Designs That Work

Hot-Dry / Mixed-Dry Climate

Hot-Dry and Mixed-Dry climates share an important climatic feature: annual precipitation is less than 20 inches. There is an important climatic difference, however, between the two: Hot-Dry climates do not have a monthly average outdoor temperature that drops below 45°F; Mixed-Dry climates do. See below for the complete discussion of the significance of this threshold.

Moisture is, in general, not as much of a problem in these climates as the others: even if the building gets wet, the dry and often hot conditions will usually quickly dry the building. Nevertheless, a brief period of heavy rain can deposit several inches of water onto a poorly designed or constructed building. Improper irrigation can replace precipitation as a moisture source, and leaks, well, they happen. If water collects in an area that cannot quickly dry, deterioration of the building can occur.

Intense solar radiation is a problem in these climates, specifically in terms of managing the cooling load on the building and in terms of ultra-violet degradation of many building materials such as wood, paint, plastics. An associated phenomenon (actually the lack of precipitation) means managing the fire risk of the structure, particularly in terms of exterior claddings and landscaping.

The coldest-month-of-the-year average monthly temperature threshold of 45°F is determinant in both roof and wall assemblies. It is critical for building design because of the following phenomenon:

The dew point temperature of air at 70°F, 35% RH is approximately 40°F. Consequently, interior air at 70°F that has a relative humidity of 35% (realistic condition in a tightly constructed house) will condense on a surface that is 40°F or cooler. How much water will condense on the interior surface of the exterior sheathing (the first condensing surface) is a function of the dew point temperature of the interior air, the amount of air leakage into the wall, and the vapor permeability of the materials in the wall. Small amounts of condensation will not be a problem if the wall allows drying to the exterior.

Adding a 5°F "buffer" or safety factor to the interior design temperature/humidity dew point (40°F dew point, 70°F, 35% RH) yields the 45°F monthly threshold temperature.

Using a thin layer of an insulating sheathing (R-4) will warm the temperature of the first condensing surface, decreasing the chance that significant condensation will ever occur in the wall assembly. For this reason we recommend an insulating sheathing in Mixed-Dry climates.

We have chosen three building profiles that we think represent building science and regional building practices in Hot-Dry and Mixed-Dry climates:

The "Albuquerque" one-story, stick-framed, slab-on-grade, stucco exterior, vented attic, asphalt shingle roof (with information on optional conditioned crawlspace). This represents a Mixed-Dry climate.
The "Sacramento" - two-story, stick-framed, post-tensioned slab-on-grade, first story with fiber-cement siding/second story with stucco, vented attic, tile roof (with information on a one-story with unvented, conditioned attic). This represents a Hot-Dry climate.
The "Tucson" - one-story, stick-framed, slab-on-grade, stucco exterior, flat ceiling (no attic), membrane roof. This also represents a Hot-Dry climate (with information on moving this building to a mixed-dry climate).

For information about a production builder from each of these two climates, view our Case Studies. These explore builder experience with the BA program and discuss the reasons for the specific design and construction details that Artistic Homes and Pulte Homes employ. To see detailed drawings on Advanced Framing and Air Sealing Techniques. Additional information on construction methods and alternative designs is included in the Builder's Guide Hot-Dry and Mixed-Dry Climates at the Building Science Press store.