I have an octagonal timber frame house with a metal roof. I don't have the diameter, but each wall is four meters/13 feet long. It is an open air house, with wooden slat walls coming up about a meter/3 feet, and metal screen for the rest. The beams are four meters tall, and set directly in the ground. For a floor there is a concrete slab with tubes for drainage underneath. The floor was completed while I'm out of town so I don't know exactly how thick it is, but it is as least 7.5cm/3". The ground underneath the floor is mainly red clay.
I want to make the interior walls out of cob. Do I need to put a moisture barrier between the concrete and the cob, or put a layer of stone before I put the wall?
The house is located on a hill in the rainforest of Costa Rica. The ceiling is over four meters, and the interior walls will not go to the top. There will be a circular room in the middle with a height around 2.5m/8' and then a few half walls(around 1.5m/5' tall) branching out from that. A lot of airflow, but also high humidity.
Yes I would put a capillary break between the cob and interior/exterior slab in a high humidity and rain environment. Stone would also provide a thermal break and allow the interface to aerate to where ever it finds lower humidity and not get trapped...Wood would also do.... if you can do either or a hybrid without loosing significant conditioned air and allow the gap to breath and stop any condensation from wicking up the wall. The clay in the COB depending on type will hold and release water/vapor to a point of saturation, so do what you can as a design goal at this low location to keep that from happening, and we do not know what microbials could be lingering there so a vent of fresh air is a good thing even if it has high relative humidity it is better than stagnant air and/or plastic fungi producing poly. ...
Some materials like the unknowns in slabs are too risky to mate with unknowns in COB so we create an isolater like inert stone or anything that would react or mate better to reduce the unknowns, and at the same time stop capillary up take or suction should it occur....Other natural material isolaters are carbon, basalt, pumice, scoria, lava rock, fiberglass, etc