Here is an excerpt from that report I mentioned.
Here is the link to the entire paper, which is about 53 pages long.:
"Considering particular ventilation strategies, the combination of continuous ridge and
soffit vents is generally accepted as providing the greatest assurance of even and
The following descriptions of passive venting strategies are based more on anecdotal
evidence and a consensus of practitioners than on actual measurements (see Forgues,
continuous ventilation air flow. Unfortunately, this approach is also the most
expensive, often comparable to the cost of insulating. A typical attic retrofit includes
some combination of gable vents, roof louver vents, soffit vents, and turbine vents.
1985 for a more detailed discussion of attic venting strategies):
" Gable vents, usually consisting of a set of horizontal louvers shaped to discouragesoffit vents. Because roof louvers usually provide only 40 to 80 square inches
the entrance of precipitation, are installed on vertical surfaces in the attic. They
can provide high flow rates but depend strongly on wind direction. They are
inexpensive and are often the only existing ventilation,
however they do not usually provide good air flow at lower portions of the attic unless combined with
soffit vents. They do not get covered with snow but may allow entry of winddriven
snow in some circumstances if not properly re-designed when installed
(some contractors fasten a thin layer of fiberglass over the opening and fabricate
a drip tray with flashing in order to avoid snow entry). Contractors report
commonly finding plastic over gable vents installed by homeowners to solve this
problem. The large air flows through gable vents have also been reported to
create suction at roof louvers and thus draw snow into the attic. Roof louvers, sometimes called mushroom vents, are mounted on sloping portions
of roofs near the peak. Used alone, they can leave dead zones and may create
localized moisture problems as moister air escapes through them.
They are more effective if used high near the ridge and in combination with low venting such as
of net free vent area each, it is often impractical to install enough of them tostack effect (i.e. buoyancy-induced) flows within the attic."
meet code. In regions with high snow loads, roof louvers may become
completely covered in snow rendering them useless during the coldest weather.
m Soffit vents are installed in soffits, of course. They may be continuous or may
consist of a series of individual vents, particularly common in retrofits. They
depend less on wind direction than most other types of venting and can provide
nearly continuous ventilation air directly up along the roof deck when used in
combination with higher venting, such as ridge or roof louvers, which promotes