While calculating the number of soffit vents I am going to install in my house, I am operating on the priciple that I should have equal intake square footage (from the soffit vents to be installed) as exhaust. Now my question is do my existing gable vents count as intake or exhaust? And just to be sure I am using the right term, by gable vent I mean the two triangular vents at each end of my house that are right below the peak of the roof. It is my guess that after the installation of soffit vents that these gable vents would function as exhaust, but I want to verify that.
You might consider them as half and half for this exercise. As an exercise in logic: The gale end vents will probably act in various ways depending on wind pressure. Soffit and ridge vents would usually act consistently based on the chimney principle. The gable end vents, because they are large and in a vertical surface, would act differently with different wind direction.
In any case, look at the Building Science Corporation web site for discussion of venting. This is researched material. TB
Complicated fluid dynamics. At a layman's level, yes, they behave more like exhaust than intake in greater than 50% of the cases. But, the way gable vents work is that air blowing towards one vent creates positive air pressure and the air moving away from the other vent creates negative pressure so one vent is intake and the other is exhaust. Maybe on a perfectly calm day they would both exhaust. Now a ridge vent work similarly. Air blowing over the top of the vent creates a negative pressure area on one side of the vent that sucks the air out of the attic. The replacement air is supposed to come from the soffit vents, thus creating the cold air flow along the rafters that is the design effect. But with the addition of gable vents this make up air will more likely come from the gable vents as intake, not from the more distant soffit vents. That is why it is recommended to do one or the other but not both when it comes to attic venting. And none of this is true all of the time. There are many more variables like orientation of the house, pitch of the roof, design characteristics of each of the vents- alone and in combination, and more. Confused? Join the club.
It is really a complex issue. Most of the time (assuming they are located high at the peak) they will act as exhaust. Depending on the wind, temperature of air and deck temperature who knows at any given moment. On occasion they can act as a short cut reducing ventilation.
"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).
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.
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.
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.
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. MLD
Thanks to everyone for their input. Of course, I am now more confused than ever. :)
So here is my exact situation. I have an older home in a warm, humid climate (Charleston, SC). Currently, I have gable vents and 2 14" wind turbines for ventilation. It is not enough as evidenced by extreme attic temps in the summer and some curling shingles (especially on the western end of the house, which is oriented such that its long axis is roughly NE to SW).
I understand that ridge vents are a great option, but adding them to my roof is cost prohibitive for me at the moment. My thought process was that there is probably little air flow in the lower attic due to all the current venting being up high. Thus, I thought soffit vents would be the answer. It appears, however, that there is a possibility that I could make the situation worse by doing so? Is that right? Should I install soffit vents and block off one or both gable vents? Any further advice would be greatly appreciated.
TIA,
James
"BP" wrote in news: snipped-for-privacy@conversent.net:
On Mon, 23 May 2005 17:52:07 -0400, James Bond scribbled this interesting note:
Composition asphalt shingles on the South and West sides of a house (in the lower latitudes of the northern hemisphere) will always wear faster than those on the other sides of the house because of the exposure to the sun. This is not due to heat in the attic, rather the solar radiation breaking down the organic compounds in the asphalt.
I'd just install the additional soffit vents and be done with it. If, on the other hand, you were to install ridge vents and soffit vents, then you'd want to close off the gable vents because the ridge vents draw from the source closest to their level. Or you could install a thermostatically controlled power vent in one of the gable vents, on the inside, that would force air through the attic and out the other end of the house. There are many, many options.
-- John Willis (Remove the Primes before e-mailing me)
That is right, but it is not common, in most case you will improve the results. A good roofer (not all are good) can tell you quickly with a high degree of accuracy.
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
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. MLD
On Tue, 24 May 2005 01:54:31 GMT, "Oscar_Lives" scribbled this interesting note:
Do a little more reading on the subject. While it is true that you can't have too much properly designed ventilation, the key is "properly designed."
For example, if you have ridge vents, it is best to close off all other vents except the soffit vents, if you want air to be drawn in from the soffits. Why? Has to do with fluid dynamics. There are plenty of sites where you can read up on the issues involved.
Here's one that does a good job if discussing many of the factors involved:
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Have a good read...
-- John Willis (Remove the Primes before e-mailing me)
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