The concept of a “dry house” represents a fundamental principle in modern building science focused on long-term structural integrity and performance. This approach recognizes that moisture can infiltrate a home in three distinct forms: liquid water, water vapor, and air-transported moisture. Implementing this principle requires a holistic strategy for the building’s exterior shell, known as the building envelope. By actively managing how water, air, and vapor move into and through the structure, builders create a resilient home environment.
Defining the Dry House Concept
A dry house refers to a building where the exterior envelope is designed to control moisture content within its structural assemblies. The primary goal is to maintain a moisture balance where the rate of water removal consistently exceeds the rate of entry into the walls, roof, and foundation. This strategy acknowledges that some water will inevitably breach the exterior cladding. Therefore, the assembly must be constructed with the capacity to dry out, either to the interior or the exterior. Durability is achieved through a layered system that prevents the sustained saturation of materials that leads to degradation and decay.
Principles of Moisture Control
Achieving a dry house relies on the precise installation of four control layers, each addressing a different mechanism by which moisture moves into the building assembly.
Water Control Layer
The primary control layer is the water control layer, often called the drainage plane, which manages liquid water from rain and snow. This layer includes external claddings, flashings, and a water-resistive barrier (WRB) applied over the sheathing. The WRB acts as a secondary defense, shedding any bulk water that penetrates the outer siding and guiding it down and out of the wall assembly.
Air Control Layer
The air control layer targets the movement of moisture-laden air through convection. Air leakage transports significant amounts of water vapor into wall cavities, where it can condense on cold surfaces. A continuous air barrier, sealed with tapes and gaskets, stops this airflow, which is a far greater source of moisture problems than vapor diffusion alone. This barrier is applied across all six sides of the building envelope.
Vapor Control Layer
The vapor control layer, or vapor retarder, regulates the slow, molecular movement of water vapor through solid materials—a process called diffusion. Its strategic placement depends heavily on the home’s climate zone, as it must be placed on the warm side of the wall assembly to prevent condensation. In cold climates, a vapor retarder is typically placed toward the interior. In hot, humid climates, it may be placed toward the exterior or omitted entirely. Some modern “smart” vapor retarders can change their permeability based on humidity levels, allowing the wall to dry in different seasons.
Drying Potential
The concept of drying potential ensures that if a wall assembly does get wet, it can dry out before damage occurs. This requires selecting materials that allow moisture to escape to either the interior or the exterior. Assemblies must be designed to be vapor-open on at least one side, preventing moisture from being trapped between two impermeable layers. This balance between limiting moisture entry and maximizing drying capacity ensures the long-term durability of the structure.
Implications of a Moisture-Managed Home
Successfully managing moisture within the building envelope results in three significant long-term benefits.
Structural Durability
Structural durability is enhanced by preventing the degradation of building materials. Wood rot and decay require a sustained moisture content above 16%. Controlling this environment preserves the load-bearing capacity of studs, beams, and joists. A dry house also prevents the corrosion of metal fasteners and the deterioration of foundation materials that can compromise the home’s framework over time.
Health and Indoor Air Quality
Moisture management directly supports better health and indoor air quality for the occupants. Mold and mildew require moisture to thrive, and keeping surfaces dry removes the necessary condition for their growth. Preventing biological growth inside the wall cavities and living spaces reduces the risk of respiratory issues and other health concerns associated with mold spores.
Energy Efficiency
A dry house improves energy efficiency by protecting the thermal insulation layer. When insulation materials become saturated with water, they lose their ability to resist heat flow, which is measured by their R-value. The air control layer, integral to moisture management, also prevents conditioned air from leaking out and unconditioned air from leaking in. This significantly reduces the energy required for heating and cooling. Maintaining dry materials and an airtight envelope ensures the home’s thermal performance remains consistent.