Quick Attic Ventilation Question

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James Bond wrote:

"Exhaust," worst-case, on a still day, when 16.6Asqrt(HdT) cfm should flow with equal vent area A ft^2 and height diff H feet and indoor-outdoor temp diff dT (F).
Nick
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Can someone explain what a soffit vent is compared to a gable vent?
- Magnusfarce

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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.
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Magnusface,
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 have soffits.
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.
Gideon
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Can someone explain what a soffit vent is compared to a gable vent?
- Magnusfarce

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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. MLD
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WRONG.
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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 etc. MLD
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
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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. :)
James

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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. MLD

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WRONG. The Ultimate vented attic would be a suspended roof open on all sides and with a huge hole in the top.
You cannot have TOO MANY vents.
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See reply inserted-

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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