temperature between the attic and the outside create an
air movement independent of the wind, and also a more
positive movement when there is wind. Turbine-type
ventilators are also used to vent attic spaces (view C).
Where there is a crawl space under the house or
porch, ventilation is necessary to remove the moisture
vapor rising from the soil. Such vapor may otherwise
condense on the wood below the floor and cause decay.
As mentioned earlier, a permanent vapor barrier on the
soil of the crawl space greatly reduces the amount of
Tight construction (including storm windows and
storm doors) and the use of humidifiers have created
potential moisture problems that must be resolved by
adequate ventilation and the proper use of vapor
barriers. Blocking of soffit vents with insulation, for
example, must be avoided because this can prevent
proper ventilation of attic spaces. Inadequate ventilation
often leads to moisture problems, resulting in
unnecessary maintenance costs.
Various styles of gable-end ventilators are available.
Many are made with metal louvers and frames, whereas
others may be made of wood to more closely fit the
structural design. However, the most important factors
are to have properly sized ventilators and to locate
ventilators as close to the ridge as possible without
Ridge vents require no special framing, only the
disruption of the top course of roofing and the removal
of strips of sheathing. Snap chalk lines running parallel
to the ridge, down at least 2 inches from the peak. Using
a linoleum cutter or a utility knife with a very stiff blade,
cut through the rooting along the lines. Remove the
roofing material and any roofing nails that remain. Set
your power saw to cut through just the sheathing (not
into the rafters) along the same lines. A carbide-tipped
blade is best for this operation. Remove the sheathing.
Nail the ridge vent over the slot you have created, using
gasketed roofing nails. Remember to use compatible
materials. For example, aluminum nails should be used
with aluminum vent material. Because the ridge vent
also covers the top of the roofing, be sure the nails are
long enough to penetrate into the rafters. Caulk the
underside of the vent before nailing.
The openings for louvers and in-the-wall fans
(fig. 5-31, view D) are quite similar. In fact, fans are
usually covered with louvers. Louver slats should have
a downward pitch of 45° to minimize water blowing in.
As with soffit vents, a backing of corrosion-resistant
screen is needed to keep insects out. Ventilation fans
may be manual or thermostatically controlled.
When installing a louver in an existing gable-end
wall, disturb the siding, sheathing, or framing members
Figure 5-32.Inlet vents.
as little as possible. Locate the opening by drilling small
holes through the wall at each corner Snap chalk lines
to establish the cuts made with a reciprocating saw. Cut
back the siding to the width of the trim housing the
louver (or the louver-with-fan), but cut back the
sheathing only to the dimensions of the fan housing. Box
in the rough opening itself with 2 by 4s and nail or screw
the sheathing to them. Flash and caulk a gable-end
louver as you would a door or a window.
Small, well-distributed vents or continuous slots in
the soffit provide good inlet ventilation. These small
louvered and screened vents (see fig. 5-32, view A) are
easily obtained and simple to install. Only small sections
need to be cut out of the soffit to install these vents,
which can be sawed out before the soffit is installed. It
is better to use several small, well-distributed vents than
a few large ones. Any blocking that might be required
between rafters at the wall line should be installed to
provide an airway into the attic area.
A continuous screened slot vent, which is often
desirable, should be located near the outer edge of the
soffit near the fascia (fig. 5-32, view B). This location
minimizes the chance of snow entering. This type of
vent is also used on the overhang of flat roofs.