Strawbale vs. Cob

Which material works best for which application...

I often get asked this question:  Which is better to use for my natural building, strawbale or cob?

The answer is simple: IT DEPENDS!  (of course!)
To help sort out if strawbale or cob walls are better suited for your application, I'm going to describe the basic properties of each material, and then how to use those attributes to your greatest advantage.

strawbale detail around a beam

STRAWBALES insulate.  And they are thick, so they insulate really well.  What this means is that a wall built of strawbales slows down heat energy traveling from one side of the wall to the other.  A good insulator acts like a down jacket that keeps your body heat inside the jacket instead of getting disbursed to 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.  If you use an air conditioner in summer, insulation will keep the heat outside, so again you need less energy to keep cool.

sculpted cob thermal mass wall

COB is a thermal mass because of its principal ingredients: clay & sand.  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.  When the air temperature around the cob is lower than the temperature of the cob itself, it 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).


So how to best use these characteristics to your advantage?
The answer depends on your climate and what you are building.  If you are using energy to change the inside temperature and keep it something different from the outside temperature, then you generally want a good insulator...ie, strawbales.  If you live in a mild climate where the temperature swings are day-to-night instead of seasonal, then a thermal mass exterior wall generally will help to average out those temperature swings...ie, cob.  Thermal mass can also provide a highly beneficial interior element in conjunction with passive solar design, to capture heat from the Southern sun in the winter (when the sun is low) but remain shaded when the summer sun is high in the sky.

STRAWBALES work best...

• as exterior walls anywhere you are trying to keep the inside temperature different from the exterior temperature; the insulating strawbales will help keep the temperature exchange to a minimum, so the energy used to change the inside temperature will be minimized

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 use a combination of strawbale and cob.  Because I design for a climate that requires several months of heat in the winter, I use the insulating strawbales for exterior walls.  This ensures that only a minimum amount of energy is needed to heat the spaces and the heat stays inside.  I then position some cob element to the interior.  Either it surrounds a wood-burning heat source or it is positioned so that low winter sun shines on the wall from the South.  That same cob element is shaded and cool in summer.  This way the cob helps to regulate the interior temperature in both winter & summer.