MIL-HDBK-1003/19
All windows, including those facing south, should have at least two
glazing layers, and in the harsh and very harsh regions, triple or even
quadruple glazing should be considered. Especially in the warmer climates,
drapes or better still, movable opaque covers or shades, as described in
Thermal Shutters and Shades, are recommended as means to prevent unwanted
sunlight from entering the windows during the summer.
4.3.3 Passive heating system characteristics. The interaction between
a passive heating system and its environment is a complex process that
involves many subtle phenomena. The complexity of the interaction makes it
difficult to determine exactly what type of passive system will perform best
in a given climate. Ultimately, detailed design analysis calculations of
the type to be described later in these procedures may be required to make
the final decision. However, a few generalizations may be cited that are
useful for selecting candidate systems during the schematic phase of design.
The general rules for system selection are based on the steady state
conductance (Uc) of the passive solar aperture. The aperture conductance
is the amount of heat that would be lost through the solar aperture if the
outside ambient temperature were maintained at 1deg.F below the indoor
temperature for a period of one hour; the units of Uc are
Btu/deg.F-ft2-hr. It is generally true that systems with low values of
Uc are better suited for use in areas having relatively severe winter
climates than are systems with larger aperture conductances. The climate
regions based on the importance of conservation measures that are
illustrated in figure 7 provide a convenient measure of winter severity.
The selection process based on aperture conductance may be further refined
by the observation that it is also more important to have a small Uc in
regions that receive relatively little sun; the solar availability contour
map in figure 8 is useful in making this secondary assessment. In summary,
passive solar systems having low aperture conductances are recommended for
use in regions having severe winter climates with little sunshine. The
steady state aperture conductances of thirteen representative passive solar
heating systems is presented in table I to aid in the preliminary selection
process.
The first system in table I, a single glazed direct gain building, is
not recommended in any climate region because of the large aperture
conductance; even in a mild winter climate where the heating load may not be
a problem, the summer cooling load can be seriously aggravated by single
glazed apertures. Systems 2 through 5, or any other system with comparable
values of Uc, are well suited for use in the mild climate region. In the
moderate region, systems 2 through 9 are appropriate, and in the harsh
region systems 4 through 14 may be considered. Finally, in the very harsh
region, systems 9 through 14 will yield the best results. Within each of
the principal climate regions, the recommended systems having the larger
conductances are more appropriate in the sunnier subregions. These
guidelines may be useful during the initial system selection process, but
the designer should feel free to also consider other systems. In
particular, a small amount of direct gain is almost always an asset when
combined with other systems having lower aperture conductances. Of course,
in the colder regions, it is desireable to place more layers of glazing in
the direct gain apertures than would be used in milder climates.
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