the ground matters more, but although the actual azekura construction may not work the way people would like to believe it does, it nevertheless does slow down the rate of moisture entry on wet days and the rate of moisture exit on dry days. So rather than maintaining a constant humidity, the amelioration of sudden changes in humidity does improve the storehouses’ performance in preserving precious objects.

It should be mentioned that the doors of these storehouses are sealed shut and opened only on ceremonial occasions and then only in autumn when the humidity is both low and stable but this too shows an awareness of the link between humidity and preservation.