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  1. #1
    Join Date
    Jul 2008
    Posts
    33

    3 sq. ft. of ventiliation for 2000 sq. ft. attic?

    I'm a homeowner in Dallas with a 2000 sq. ft. single story ranch style home built in 1993. The A/C electricity bills are killing me. The air handler and A/C ducts are all in the attic, which I've measured above 125 when the outside temp is 100.

    My understanding is that for a ~2000 sq ft attic area, I should have a ventilation ratio of 150:1, or about 13 sq. ft. free vent area (FVA). I have two 9 inch round vents at the peak of the roof which together give me about 1 sq ft FVA, and ten 4x16 inch soffit vents (28 sq in FVA each) which gives me about 2 more sq ft FVA. So I have a total of 3 sq ft of vent area when I should have 13...are my calculations off somewhere or did they build the house with way too little attic ventilation? Is this even in code? If anyone has any thoughts on this, I appreciate your expert guidance. What should I have installed to correct this problem and give me the most bang for my buck? More soffits, a ridge vent, more round vents at the top of the roof, or something else entirely?
    Last edited by soupcxan; 07-18-2008 at 11:27 AM.

  2. #2
    Join Date
    Jul 2004
    Posts
    322
    120+ is not unusual. The amount of venting you describe is also common but may not be adequate. There are many ways to address your problem. I suggest ruling out obvious problems first. If the equipment is in the attic you want to make sure leakage at the ducts and equipment is minimal. Make sure your register boots have a good seal. The pressures are greatest at the air handler; even a small leak on the return (negative pressure) side of the air handler can pull a lot of superheated air into your system. Attic insulation, radiant barrier, better attic venting, cleaning the AC equipment and checking the charge are all possibilities. This forum has lots of info. on all these topics. Keep your blinds and curtains closed. Turn off your computer when not is use. Go with the mini-spiral bulbs. Clean your refrigerator coils. Keep your dryer lint trap and exhaust clean. On the attic venting, make sure the soffit vents aren't clogged. One simple test is to make sure you see light in the attic, along the roof decking, where you know the soffit vents are located. Make sure the roof turbines are turning. At 125 degrees they should turn by themselves even if there is no breeze. If not turning, they come in standard sizes and you can pull off the top and replace it without having to replace the flashing.

  3. #3
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I believe there have been published studies from FSEC (Florida Solar Energy Center) where they tried to compare energy usage with 1:150 and 1:300 ventilation. The surprising things is the study failed to establish any energy savings from better ventilation in that range. May be best to look to other things to reduce your bills. Any duct leakage would be A-Number-One priority.

    Adding roof ventilation is fine, just don't expect it to pay off very well in bill savings.

    Best of luck -- Pstu

  4. #4
    Join Date
    Jan 2008
    Location
    California
    Posts
    108
    Code in Florida could certainly be different than it is in California, but I believe that a combination of 'high' and 'low' vents allows the ratio to go to 300:1, the idea being that rising heat escaping will create passive ventilation in the attic.

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