Tuesday, March 26, 2013

Crack Loss

For the purposes of this article, crack loss describes the quantity of heat lost from a building due to kinetic energy pushing and pulling air through gaps in the exterior envelope. Of course, crack loss is not the only means by which heat is lost from a building, but it contributes the most effective means by which heat is lost from contemporary wood framed residential construction.

Kinetic energy in simplest terms is the energy of motion; anything that moves has kinetic energy. Air has kinetic energy when it is moving, and inside a house it moves almost constantly. When the air inside a home is heated through convection, radiation or conduction, an imbalance is created between the air temperature inside a house and the air outside a house, because we heat our houses when it's cold outside. The physical nature of our environment abhors imbalance, so when we turn the heat off, the air inside the house will always drop in temperature to match the temperature of the air outside to regain balance. This natural balancing is unavoidable at the scale of our observable environment.

Natural phenomena are also inured to the path of least resistance, and an observable crack, hole or other gap that exists between unbalanced temperatures provides the path of least resistance to reestablish balance. For example, the imbalance between 30ºF outside and 65ºF inside a house is an unbalanced temperature phenomenon, and an observable* crack between a window sash and frame
1: Offices 2: Vestibules 3:Industrial 4: Houses 5: Public Buildings
offers a path of least resistance to moving heated air that cannot avoid cooling to balance its temperature with the air outside. Windows and doors are part of a building envelope, and often offer direct paths for air movement from a heated condition to a cool condition, even if closed. This is due to the fact that the seals they employ are often inadequate when new, and almost all degrade relatively quickly through use.
Exhaust fans found in bathrooms and kitchens also offer direct paths, but are even more effective at aiding natural balance because the push large quantities of heated air directly to the outside and tend to create localized negative pressure gradients within a space which can draw-in cooler air from the outside via gaps at windows and doors, and the building materials and components that comprise the exterior walls of a house.

When viewed in isolation, the gaps between building materials and components may appear insignificant, especially because they generally do not align to create direct paths between inside and outside. But this apparent misalignment is a false friend because air movement is generally not restricted by changes of direction like human vision. In other words, the labyrinth of gaps between building materials from inside to outside my be difficult to observe with the eye, but they exist and air with pass through them. It's unavoidable, and in addition, the gaps may be thought of in the agregate. In other words, if one was somehow able to gather all the gaps into one large gap, the aggregate would be on average like having a window open, and sometimes like having a door open in a really leaky house.
*For the purposes of discussion, let's say anything we can see with any device is observable.  Bacteria would be observable, but quarks would not be observable.

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