Adding thermal mass increases the ability of a material to store heat energy. Brick, adobe, rammed earth, and earthbags are all great examples of mass used in building. To understand how mass works think about an adobe dwelling in the desert region (New Mexico, Arizona). The heat of the day travels through the walls at a slow rate (1 inch per hour), so that by the time the night approaches the warm air releases into the interior space. In other words, mass is great for room temperature regulation. Heat moves from hot to cold so whenever your space feels too warm the heat is absorbed in your mass; likewise when your space feels to warm the heat absorbed in your mass will be released into the room.
At Aprovecho, we built a cob bench in proximity to a stove and window. The fire heat from the stove and sunlight from the sun will soak and become stored heat. Cob can be sculpted into whatever you please. In this case, a bench was most appropriate because we needed more seating.
To begin we made sure that the house could carry a heavy load in the area we wanted to build. After looking underneath the house we determined that there would be sufficient support. Next, we laid out the size of the bench using leftover cinder blocks and marked the height of the seat on the wall using a chalk line. The infill was a mix of urbanite and cob. The urbanite was added because it lessened the amount of cob that needed to mix and made the project move along faster. When we got closer to the top of the seat we started to use just cob, following the same techniques as the cob wall (Introduction to Cob: Garden Wall). For the backboard, we drew the edge and “peppered” the wall with a bunch of screws for stronger integration and support. The mix was straw coated with clay. In this case, the clay was slaked and applied heavier than light straw clay insulation. When coating the straw, keep the long pieces of straw all facing the same direction. This increases tensile strength and makes it easier to apply to the wall. Take a “straw sausage” and work it on the wall, wrapping the straw around the screws and spreading the clay using your palm and fingers.
For base plaster our ratio was 2.5 parts sand to 1 part clay with a few handfuls of chopped straw. Finish plaster was the same ratio with no straw, except the sand size was much finer and we added light green pigment. The cinder blocks were tiled and grouted to finish off the bench.