Designs That Work
Hot-Humid Climate
Introduction - Building Science for the Hot-Humid Climate 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.
- 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.
- 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).
- 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.
- 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.
- 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.
- 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.
- When the rate of wetting exceeds the rate of drying, accumulation
occurs.
- When the quantity of accumulated moisture exceeds the storage
capacity of the material or assembly, problems occur.
- The storage capacity of a material or assembly depends on time,
temperature, and the material itself.
- 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).
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