Designs That Work

Mixed-Humid Climate

Introduction - Building Science for the Mixed-Humid Climate House

An understanding of the regional climate is the starting point for the design of affordable, high-performance homes. Applied building science is the next step to create houses that are safe, healthy, durable, comfortable, and economical to operate. For the Mixed-Humid Climate Case Study House, this means understanding and managing the way that four things move on or through homes:

• Water,
• Vapor,
• Air, and
• Heat

Section Two of this package, The Basic Mixed-Humid Climate House, focuses on these four phenomena. The greatest risks for moisture-related problems are discussed and where possible, the reasoning behind the selection of enclosure assemblies is given. The house design is based on extensive experience with what works and what does not work, from forensic investigations of building failures, and from the results of test houses and thousands of houses constructed by builder partners of the Building America program.

To bolster your own professional judgment and building common sense, the following ten building science principles are offered. It should not be a surprise that all of these principles are at least indirectly related to moisture. Even in hot-dry climates, moisture events related to occupant activities, leaks, and singular climate events can bedevil the performance and durability of today's homes.

1. Our efforts to save energy and reduce the flow of heat through building assemblies have reduced drying potentials and, therefore, increased the importance of controlling moisture flow through building assemblies.
    
2. Ideally, building assemblies should be designed to dry to both the interior and exterior. In heating climates, the primary drying potential is to the exterior (but not necessarily exclusively so); in cooling climates, the primary drying potential is to the interior (but not necessarily exclusively so); and in climates with both heating and cooling, some drying potential in both directions is typically a good idea (but not necessarily exclusively so).
    
3. Building materials last longer when their faces are exposed to similar or equal temperature and humidity. This is why the ventilation of claddings, particularly those that store moisture (reservoir claddings), can be important.
    
4. Drainage planes, air barriers, and thermal barriers must be continuous to be truly effective. Being able to trace each of these on a full elevation drawing without lifting your finger (or pencil or pointer) from the elevation is a good test of continuity.
    
5. In moisture control, the priority is liquid water first, particularly when it comes in the forms of rain and groundwater. In these forms it is referred to as "bulk" water. Following are air-transported vapor and then diffusive vapor, all other things being equal. It's always a question of quantities and rates, of wetting and drying, and the tolerance of materials (individually and in combination) for each and all of the above.
    
6. Three things destroy materials in general and wood in particular: water, heat, and ultraviolet radiation. Of these three, water is the most important by an order of magnitude.
    
7. When the rate of wetting exceeds the rate of drying, accumulation occurs.
    
8. When the quantity of accumulated moisture exceeds the storage capacity of the material or assembly, problems occur.
    
9. The storage capacity of a material or assembly depends on time, temperature, and the material itself.
    
10. The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow (except in the case of air flow in severe cold climates during cold periods where interior moisture levels are high).