MIL-HDBK-1011/2
Overhanging eaves may provide
necessary solar shading for
windows and building surfaces. Refer to
para. 4.2.3.1.
Figure 21
Attic Section with Three Possible
Locations for a Radiant Barrier
4.5.2.3
Ventilators. Higher indoor air
movement can be obtained
with proper cross-ventilation than with
roof openings. Therefore, roof
ventilators should not be considered as
alternatives to proper wall
openings but should be used in conjunction
with proper wall
openings to obtain well ventilated interior
spaces. Only part of the
windward slope of a steeply pitched roof is
under positive pressure.
Low pitched and flat roofs are subject to
suction over their entire
areas when:
EQUATION:
X = < 1.2 length
))))))))))
feet
where:
length
=
Length of the building in the windward direction
X
=
Area of the windward face
Under these conditions a stagnant zone exists over the entire roof due to flow
separation occurring at the windward eave. The result is that the entire roof
is more or less under suction and is a good location for exhaust outlets.
With high pitched roofs and when the building length is greater than 1.2 times
the area of the windward face, the stagnant zone exists mainly downstream from
the ridge while a portion of the windward side of the roof is under positive
pressure. The critical roof pitch at which the point of flow separation is
displaced from the windward eave to the ridge depends to a large extent on the
wall height but may be taken between 18 and 25 degrees for wall heights from
12 to 15 ft (3.6 to 4.6 m) respectively (see Figure 22).
Ventilator Placement. Use the strong negative pressure areas near
4.5.2.4
the ridge as exhaust locations. Placement of exhaust openings on high pitched
roofs is more critical because of possible positive pressure zones, which
should be avoided. Ventilators or wind scoops with openings facing the wind
can act as effective inlets but water infiltration must be considered in their
design and location. Possible problems with privacy, rain, and noise from
ventilators must be identified and resolved.
4.5.2.5
Ventilator Performance. Wind tunnel studies have shown that the
performance of common turbine roof ventilators is only slightly better than
that of an uncapped pipe. All other tested pipe designs proved more effective
than the turbine type. The highest performance for a simple ventilator was
produced by placing a canted flat plate over the pipe (see Figure 23).
34
Previous Page
