For a brief time in the 20th century, we achieved a high degree of occupant comfort in our buildings by using fossil fuels relatively indiscriminately; we filled leaky homes with conditioned air at a rate slightly faster than it exited. We were able to reliably meet thermostat set temperatures if we used a lot of fuel to do so.
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When the price of all that fuel started to rise dramatically in the 1970s, our first reaction was to add insulation to buildings to slow down the loss of heat. However, these new, more insulated houses were still relatively leaky and did not meet predicted energy savings. By the late 1980s, we were beginning to attempt to both insulate and seal buildings against gross air leakage. These buildings were, indeed, more energy efficient. However, they also experienced many forms of moisture-related failure. The efforts to solve these failures are at the heart of building science.
The rest of this home design will attempt to make clear the reasons for those failures and what has been learned to ensure that we can, indeed, create comfortable and efficient indoor spaces without causing negative human health or building durability issues.
This is the most critical of all the control layers in the building, because if vulnerable parts of the building are damaged by being repeatedly exposed to wetting, it is unlikely to meet any of our remaining criteria for a successful building.
Our buildings are exposed to water in many different forms and from every possible direction. The role of the water control layer is easy to understand. It is there to shed and repel all water from the outside.