Howdy all. I am hoping to get some advice about attic ventilation, specifically the importance of the ratio between inflow at the soffit and outflow at the ridge vent.
I live in Austin, TX, and the square footage of our house is 2070 sq. ft. The house was built in 1965, when the idea of having adequate insulation and ventilation were apparently mere flights of whimsical fancy. The original amount of venting in the attic was less than 1 sq. ft. But we just had a new roof installed, and had better insulation and radiant barrier decking put in. In addition, the new roof has a ridge vent, and I am putting in vents along the soffit to create some better circulation.
From what I have been able to gather, the appropriate (or at least minimally adequate) amount of ventilation for an attic of about 2100 sq. ft. should be 7 sq. ft. or 1008 sq. in., divided equally between inflow and outflow. The NFA of the intake ventilation is now 582 sq. in. The ridge vent, however, is 75 ft. long, and thus has an NFA of 1350 sq. in. I have seen in several places that the intake ventilation should be equal to or slightly greater than the amount of outflow ventilation, but I have not been able to find out why this is so important. Common sense would suggest that the total amount of circulation cannot be greater than the amount of inflow, so the extra amount of ridge venting would be superfluous. But is there some other, potentially more troubling problem with having more more ridge venting than soffit venting?
Any thoughts or advice on this issue will be welcome.
Thanks!
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I have seen in several places that the intake ventilation should be equal to or slightly greater than the amount of outflow ventilation, but I have not been able to find out why this is so important.
Each of the four model building codes—The BOCA National Building Code, International Building Code, Standard Building Code and Uniform Building Code—require attic spaces to be ventilated. Generally, building codes require that a minimum net free ventilating area for attic vents be a 1-150 ratio of the area of the attic space being ventilated. The four model building codes also generally allow for a reduction of the ventilation ratio from 1-to-150 to 1-to-300 if attic vents are balanced on a roof and/or a vapor retarder is installed on a ceiling assembly's warm side.
NRCA recommends designers of steep-slope roof assemblies provide attic ventilation by using static, balanced ventilation systems with a minimum amount of 1 square foot (0.09 m2) of net free ventilating area for every 150 square feet (28 m2) (1-150 ventilation ratio) of attic space measured at the attic floor level.
A balanced ventilation system uses ridge vents or provides about one-half of the total ventilation area at a roof assembly's low points, such as soffit vents, and the remaining area at or near the ridge. Balancing ventilation in this manner allows for air intake to occur at the low points and exhaust to take place at high points. Air movement from low to high points is aided by natural convection.
In place of balanced ventilation systems, forced or mechanical ventilation may be appropriate. Ventilation measuring 1 cubic foot per minute per square foot (0.3 m3/minute/m2) of attic space at an attic floor effectively is equivalent to the 1-150 ventilation ratio for static ventilation.
For large-volume attics, such as attics with roof slopes greater than 8-in-12 (34 degrees), consideration should be given to increasing attic ventilation.
OK - there's your goals to shoot for. Then there's real life applications which can't always be made to fit the codes within the realm of common sense. Very seldom do attic venting systems match those perfectly balanced specs.
It would be silly to install less ridge vent just to balance it to the available soffit vent. Get as close to the code as possible. More ridge vent than soffit vent is not the end of the world, but ridge vent with no inlet will perform poorly if at all.
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Seeyou:Thanks for getting back to me so quickly. As you point out, the real world and what's practical often don't match the code. I am currently installing 55 undereave vents to provide the inflow. For each one, this entails cutting a hole in the soffit siding, sticking my drill up through the jagged hole to cut another hole through the frieze block, and then screwing on the vent. Our house was clearly a spec house, built on the cheap, and no thought was ever given to having to do any renovations or repairs down the line. Since our roof has a pitch of 2:12, the only time we could install batts in the attic was when the roof and decking were removed. Actually, using the term "attic" is perhaps being overgenerous.So… if there is no potential problem (other than reduced performance) from having more ridge vent than soffit vents, I probably won't go beyond the 1:300 ratio. It's hardly ideal, and arguably not even adequate, but it is still much, much better than before.Thanks again.
As you say, having an unequal volume of venting at the ridge causes no harm other than reducing performance. Do what you can do, it is better than it was.
Did you manage to install baffles to keep the insulation down at the eaves?
if there is no potential problem
Other than the can of worms of how much 99º air do you let into an attic to "cool" it question. (Even more fun over here in the Brazos Valley, with a pair of rivers and geography to hold in high humidities, too.) From my perspective, your question is "I've opened two windows a 1/4" on the down windside, and I have only one window i nthe windward side open 1/8--will opening it to a 1/4" make a difference?" (Did I mention a can of worms?<g>)
Which is mooted a bit by that 2/12 pitch. Are there any hvac ducts up there? That's going to be a very shallow space up there.
I'll guess that there's not a lot of depth of insulation between the ceiling joists, either (since that would be as hard to place as soffit vents).Occupational hazard of my occupation not being around (sorry Bubba)
No biggie if you can't "balance" things. Just do the best you can. Try to avoid "dead" spots where no air will be flowing.
Thanks guys! This is the information I was looking for. I couldn't think of any reason why having more outflow venting than inflow venting was bad (other than it being useless), but it's nice to hear from the experts. In response to some of your questions:Rather than using baffles, we simply avoided placing insulation along the edges and the frieze block. Hardly a good solution, but to do otherwise would have involved trying to attach the baffles before the pieces of decking were placed. If I had done the roofing myself, I probably would have done this, but trying to explain this to the guys who were laboring in the August Texas heat (and whose command of English was only moderately better than my command of Spanish) was not practical. However, they did come up with the commendable idea of rearranging some of the loose rock wool in the attic along the edges to provide some minimal amount of insulation. I did like the comment about trying to "cool" the attic with 99° air. :) But before the new roof and venting, the average temperature in there was easily 120° or more. Since I not infrequently go up there to run new wires or fix some other problem, even 99° seems like a godsend.As for the ductwork, there is HVAC up there and it does undoubtedly leak somewhat. It has been patched as well as possible, but that is not saying much. Because of the 2:12 pitch and the ducts, trying to move around up there is a job for midget contortionists. Thanks for the feedback!