A soffit vent is under the lower eaves, at the lowest point in the
attic. They can be either narrow and continuous or else individual
larger ones. Gable vents are on the ends of the attic, near the top.
Some homes have soffits and others don't. Soffits exist
when the roof overhangs the house. Suppose that your
house has a gutter and downspouts on the front of your
house. If those gutters are flush with the front of your house,
then you have no soffits. If on the otherhand, the roof overhangs
the front of the house and your gutters are offset a foot or so
in front of the vertical surface of the front of your house, then you
Soffit vents are screen covered opening in the overhang area.
If you are on the ground looking up, then you will be looking up
at those oval or rectangular openings. Building a house with the
soffit overhang is more expensive than building one without. Hence,
many homes lack soffits.
Now move around to the side of your house which is shaped like
a triangle on top of a rectangle. The pointed area at the top of that
upper triangle is the gable area. Many homes have large louvered
openings in that gable area for attic ventilation. This should provide
a good (and free) cross ventilation of the attic.
Both types of venting are very useful and they provide good passive
(ie, energy free) attic venting. Many astute homeowners add soffits
to their homes when they need to add a new roof. This means that
the roof must be extended on two sides, but the increase in attic
venting is often worth the effort. Of course, good venting can still
be achieved without soffits. The killer situation is a home which has
no soffit venting and the only other attic venting is extremely undersized
gable venting. In that case, the roof sheathing (plywood) will often
be worthless after 30 years or so.
Can someone explain what a soffit vent is compared to a gable vent?
Make sure that soffit vents and gable vents are compatible and can live
together. It's possible that they might counteract each other and/or
disrupt the intended air venting. They will interfere with each other if
you have a ridge vent or roof vents.
Don't be too hasty with your answer. The following does not come from the
article that I first read but it does point out that there can be conflicts
if you have too much of a good thing. Basiically, what I get out of this is
that vented air will get it's source from the nearest opening (least
resistance) available. This means that if there are gable vents the flow
from the soffits will not be too effective. The first article had several
schematics that showed the air flow with different systems (gable, soffit
and one that had both). The system with the combination of the gable and
soffit acted principally as a gable only vented system. Recommendation was
to block of the gables if there are soffits. Pay your money and make your
choice--but do a bit more research first. Google roof vents, gable vents
Watch out for unintended consequences!
Murphy's law rears its head in the strangest places... your attic, for
instance. Who would think that beautiful powered ventilator might do
absolutely nothing to cool your attic? If you have lots of ventilation
already, adding a powered ventilator may be a waste of time and money.
This is especially true if you have a soffit-ridge vent system, the most
efficient natural ventilation system. Face it... ventilators are stupid
machines! They will draw air from wherever they can with the least effort.
This means that a ventilator will draw air from ridge vents, gable vents or
soffit vents. Fine. But if the fan draws much of its air from a nearby
gable or ridge vent, there is no benefit to anyone except the electric
company! So placement of the ventilator is essential... as well as careful
consideration of whether or not it is a wise investment in the first place
Okay, so I see it is a very complicated issue. So for my (perhaps) last
question, is there some technique or resonably purchased or rented device
that allows me to assess how air is flowing in my attic now and after
installing soffit vents?
Again, thanks for all the very informative advice. Great group of people
around here. Since I now have this old home, I am guessing I will be
spending some time around here. :)
If there is a measurable amount of air flow then I guess with a lot of
patience and some way to generate smoke you might be able to make some kind
of observation. I can recall on a sand building contest someone made a
dragon and made smoke coming out of the nostrils by blowing out something
like baking powder using a bulb and plastic tubing.
Standalone gable vents serve both as supply and exhaust venting. They also
provide cross ventilation, especially if aided by the exterior wind
conditions. Adding soffit vents makes the equation less accurate. In that
case, they act more as exhaust since you're adding supply.
Best attic vent system for warm climate areas, in my opinion, is a
continuous ridge vent for exhaust. Hardisoffit would serve as the supply
whereever soffit is used.
You need a minimum of 1" clearance between the attic ceiling insulation and
the sheathing for proper soffit supply ventilaiton. If this is blocked,
adding soffit vents is futile.
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