I live southeast Texas and have similar heat levels to contend with. When you live this far south, attic ventilation takes on a whole new complexion. Further north, attic ventilation is primarily for moisture control and to a lesser degree heat evacuation. When the temperatures hover between 95 and 105 for weeks on end, the levels of ventilation used in northern climes don't cut the mustard. If you investigate houses built before the common use of air conditioning you will find larger eave vents (huge by todays standards) and large gable vents which moved much more air volume than turbine vents or ridge vents. Two basic principles to remember when dealing with large scale heat removal are 1) the more air removed, the more heat removed (when you sweat do you cool down faster with a light breeze or a gale?) and 2) larger openings move more air.
As an example, take a 1500 sf house 25' x 60' with a 6:12 gable roof 60' long. The attic volume is about 4700 cubic feet.
Assume the house to be fitted with three turbine vents on the roof and 30 soffit vents (4' centers both eaves). The soffit vents let in the cooler air and the turbine vents let out the heated air. A 12" base dia. turbine vent is 0.78 sf x 3 each = 2.34 sf. The soffit vents are usually 4"x10" (or smaller) stamped vents with about 25% free area which, for 30 vents, translates to 2.00 sf. The smaller of the two areas is the controlling opening size. Thus there are 2 sf of area to ventilate 4,700 cubic feet of attic. If the heated air is moving at the velocity of 2 feet per second (generous) the system will remove 240 cfm of air. With the attic volume of 4,700 cf divided by 240 cfm, it will take 19.58 minutes for one air change (3 air changes per hour). That is not going to cool the attic very well.
Ridge vents advertise openings of 12 to 20 square inches per linear foot. Converting, 20 si = 0.139 sf x 60' =8.33 sf for the entire ridge. That is nearly four times the area of the 3 turbine vents! Remember, equally large (or larger) soffit vents areas must be provided to supply the air for the ridge vents.
Traditional gable vents are 10' to 12' long with about 60% free area which calculates out to 21.65 sf per vent. With one vent at each end of the roof you have 43.3 sf of vent area. That is FIVE times greater than the ridge vent. NOW we are moving some air and heat!
To cut costs, builders have been installing ever smaller vents until, in the past decade, many houses have been built with no vents at all. In the 80's and 90's many builders used 2" dia. aluminum disk soffit vents with stamped slots totalling 0.5 square INCH of area. At every 4 feet that would be 0.10 sf for the whole house! That is so insignificant as to be criminal.
If you want to cool your house, install anything you like as long as it has large FREE area to pass large volumes of air. Don't be taken in by slick advertising. Remember that FREE area is much less than the NOMINAL size of the vent. What works ok in Illinois isn't necessarily sufficent for south Texas. Learn from the old homes in your area (1800's-1940's). They HAD to have better ventilation, wereas we just let the AC run longer.
Also remember that building codes in Texas now establish minimum ventilation standards. Note the word MINIMUM. This does not mean that the sizes required are high performance, it means that they are the LOWEST that the code will allow.
Regards |
Member Since
06/24/2004
Total Contributions
214 Posts
|
Moderated by Handyman 
