Hot-Dry and Mixed-Dry Climate Zone
Hot-dry and mixed-dry climates present several challenges for home building.
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The intense solar radiation imposes a large thermal load on houses that can increase cooling costs, affect comfort, and damage home furnishings. This section includes some of the best methods to minimize the impact of solar radiation on the building, its mechanical system, its occupants, and their furnishings.
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Annual precipitation in these climates is less than 20 inches. Nevertheless, a brief period of heavy rain can deposit several inches of water onto a building. Improper irrigation can be a major moisture source, leaks can cause significant damage, and indoor sources of moisture can be a problem. If water collects in an area that cannot quickly dry, deterioration of the building will occur. Managing fire risk, particularly in terms of exterior claddings and landscaping, is an important issue associated with the dry climate.
Hot-Dry and Mixed-Dry Climate Best Practices
Housing types vary greatly in the hot-dry and mixed-dry climates.
In the face of this diversity, no single set of measures will achieve the 30 percent energy savings in space conditioning and water heating that qualifies a home as ENERGY STAR®-qualified. The principles included in these best practices need to be adjusted for different circumstances. A building scientist, such as a home energy rating professional, can help homeowners determine which combination of best practices is most appropriate.
These best practices are derived from Building America's research on tens of thousands of homes. Building scientists have tried and tested these measures on actual homes in the field. While not every measure will be right for a specific home, achieving high-efficiency performance and a healthy indoor environment depends on making informed decisions about interactions among all aspects of the building system.
Special Considerations for the Hot-Dry and Mixed-Dry Climate Zone
Site Design
The hot-dry and mixed-dry climates are dominated by cooling rather than heating needs. Avoiding summer cooling is more important than encouraging solar gains for winter heating. Planners should do all they can to avoid the entry of solar energy into houses in summer. Site planners have two important tools to help avoid solar heat gain: lot orientation and shade trees.
Lot Orientation
When the house has clear single glazing, which is not recommended, interior shades, overhangs, and various combinations of shading devices significantly reduce energy costs. Naturally, a completely shaded house has the best performance in a hot climate.
Xeriscaping
In the low-water environments that dominate much of the Southwest, lots should be landscaped to take advantage of plants that use less water than traditional turf-dominated approaches. Many communities have been faced with increased demands on existing water supplies. Consequently, there is a greater focus on water conservation, not just in times of drought, but in anticipation of future population growth. Water can no longer be considered a limitless resource. Conserving water through creative landscaping has engendered the new term, xeriscape. The term is taken from the Greek xeros, meaning dry, in combination with landscape.
The goal of a xeriscape is to create a visually attractive landscape that uses plants selected for their water efficiency. Properly maintained, a xeriscape can easily use less than one-half the water of a traditional landscape. Once established, a xeriscape should require less maintenance than turf landscape.
By grouping plants with similar water needs together in specific zones, a xeriscape landscape can use water more efficiently. Low-water-use plants should be grouped together, away from high-water-use plants and turf. Take advantage of warm or cool microclimates (climatic conditions influenced by the placement of walls and shade trees) to create areas of interest and diversity.
A well-planned and well-maintained irrigation system can significantly reduce a traditional landscape's water use. For the most efficient use of water, irrigate turf areas separately from other plantings. Other irrigation zones should be designed so low-water use plants receive only the water they require. Proper irrigation choices can also save water. Turf lawns are best watered by sprinklers. Trees, shrubs, flowers, and groundcovers can be watered efficiently with low-volume drip emitters, sprayers, and bubblers.
The information presented here was adapted from the City of Albuquerque's Web site.
Foundations
Slab foundations are a common foundation system in the hot-dry and mixed-dry climates . Building foundations should be designed and constructed to prevent the entry of moisture and soil gases such as radon.
Slabs and Basement Floors
Where gravel is scarce, builders often pour slabs onto sand. When sand or other native fill is used, a 3- or 4-inch perforated and corrugated pipe loop can be use for both drainage and radon control.
Slabs in the hot-dry portion of these climate zones, are generally not insulated, even at the perimeter, because of the low overall heating load. Slabs in the mixed-dry climate should be insulated at the perimeter with one inch of borate-treated foam board insulation or rigid glass fiber insulation.
Walls
Wood Frame Walls
Best practice for frame wall construction involves advanced framing techniques. However, more traditional wall construction techniques can also achieve 30 percent space conditioning energy savings in the hot-dry and mixed-dry climate zone. More information on gaining greater efficiency using advanced framing can be found at the Building Science Corporation's Web site. If advanced framing is to be used, a detailed plan should be developed showing framing placement.
External walls with 2 x 4 framing may achieve 30 percent energy space conditioning savings in the hot and humid climate zone and should include the following features:
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Exterior finish such as stucco over paper backed with lath (paper to have perm rating of <1 perm), vinyl siding, or cementitious board.
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A housewrap installed as an air and water barrier.
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R-13 (high-density) friction-fit unfaced fiberglass insulation or blown-in cellulose insulation.
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R-13 friction-fit, unfaced batt insulation on frame walls between the garage and the conditioned space, including bonus rooms.
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Rim joists that have unfaced R-13 friction-fit batt insulation cut to fit.
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Foam-sealed or caulked top -plate and exterior wall penetrations.
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Sealed gypsum board to control air leakage through the walls, especially in penetrations to garages and porches, and where the walls meet the ceiling.
Insulation
Reflective Insulation Systems
Reflective insulation systems are recommended for the hot-dry and mixed-dry climate zones due to their effectiveness at reducing peak cooling loads, especially if ductwork or cooling equipment is located in the attic. For more information visit the residential buildings insulation section.