Using thermal mass to capitalize on passive solar design
Do you feel like you should be taking advantage of our seemingly endless hot summer days, but you’re not quite ready to invest in solar panels? Many homes in Alaska capture heat during parts of the year with passive solar design–which combines specific building features with the sun’s energy. Typically, south-facing windows and a large thermal mass work together to collect, store, and distribute solar energy during the heating season. During the summer, things like deciduous trees or awnings can block solar energy from overheating a house.
Thermal mass is key to an effective passive solar design. It means using heavy, dense building components that have a high capacity to absorb, store and release heat, such as logs, masonry, concrete and adobe, for example.
With passive solar, there is no control system that dictates the movement of heat energy, as with a boiler or furnace. To understand how this might work, picture a hilltop house on a sunny spring day. You’re in a south-facing room with a concrete floor. As the sun’s radiation penetrates the windows, it warms up the room. The concrete floor absorbs this energy throughout the day.
At night, the situation reverses. As the room’s ambient temperature drops below the temperature of the floor (the thermal mass), the heat energy stored in the concrete radiates back into the room, stabilizing the temperature and offsetting – or at least delaying – the need for the boiler to turn on. In effect, thermal mass acts as a heat battery, storing solar radiation until the sun disappears and then releasing it back into the room. A properly designed passive solar system saves energy because the thermal mass can store excess heat during the day and allow it to offset nighttime heating loads.
Although thermal mass is often in the form of a concrete floor, there are other ways it can be incorporated into a home—such as a wall that receives lots of sun or a masonry bench in the sun’s path.
As days lengthen during spring and summer, the large south-facing windows in the example above can allow too much solar radiation in. Some people install awnings or curtains, or plant deciduous trees to shade the windows.
Thermal mass also helps prevent overheating, especially in early spring before deciduous trees have leafed out. A room that might have become uncomfortably warm during the day instead uses thermal mass to absorb this extra heat, then releases it to the room later when outdoor temperatures have dropped. Overall, the thermal mass acts to smooth out temperature swings inside your home, and can be an easy way to reduce your heating load without a lot of extra cost.