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Why You Shouldn’t Mix Attic Ventilation

Why You Shouldn’t Mix Attic Ventilation

The importance of attic ventilation cannot be understated.  A brand new a perfectly finished roof will still rot from the inside out if there is not enough ventilation.
In the Pacific Northwest, ventilation is even more critical because of the cold temperatures and large amount of moisture.  When replacing an old roof, it is common to find most of the wood sheathing structure of the roof deck to be damaged from condensation caused by lack of attic ventilation.
The roofing industry has used many different tools over the years to try to solve this issue, and we have settled on a very effective method.  But when that method is strayed from, ventilation problems can return.
It may make intuitive sense that when there isn’t enough ventilation, to just add more.  However, depending on what is added, the ventilation can be made worse.
The most common 5 types of ventilation you’ll see are:

It is difficult to easily classify each of these as exhaust only or intake only, because depending on construction, most vent types can act as one or the other.
The key for proper attic ventilation is these 4 main principles:

  • Have only one exhaust type and one intake type
  • Never have more exhaust than intake
  • Make sure intake is at the lowest point of the attic and exhaust is at the highest
  • Have enough total ventilation

Attic ventilation only works right when the entire attic is vented.  If there are dead spots of air, then problems can still arise.  Mixing ventilation types can cause dead spots.
For example, a great strategy is to use continuous ridge vent as exhaust and bird blocks as intake.  This allows outside air to flow from the lowest edge of the attic up to the highest peak, as long as there are no obstruction inside the attic (like insulation blocking vents).
However, it’s not uncommon to see a roof with those two vent types have a third type as well: gable end vents.  Adding this third type will disrupt the flow of air in the attic, where the gable vent will act as either intake or exhaust, depending on the circumstances.  Then dead spaces of air will exist in the attic and lead to potential rot on the wood structure.
Another strategy you may see from time to time is louver vents along the peak of the roof as well as in the middle.  Even if the amount of intake ventilation is perfect, this will still result in air flowing in and out of only half the roof much of the time, from bottom to middle or from middle to top.
While there are many considerations necessary for ventilating an attic so that it doesn’t lead to rotten wood sheathing, if you use only one type of intake and one type of exhaust, you’re on the right track.
Please call us now ((253) 544-3651) or fill out our quick estimate form if you need a roof replacement and want to make sure your attic will be vented right and won’t lead to big headaches down the road.

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