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:
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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
EEBA Bookstore.
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