TM 5-852-9/AFR 88-19, Vol. IX
during the summer. The need for windows should be weighed carefully because there is a lot of unwanted
sunlight in summer and cold infiltration in winter. As a rule, window area should not exceed 10 percent of
the floor area served by the window(s) in a given room or area. Another factor which must be considered is
that continuous daylight during summer months necessitates use of opaque window shades for some
functions. If passive solar heating is being considered because of the high heating degree days, however, more
windows on the sun exposed side are allowed to achieve this design. The number of operable windows should
be held to a minimum to reduce cold infiltration and heat loss. AFR 88-15 requires operable windows in
sleeping rooms for use as emergency and secondary exits. Operable window area shall be in conformance
with NFPA 80 and 101, and U.B.C. Low air infiltration rate and frames that provide a thermal barrier are
essential for window design in cold climates. Windows that do not have these inherent qualities frost up and
freeze. The result is an ice buildup which melts and drips, transmits cold air, allows heat to escape, and is
most unsatisfactory. Aluminum windows with an integral thermal break and metal clad wood windows
(aluminum cladding with baked-on factory finish) are good design choices. Both require minimum
maintenance, painting, etc. Vinyl clad wood windows should be specified with caution since some vinyl
cladding becomes brittle when subjected to low temperatures. The composite "U" value of approximately
0.54 for units fabricated from both types of materials is satisfactory. The American Architectural
Manufacturer's Association's Voluntary Specification for Aluminum Prime Windows, AAMA 101, and the
National Woodwork Manufacturer's Association's Industry Standards for Wood Windows, I.S. 2-80, both
require the air infiltration rate not to exceed 0.5 cubic feet per minute (cfm) per foot of crack of all operable
sash when tested in accordance with American Society of Testing Materials, ASTM E 283. In the arctic, the
tested air leakage rate should not exceed 0.15 cfm per lineal foot of crack for a pressure difference of 0.3 in.
of H2O across the window. In the subarctic, the same test should not exceed 0.25 cfm per lineal foot of crack.
The gap between the window unit and the rough opening should be thoroughly sealed so that air leakage
around the unit won't negate the benefits of a low leakage window.
b. Adding a weather-stripped storm sash to existing windows can improve the U-value of the window
system. However, most will not significantly improve the air leakage characteristics of existing windows.
When occupants neglect to close storm windows during cold weather, they nullify the benefits of storm
sashes. Avoid using windows with double prime sashes, either sliding or double hung, in cold-dry arctic and
subarctic regions, because occupants are likely to leave one set of sashes open and nullify the potential
benefits of the window design. In wet, windy areas, however, such double sash windows are suitable because
they can provide ventilation without allowing water to enter. The occupant simply opens an outer sash such
that air encounters a closed inner sash behind it and is diverted sideways to an open inner sash behind a closed
outer sash. Modern window units glazed with insulated glass and equipped with effective weather-stripping
have been used very satisfactorily on many buildings in the arctic and subarctic to reduce heat loss,
condensation, and frost buildup. Triple or double insulating glass, with a single pane piggyback unit in a
removable weather-stripped frame attached to the interior face of the sash, should be used in areas that have
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