MIL-HDBK-1003/19
4.3.4 Sizing overhangs. The purpose of a fixed overhang is to reduce
unwanted solar gains during the summer while allowing the low winter sun to
illuminate the solar aperture and provide heat to the building interior.
Sizing an overhang is a difficult problem because the heating season is not
symmetrical about the winter solstice, but tends to be displaced toward the
new year. Therefore, a design that provides adequate protection from
overheating in the fall may tend to reduce the amount of solar energy
available for needed space heating in late winter or spring. Since an
overhang does not provide protection from sky diffuse or ground reflected
radiation, it is often necessary to provide additional countermeasures to
prevent overheating during the cooling season. For this reason, the
currently accepted design practice is to size an overhang such that the
performance of the passive heating system is minimally affected, and employ
additional countermeasures against overheating as required. The sizing
procedure introduced below is based on "How to Design Fixed Overhangs", by
Andrew Lau.
The contour map presented in figure 9 gives the last month for which
full illumination of a solar aperture facing within 20 degrees of true south
is desired. This map is one of several presented by Lau and represents a
direct gain building with an aperture size of 15 percent to 25 percent of
the floor area on a moderately well insulated house (R-19 to R-30 roof, R-11
to R-19 wall, 0.5 to 0.75 air changes per hour). Use of the map in figure 9
will yield conservatively sized fixed overhangs in that there should be no
degradation of passive solar performance during the heating season although
there may be some tendency toward overheating in the fall. Movable shading
devices should be employed to control overheating due to asymmetry of the
heating season.
After determining the last month for which total illumination of the
aperture will be allowed, it is an easy matter to fix the overhang geometry.
The overhang length is denoted by X and the separation is given by Y, as
indicated in figure 10. The ratio X/Y is related to the latitude (L) minus
the declination (D) and this relationship is represented graphically in
figure 11. The quantity (L-D) may be read from one of the four contour maps
in figure 12 that represent the months of January, February, March, and
April. Briefly summarizing the sizing procedure, the user first determines
the last month of total illumination from the contour map in figure 9; then
he reads (L-D) from the contour map for that month from figure 12; finally,
the length to separation ratio is obtained from the plot in figure 11.
Summer shading is enhanced by selecting the largest practical overhang
separation and then calculating the length from the ratio X/Y. Constraints
on building geometry will generally limit the overhang separation.
4.3.5 Insulation levels. starting point values for thermal insulation
are recommended on the basis of principal climate region and building size,
and geometry. The R-values (thermal resistance in deg.F-ft2-hr/Btu) of
walls, including installed insulation and other layers, should lie in the
intervals indicated in figure 13 for small (1500 ft2), one story, single
family detached residences. The values suggested in figure 13 are
consistent with the results of a study presented in DOE/CS-0127/3, Passive
Solar Design Handbook, Volume Three, on the economics of mixing conservation
and passive solar strategies that was conducted for the United States
Department of Energy.
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