BSP-031: Designs that Work: Hot-Humid Climate (Lake Charles, LA)

The Habitat Congress Building America Case Study Houses are designed to be climate-specific, affordable, energy-efficient housing prototypes.  As a Building America house, the design also works towards the following objectives:

• Produce homes that use 30 to 50 percent less energy.
• Reduce construction time and waste by as much as 50 percent.
• Improve builder productivity.
• Provide new product opportunities to manufacturers and suppliers.
• Implement innovative energy- and material-saving technologies.

To reach these objectives, the basic hot-humid climate house plan presented in this package uses a systems engineering approach.  This means that a significant amount of analysis and refinement has gone into the design.  The Building America design team has considered the interaction between the building site, envelope, mechanical systems, and other factors, recognizing that one feature of the house can greatly affect others. The team has then evaluated its design, business, and construction practices to identify cost savings, which have then be reinvested to improve energy performance and product quality.

There are two influences on this process that should be explained before you examine the hot-humid climate house plan: an understanding of the regional climate, and building science knowledge and experience.

Climate-Specific Design

Houses should be designed to suit their environments.  In the home-building industry, we have accepted that design and construction must be responsive to varying seismic risks, wind loads and snow loads. We also consider soil conditions, frost depth, orientation and solar radiation. Yet we typically ignore the variances in temperature, rainfall, exterior and interior humidity and their interaction.

The Habitat Congress Building America houses are designed for a specific hygro-thermal region, rain exposure and interior climate.  This means that the building enclosure and mechanical systems that are recommended in this package are generally suited to the hot-humid climate region.  You can find a description of the North American annual rainfall and hygro-thermal regions on the climate maps that follow.  Notice that while there are similarities between regions, there are also differences.  It is cold and dry in Wyoming; it is cold and somewhat wet in Wisconsin. Local climate may also differ significantly from the regional climate descriptions, and if so, the differences must be addressed when implementing the house design provided here.

Building for a Hot-Humid Climate

A hot-humid climate is defined as a region that receives more than 20 inches of annual precipitation and where one or both of the following conditions occur:

• A 67 °F (19.5 °C) or higher wet bulb temperature for 3,000 or more hours during the warmest 6 consecutive months of the year, or
• A 73 °F (23 °C) or higher wet bulb temperature for 1,500 or more hours during the warmest 6 consecutive months of the year.

The intense solar radiation in this climate imposes a large thermal load on the house that can increase cooling costs and affect comfort. The approach presented in this package minimizes the impact of solar radiation on the building, its mechanical system, and its occupants. Moisture is a significant problem in this climate, more so in those areas that receive more than 40 inches of annual precipitation. The ambient air has significant levels of moisture most of the year. Because air conditioning is installed in most new homes, cold surfaces are present on which condensation can occur. Controlling the infiltration of this moisture-laden air into the building envelope and keeping moisture away from cold surfaces are major goals of design and construction. Housing types vary greatly throughout all of the different climate zones, but nowhere is the contrast so great as in the hot-humid climate of the south-eastern United States.

Note: Don't forget that it is always the conditions that you actually experience in your area that determine the appropriate building design and construction details. The Building America Climate Zones provide simplified groupings of geographic locations that may actually vary greatly in terms of weather, and therefore should be viewed as guidelines.

Building Science for the Hot-Humid House

An understanding of the regional climate is the starting point for the design of affordable, high-performance homes.  Applying building science is the next step to create houses that are safe, healthy, durable, comfortable, and economical to operate. For the Hot-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 Hot-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 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 should 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 in importance are air-transported vapor and then diffusive vapor. 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).