As builders and contractors seek more ways to create quality air and thermal
barriers in their building envelopes, it has become increasingly clear
that in order to successfully accomplish this, the right insulation material
must be chosen from the start of the process. The expectation of your
chosen insulation is that it not allow airflow or moisture to pass through
or around the barrier.
Two popular insulation choices are spray foam and fiber insulation. A key
objective of both systems would be to achieve low air permeance, a resistance
to air flow.
Air movement through fiberglass insulation is referred to as wind-washing,
which serves to diminish the effective R factor of the insulation, and
creates the potential for condensation. Spray foam insulation by the nature
of its cell structure prevents wind-washing, and can create an air and
vapor barrier dependent on the assembly and spray foam application method.
An attic often provides the best wind-washing opportunity as air enters
the venting system and travels through the insulation. Fiber insulation
is susceptible to this as it cannot provide a 100% barrier based on limited
contour ability to prevent air and moisture movement around its edges.
Spray foam has a clear advantage based on its conformity and density properties.
It is imperative that your barrier successfully manage the flow of heat,
air and moisture. Failure to do so in key areas such as the roof and wall
cavities can cause early deterioration in building materials and moisture
buildup in walls, which can lead to mold and corrosion -- driving repair
costs literally through the ceiling.
In addition to experiencing the effects of energy and long-term repair
costs, as the insulation protection decreases you will also notice a diminishing
comfort of your home in areas such as less noise control, inability to
maintain proper indoor heating and cooling environment, along with the
danger of circulating allergens.
In a study of two-story homes by the U.S. Department of Energy,
Technology Solutions for New and Existing Homes*, wind-washing affected energy use, peak demand, comfort, potential for
moisture damage, and elevated humidity in hot and humid climates. Wind-washing
can also cause water pipes to freeze in cold climates
When choosing an insulation, always make a point to inquire if it protects
against wind-washing. In short, all insulation are not created equal.
Wind-washing: The phenomenon of air movement driven by wind pressures passing
through or behind the thermal insulation within enclosures, causing significant
loss of heat flow control and potentially causing condensation. Typically
occurs at exposed building edges, such as at the outside corners and roof
eaves because of the large pressure gradients at these locations. This
can be thought of as the “wind blowing through the insulating sweater”
effect. Source: https://buildingscience.com
U.S. Department of Energy Study: Investigating Solutions to Wind Washing
Issues in Two-Story Florida Homes: Phrase 2: Source: www.buildingamerica.gov.