Techincal question re: soffits and attic ventilation
April 2, 2011 6:23 PM   Subscribe

Technical question for architects / builders / roofers about attic ventilation in a southern zone. I'm renovating a house in south/central Texas (Brazos County), and I need some advice on how to properly ventilate my roof, as the original soffit/gable ventilation now seems quite messed up.

About 15 years ago, someone added roof peak ventilation to the house I bought last year. The roof peak ventilation is quite well installed and I have no problems with it whatsoever -- except that it doesn't seem to form the proper convection cycle that pulls air up through the soffits and out the roof peak, which is essential for proper roof surface cooling and mold prevention in our very hot, humid environment. The house is a 1600 sq ft contemporary ranch with a tri-bearing gable framing.

Using a smoke stick, I traced where the air currents go. Currently, (90 degrees ambient temp outside, full sun on the roof) the house is pulling air in the gable vents (at the peak of the gable), and out the roof peak vents. The rest of the air in the attic is either stagnant or dropping out through various perforations or leak points in the old, rotten siding and soffits. Without the proper convection pattern drawing upwards, the underside of the roof decking exceeds 125 degrees even on a relatively mild day. This means that the A/C is less efficient, the house stays warmer in the evening, and the asphalt materials on the roof will wear faster than they normally would.

The way that I understand it, air SHOULD be drawn in from the soffits, up through the baffles past the blown-in insulation, and out the peak vent. This isn't happening in my house.

The soffits are clear, but most of the house currently has VERY small soffit vents. Due to the age of the original soffit wood (~30 years), I'm having to replace all of the soffits ... some is rotten, some was eaten in a previous insect infiltration. I've chosen the perforation-vented Hardi material in the hopes of increasing the amount of air intake through the soffits (and I'm replacing all of the ancient T-11 siding at the same time with Hardi)... unfortunately, the areas that I've re-done with the perforated Hardi still shows the same convection pattern, with little to no ventilation in or out of the soffits and about 90% of the ventilation in through the gable and out through the roof peak.

Is the best approach in order to increase the CFM of air moving through the attic and achieve the proper convection pattern to:
a) install turbines below the level of the gable to increase the draw from the soffits?
b) block off the gable vents (which used to vent the peak before the roof peak vents were installed 15 years ago) so that air is forced to draw in the soffits?
c) some approach I haven't thought of yet?

I'm afraid that if I just added a fan to force air out the gables (of which there are at least four), air would still just leak in around the fan...
posted by SpecialK to Home & Garden (2 answers total) 2 users marked this as a favorite
My instinct tells you to try blocking the gable vents, especially if you are going to do the perforated panel thing on all the soffits to get more air moving. Hopefully the roof vents can take the full load of exiting air, which I assume they can, otherwise you wouldn't have this problem. In any case its an easy thing to test once you've done it. If you add a fan to the gables I would be a little afraid of reversing the situation you have now and end up pulling air through your peak vent instead of helping create convection.

Full disclosure, I am an architect, but not the homebuilder type so I'm not really any more knowledgeable about this than you I think. Used to live in Houston though.
posted by tmthyrss at 8:09 PM on April 2, 2011

I agree w/tmthyrss; block the gable vents and see what happens. Right now the air is coming in through the gables instead of the soffits because that's the path of least resistance. The currents you're seeing now are probably wind-driven rather than convection. Air isn't going to make the more difficult trip from soffit to peak unless you give it no other choice. Gable vent fans would just reverse the current bad situation unless you blocked the ridge vent.

I live in Ohio and am not a professional builder, but I have done a little bit of construction work and have also tweaked the attic ventilation systems in a few local houses.
posted by jon1270 at 3:18 AM on April 3, 2011

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