One problem that happens sometimes with passive solar houses is that they can get too hot even in the middle of winter. For instance, once when we were house sitting in Raglan, the temperature inside the house could reach almost 30 degrees in the late afternoon just from sunlight. This happened because it had lots of windows facing north that let the sun inside, but most of the energy from the sun heated the air. But overnight the heat would escape and by the next morning it would be 12 degrees inside. A graph of the temperature would look like this.
This graph shows that it’s hot during the day and cold at night. You might as well live outdoors! The reason for this is because the house had lots of windows in the right places, but not enough “thermal mass” inside to absorb the heat during the day and let it out slowly at night. An example of thermal mass is our brick patio. Even an hour after sunset the bricks still feel warm. In a way, the bricks are like rechargeable batteries because they can store the sun’s energy like a battery stores electricity. But when the energy runs out they need to be recharged.
But of course the patio is outside. We want more thermal mass inside. To get an idea of what might be a good example of thermal mass, think of water and anything that sinks in water. We might call these things “heavy.” Anything that floats in water is not so good as thermal mass, but is better as insulation. That’s what the next post will be about.
So how do we get thermal mass inside our house that is up on piles? Here are a few ways.
Our iron bath.
Our Multi-fuel Stove
Extra layer of GIB on some walls.
We put the bath, the stove and the extra GIB all along the northwest-facing interior wall as shown in the picture below.
The winter afternoon sun shines directly on them, and instead of our house overheating, it gets up to 22 or 23 degrees. Then as the house cools down at night, the heat stored in the bath, stove and GIB goes into the air space of the house. A graph of the temperature in a house that gets lots of sunlight but also has enough thermal mass and insulation would look like this.
This graph shows that it gets warm during the day but not too hot, and it cools down at night but doesn’t get cold. Goldilocks might say that this is “Just right!”
Any questions or comments?