Tom Tamlyn
President
R.H. Tamlyn & Sons, Inc.

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Properly Balanced Attic Ventilation Key to Energy Efficiency

With consumers getting hammered by soaring cooling and heating bills, the focus is on building more energy-efficient homes. While the total package includes building a tightly sealed envelope, excellent insulation, a radiant barrier such as TechShield, low-E double pane windows, etc, certainly a well-ventilated attic can play a part in lowering utility bills.

A properly balanced attic ventilation system, defined by ridge vents coupled with continuous soffit vents, will:

• meet the new International Residential Code requiring ventilation be balanced 50/50 (set to become the statewide building code for Texas in September 2001)
• lower summer attic temperatures by as much as 24 degrees which means the air conditioning equipment operates in a friendlier environment
• meet shingle warranty requirements (the typical shingle warranty states that ridge vents should be balanced by at least 9sqin of intake venting per foot at the soffits)
• meet ridge vent manufacturer ventilation requirements
• make living areas more comfortable which will reduce air conditioning usage/cost
• naturally decrease moisture which harms insulation effectiveness (without installation of expensive dehumidifying systems---just another system to maintain)
• extend shingle life and is a big factor in how long the roof will last
• protect against ice dams in northern climates
• lower utility bills an estimated 2%

All this benefit for only a few hundred dollars!

The importance of a properly balanced system is hard to exaggerate. Experts say inadequate intake ventilation causes 95% of all ventilation problems and will typically void the shingle warranty. So what is balanced ventilation? Balanced ventilation can be defined as equal amounts of air per foot coming in through the continuous soffit vents and going out at the ridge vent. (Forgive the assumption that most ventilation and efficiency experts consider this the best form of attic ventilation; there are certainly other products available to use such as power vents, 8x16 soffit vents, etc but the continuous ridge/soffit combination is considered most efficient because it leaves no areas unvented). Since the typical ridge vent has 18 or more sq.in./' of net free area, a balanced system would use a continuous soffit vent of 9sq.in./' or more (steep pitches/large attic areas will require more soffit venting). All things considered, more air coming in at the soffit will allow the ridge vent to do a better job of removing hot air at the peak. Note: If the ridge vent has more air flow than the soffit, the ridge vent could backdraft and pull hot air, debris, rain, etc into the attic rather than expel as designed. More air flow will create more air exchanges which will serve to lower attic temperatures, akin to lowering car windows or opening the front and back door of a house, creating a strong pull-through of ventilation.

So how does this stack up in the real world? An example of a simple gable design home with a 40' ridge: ridge vent net free area = 40ft x 18sqin/'nfa = 720 sq.in. of net free area; therefore, you need at least 720 sq.in. of net free area coming in at the soffits. Soffit vent net free area = 40ft(2) x 9sqin/' NFA = 720 sq.in. of net free area, so the system is balanced. What's wrong with this picture? This assumes an ideal world where no insulation blocks the air of the soffit vent and you can run the soffit vent the full length of all available horizontal soffit areas. But builders typically break up the front of the home with design features, leaving maybe half of the front or less available to ventilate. In this situation, as long as half of the front could be ventilated, you can still balance with a 12.7sqin/' NFA continuous vent; the calculation would be (40 + 20) x 12.7 = 762 sq.in. of net free area. Having more air flow at the soffit than ridge creates no problems. These net free area calculations are based on .030 vent material thickness as standard. Be careful with products providing airflow resistance inherent in product designs. We recommend running ridge and continuous soffit vents from end to end in all available horizontal areas for best aesthetics and ventilation (stop ridge vent cut channel 6" from end).

We sincerely hope this helps builders and designers determine proper ventilation for their homes and maybe save homeowners a few bucks this summer. We have much more information on ventilation and also have ventilation calculators available free to all in the industry. Please consult building officials, ridge vent manufacturers, shingle vent manufacturers and others for their feedback on this important topic.