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Extension > Environment > Housing Technology > Indoor environmental quality > Air sealing, moisture control and durability: What's the connection?

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Air sealing, moisture control and durability: What's the connection?

By Richard Stone, Extension Educator in Housing Technology
Reviewed 2010

What is all the concern with framing and air sealing details really about? Not that long ago, homeowners had very different expectations of their homes. Heating fuel was much cheaper, air conditioning was considered to be an extravagance, and the definition of comfort was down the scale a few points from where it is today. The building industry produced homes that delivered the comfort that was expected and provided a durable investment for generations of homeowners.

After a couple of energy shortages and a lot of changes in the materials and processes used to construct new homes, it has become more challenging to build that durability into our largest investment. Today, a builder's skill set needs to include a strong understanding of building science principles so they can integrate new materials and changing processes without sacrificing durability and craftsmanship. What are some of these changes and how do they affect homes?


Photo by Joe Nagan, Home Building Technology Services

Attic spaces have changed. Driven by energy costs and comfort expectations, insulation levels have increased by stages. In the older homes, minimally insulated or un-insulated ceilings resulted in attic spaces that were warmer in the winter. Warm, moist air that leaked into attics escaped to the outdoors before finding cold surfaces that would allow condensation. Attics in new homes with higher levels of thermal insulation are much colder in the winter. Warm moist air leaking into these spaces drops the moisture on cold attic surfaces before it can be vented to the outdoors. This keeps the structural materials wetter for extended periods of time, creating the potential for durability problems.


Photo by Richard Stone, University of Minnesota

Unsealed bypasses in interior walls and in ceilings allow warm moist air to leak into the attic. Holes cut for plumbing, electrical, and ventilation ducts must be carefully sealed to reduce the potential for moisture damage to the structure. Heating and cooling ducts that are located in the attic, or wall cavities used as ducts are under mechanically induced pressures when the air handler is operating. Unless they are carefully sealed at all seams and joints, they can cause large amounts of heated and humidified air to be forced into cold attic spaces or wall cavities, wasting energy and money and risking moisture damage to the home.

Wall cavities have changed, too. The tightness of building shells has increased as sheet goods replaced dimension lumber in the construction of new homes. Many of the gaps and cracks between boards that allowed older, un-insulated, or minimally insulated walls to dry are gone.

Surface temperatures on both sides of wall assemblies have been affected by increased levels of thermal insulation. Moisture that was previously carried to the outdoors through leaky wall assemblies can now be trapped in cold building assemblies made of composite materials that are more susceptible to moisture damage than dimension lumber. Unsealed bypasses can allow moisture to accumulate and sit longer in places where it can affect the durability of structural components of the home.


Wisconsin Focus on Energy photo

In the past, accumulated moisture from the winter would dissipate during the summer. That equation has changed too. Air conditioning homes changes the direction of heat and moisture flows in the summer so warm, moist air from outside moves toward the cold surfaces that enclose cooled indoor air. This is challenging the housing industry to design and build assemblies that can dry in both directions preventing the accumulation of water in building cavities during winter and summer.

With all these forces working on and within our homes, we need to take control of our buildings by constructing tighter building shells to control air and moisture flows. In a tight building, heating and cooling efficiencies can be maximized and properly designed ventilation systems can effectively manage moisture levels and indoor air quality. During commissioning, quality assurance testing of building tightness, heating and cooling performance, and ventilation flows is completed in homes already being built to these standards. Builders that have embraced this technology and put it to work for themselves know more about the performance of every home that they produce. That knowledge provides them with the ability to continually refine the quality of their homes and confidently position their companies to lead the housing industry into the future.

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