Is It Possible to Over-Vent a Roof?
In regards to roofing ventilation, at first glimpse the common thought would be, “the more the merrier.” After all there’s nothing quite as relaxing as opening all the house windows and getting fresh air on a beautiful Spring day. It’s the practice of; “you can never have too much air-flow” that many roofing contractors are following when re-shingling a roof – leaving the old vents while also adding ridge cap ventilation. The truth is, once an attic has achieved a certain amount of oxygenating adding extra has no significant advantage and can in fact be detrimental.
The Reason For Ventilation
It’s easy to understand why an attic or roof deck would need some air flow in a hotter climate. The attic becomes a sauna that traps rising heat and wreaks havoc on a home’s thermostat, constantly telling it to drop the temperature which runs the A/C constantly. Surprisingly enough roof ventilation is also important in cold and wet climates to prevent the building up of ice dams from melting snow and to disperse any moisture that has risen from the living quarters.
So, What if You Don’t Vent?
Putting off any roof damages or changes to utility bills for a minute, the most important reason to vent a roof is because it may be a requirement from the roofing manufacturers to maintain the warranty of their product. If the manufacturer finds out you’ve failed to vent your attic and roof properly they’ll find you negligent and will likely be off the hook for any recourse even if the material fails four years into a 30-year warranty. The reason that the manufacturer isn’t liable is because improper venting directly contributes to the roof failing as temperatures that reach 150° and higher damage shingles and underlayment and also trap moisture which rots the plywood underneath. The health effects and utility costs are damaging as well as mold and mildew forms and hot air is forced back into the home which causes higher A/C costs.
How Much Ventilation Is Recommended?
Installing a ridge vent doesn’t immediately solve the problem of poor roof ventilation. Think of your attic like you would a car engine – it needs a good supply of air for combustion to operate but also must disperse that flow through an exhaust. Warm air will simply sit in your attic unless a source of flow is created which is typically done with soffit ventilation. Air enters the attic underneath the soffit or somewhere close to the lowest edge of the roof and cycles out near the top in a loop. According to the Federal Housing Administration 240 square inches of net free vent area is recommended for every 1000 square feet of attic. Also, the length of the intake (soffit) should be greater than or equal to the amount of exhaust (ridge).
The job of the intake vents is to re-supply the air that the exhaust vents are releasing at a constant basis. If all the air is leaving before replacements can arrive than the attic becomes stagnant and the atmosphere heats up bringing with it moisture, thus negating the intentions of the ventilation to begin with. It’s best to think of attic ventilation as a two-part system – air comes through the soffit intake and out the ridge exhaust. When you start to add a third party like static roof vents or metal domes the system is interrupted and shortcut. Too much air flow can also bring with it debris or can push around insulation.
In summation, installing extra ventilation on a roof is simply counter-productive and in most cases damaging. By avoiding excess you save the installers time and ensure that your attic air-flow is a cohesive, well-designed ‘system’ that operates as intended.