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Yes, You Can Achieve Airtightness and Keep Wall Cavities Dry


Author: Will Lentlie, Group Leader, System Solutions

The growing demand for energy-efficient homes, whether spurred by code revisions or the desire to cut heating and cooling expenses, is driving innovations in building materials and methods. A leading concern for builders is how to achieve airtightness and energy performance without compromising moisture control. These needs can seem at odds; after all, don’t buildings need to “breathe” in order to prevent trapped moisture, mold growth, and rot?.


The truth is buildings don’t need to breathe, and both energy efficiency and sound moisture management can go hand in hand. CertainTeed’s ONE PRECISION ASSEMBLIES (OPA) proves this with its weathertight wall, floor, and roof panels engineered to meet and exceed various sustainability requirements, typically consuming less than half the energy of other new homes.
 The key to increasing energy efficiency while warding off mold and rot is utilizing the right control layers in your building envelope. To understand why, you must first understand how mold growth and moisture damage occurs.

What causes mold growth and moisture damage?
Various sources cause moisture to penetrate or become trapped within walls and roofs. For example, rainwater and groundwater can seep in from outside. And from inside, cooking, laundry, house plants, and more put water vapor into the air, increasing humidity. 
To control the movement of moisture from all these sources, your building layers need to control heat, air, vapor, and water. OPA’s building scientists look at these measures and more to predict when and where moisture may become trapped and mold may grow.

Humidity, of course, is the measure of how much water vapor is in the air. Absolute humidity is the total mass of water vapor in the air. This does not vary with temperature. Relative humidity, however, changes with air temperature. Relative humidity is the ratio of how much water vapor is in the air to how much water vapor the air could potentially hold at a given temperature. According to ASHRAE 160, Criteria for Moisture-Control Design Analysis in Buildings, mold tends to grow when relative humidity is above 80% and the temperature is above freezing.

Membrain graphic explaining affects of humidity

Dew Point
The colder the air, the less water vapor it can contain, increasing relative humidity and causing water vapor to condense. That’s why dew forms on cold surfaces, like a bathroom mirror after a shower. The dew point of the air in and around a building is the temperature at which water vapor will condense and form dew. 
Vapor Pressure
Vapor pressure, or the pressure that drives vapor from warm and humid to cold and dry, also changes with temperature. Building scientists use vapor pressure to determine the direction and speed that vapor will move. 
When a building is designed, constructed, maintained, or renovated improperly, humidity, temperature, and vapor cannot be controlled, and moisture-related damage is practically guaranteed. OPA uses the latest advances in precision manufacturing to ensure that building enclosures are constructed to the highest standards of airtightness and vapor control. The CertainTeed-engineered enclosures combine  best-in-class structural systems, insulation, windows, doors, weatherproofing, moisture management, and finishes to ensure exceptional performance at every layer.

Infographic explaining the vapor permanence ratings

Controlling moisture in mixed climates
Because water vapor is affected by temperature, the moisture management needs of a building can vary throughout the year. In mixed-climate regions where people turn up the heat in winter and crank the AC in summer, the direction water vapor moves changes with the seasons.
In winter, vapor moves from the warm and humid inside to the cold and dry outside, posing a risk of condensation on the cold sheathing. Then, as temperatures rise, mold begins to grow.
In summer, vapor pressure differences drive moisture through the building envelope from outside to inside, where it becomes trapped behind the vapor retarder. Excess moisture then collects on the bottom plate of the walls and causes mold and structural rot.
CertainTeed’s MemBrain™, a high-performance air barrier and smart vapor retarder, was designed to address these challenges. MemBrain adjusts its vapor permeability based on seasonal conditions, becoming less vapor permeable in winter and more permeable in summer so walls stay dry year-round. This is just one control layer included in OPA’s carefully engineered wall, roof, and floor panels.

A building envelope is only as good as its moisture management.
As you can see, selecting the right combination of building envelope layers is crucial for builders who want to optimize both energy efficiency and moisture management.
OPA’s full-service wall, roof, and floor assemblies combine CertainTeed’s proven, high performance control layers to address vapor, air, and moisture concerns in building envelopes. Plus, because the assemblies are manufactured in a controlled environment and quickly installed on your site, materials are never left sitting exposed to the elements—another source of moisture damage. Your builds are weathertight in days—not weeks.
OPA was recently awarded the Offsite Construction Award for Innovative Product or Service in recognition of our leading-edge contribution to systems-built housing. Better performing, healthier, and more durable homes that address sustainability needs are possible. We’re building them.

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