King Kong and Godzilla fought to the death. One victor. One “good guy”. They didn’t walk off into the sunset hand-in-hand at the end of the battle.
Not so for strawbale and cob. There is no epic battle, no single "good guy". For the question “Which is better to use for my natural building, strawbale or cob?” the answer is simple: IT DEPENDS! (of course...) And often the highest performance comes when you use both materials together. Strawbale and cob compliment each other extremely well, because each material possesses completely different properties. So let's understand the benefits and limitations of each material so you will know how to maximize the attributes of both strawbale and cob.
STRAWBALES insulate. Small air pockets trapped between the strands of straw slow down heat energy traveling from one side of the wall to the other. The thickness of the strawbale walls ensures that there is A LOT of insulation between your cozy interior and the outside elements. A good insulator acts like a down jacket that keeps IN your body heat instead of letting it disperse out into the cold winter air. A well-insulated house will use less energy to heat in winter than a poorly insulated house, because the insulation keeps the heat inside longer. If you use an air conditioner in summer, insulation will keep the heat outside, so again you need less energy to keep cool.
STRAWBALES work best... ...as exterior walls anywhere you are trying to keep the inside temperature different from the exterior temperature. The insulating strawbales keep the temperature exchange to a minimum, so the energy used to heat or cool the inside is minimize.
I RECOMMEND: Build exterior walls with STRAWBALES if you live in a climate where a well-insulated home is more comfortable and cheaper to heat/cool.
COB provides thermal mass. The principal ingredients, clay and sand, are thermal mass materials that store heat energy. Cob has limited insulating properties. Instead, a thermal mass is like a storage battery for heat (or cool) energy. This means cob is good at absorbing heat energy from the sun or a fire and storing that heat even once the heat source is gone. When the air temperature around the cob is lower than the temperature of the cob itself, the thermal mass releases its battery storage of heat into the air. In this way, cob can absorb a lot of heat energy and then release the heat over time, long after the heat source is gone. Conversely, a shaded thermal mass with no heat input will stay cool in the summer and absorb heat energy out of the warmer air around it (thus having a net cooling effect).
COB works best... ...as thermal mass built around a masonry heater or rocket stove (or near a wood burning stove), where the cob can absorb heat from the fire, and store the heat energy even after the fire is out.
...for trombe walls in passive solar design, with the cob thermal mass inside, where it is warmed by sun coming through South-facing glass.
...for any interior element when you are trying to keep the inside cool. This can be the same thermal mass used to keep warm in winter as long as there is no heat source warming it when you want to stay cool.
I RECOMMEND: Use COB where high thermal mass will store heat or cool energy…interior walls for passive solar strawbale homes or exterior walls in temperate climates or where daytime temperatures are warm all year round.