If both “tents” offer the same degree of thermal insulation and airtightness, neither scenario is inherently better than the other. The tent with the boulder requires a lot of thermal energy to raise its temperature, and then releases a lot of thermal energy as it cools. The empty tent requires a very small amount of thermal energy to change its temperature, and then cools quickly.
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Now translate these scenarios to a building. You wake up in the morning and turn on a heating device. In a building with little thermal mass, the temperature will reach the desired level relatively quickly. In a building with a lot of thermal mass, the desired temperature will take much longer to reach. The low-mass building will require more frequent but shorter inputs of heating energy and the temperature swings will be shorter. The massive building will require fewer but longer inputs of heating energy and the temperature swings will be longer. The ideal amount of thermal mass in a building is dependent on the climatic conditions, the type of heating system being used, the amount of passive solar exposure, the amount of insulation, and occupancy patterns (do occupants want faster or slower response times from heating and cooling systems?). More or less thermal mass is not necessarily better, but should be among the important design considerations for the building. Passive Solar The sun is the source of all energy on our planet, and as such it seems foolish to ignore the impact of the sun on our buildings. Passive solar design encourages us to think about how the sun will relate to our building, and asks us to plan how we can best exclude direct solar heat gains during cooling seasons and include direct solar heat gains during heating seasons. By doing so, we can realize significant reductions in the amount of energy we need to condition our spaces. There are two aspects to solar exposure that must be considered: Bearing angle The point on the horizon where the sun appears in the morning and disappears at night. Altitude angle The height of the sun above the horizon.
Depending on the latitude of your building site, these points will change sometimes dramatically from summer to winter. Understanding these angles allows you to strat-egize for the seasonal inclusion and exclusion of direct solar gain.
Many building shapes and orientations can accommodate passive solar design. There exists the notion that “best practice” involves creating a building with a long east-west axis that maximizes southern exposure. However, as the drawing above makes clear, a building of any shape can receive a lot of solar exposure. Regardless of whether or not solar exposure is ideal, it is worthwhile to incorporate passive solar planning, which includes gain and loss calculations, but also room layout, heat distribution, and the mass, thermal capacity, and reflectivity of building materials. It is better to make the best use of whatever amount of solar exposure and/or to take advantage of whatever exclusion aspects are available on a given site. There is no site and no building design that cannot benefit from a passive solar analysis.
Roof overhangs are the simplest form of passive solar design. As shown in the examples above, the roof overhangs are designed to allow for maximum solar gain in the windows in the winter and maximum solar exclusion in the summer. Despite the narrow southern exposure on this particular house, 25% of the annual heating energy requirements come from free sunshine, and air conditioning is rarely needed. The energy impacts are real!