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Principles & Calculations of Attic Ventilation

Principles & Calculations of Attic Ventilation

Attic ventilation has several purposes, as we will soon reveal. However, setting up a proper venting system can prove problematic without the right knowledge beforehand as there are factors you must take into consideration and specific calculations that must be done to fully benefit from the setup and keep structural damage and other issues at bay.

How year-round attic ventilation is beneficial

During warm weather

During summer, the upper rooms of the house always feel warmer as opposed to the ones placed on lower levels, which ultimately makes homeowners turn on their fans or air conditioning systems for longer amounts of time, ultimately spending more money when the electric bills come.

Normally, hot air tends to rise as it is lighter, while cold air falls due to its denser structure. However, when there isn’t proper attic ventilation in check, the heat builds up in this space, it has a downward migration, and radiates to the floor, which ultimately leads to higher temperatures in the rooms placed directly underneath it. But this isn’t the only issue at hand as, with the passing of time, excess heat can cause shingles to deteriorate, roofing materials prematurely failing as a consequence.

During cold weather

Temperatures might plunge when the cold seasons arrive, but this doesn’t mean that heated air movement can’t cause problems in the attic anymore. When temperatures drop, conditions reverse, meaning that instead of the heat from the attic to travel into your home, the heated indoor air travels from the living quarters to the attic along with moisture.

As the warm, moist air moves in this space where the air is cool and dry, moisture condenses and drips on the insulation below, which causes insulation to reduce in effectiveness. Thus, a more severe sequence of events unfolds, colder rooms witnessing higher heat loss, while the furnace has to pump more, which implies higher energy bills as well.

But not all the condensing moisture drips into the insulation, some of it getting absorbed by the structural elements of the house, which means that wood will rot faster and roof materials will deteriorate as time passes. 

The excessive amount of moisture in the air has direct repercussions on both the structural integrity of your home, as well as your health. With humidity on upraise, proper conditions are created for the appearance and spread of rot and mold. While proper ventilation measures aid solve the issue to some extent, it’s recommended to combine ventilation with the use of a dehumidification system that pulls the excess moisture from the air, aiding to enhance efficiency and keep humidity within the safe 45% – 55% range.

Dehumidifiers vary in size and power, but crawlspace dehumidifiers prove to be most efficient when it comes servicing toward a house’s structural protection. Keep in mind to use a dehumidifier that handles the size of your home and existing conditions. For this, you should check the buying guide put together at Popular.Reviews as it can help you figure out exactly what aspects to look into so that efficiency is granted by the dehumidification system.

 Principles & Calculations of Attic Ventilation

How different types of venting work

  • Natural convection: A passive venting method that uses the natural buoyancy of air as it provides inlets and outlets low and high on the roof. When the air in the attic is warmed, it becomes light and it rises, allowing it to escape through the high vents. As the warm air escapes the enclosed space, cool air comes in through the eave vents.
  • Bernoulli principle: This principle demonstrates venting via wind effect as wind can create positive pressure on the windward side of the roof while creating negative pressure on the protected side. Thus, a sufficient air stream across an opening makes enough pressure difference to draw the air out of the attic space.
  • Power venting/Attic fan: An active/mechanical ventilation approach which is accomplished in two different ways:
  1. In homes where there is no air conditioning, you can control the temperature in this space to a certain extent with the use of attic fans. Usually, people opt to mount the fans on the ceiling in a central hallway. The outdoor air is pulled inside through open windows and it is exhausted through the attic. Just make sure that there are enough outlets so that the fan doesn’t confront absurdly high pressure. However, for more cost-efficient ventilation you can go solar, as Optima Institute comparison between solar attic fans and electric models highlights, literally no costs being added to the power bills through the use of the Eco-friendly alternative. There’s the added perk that you could benefit from a tax credit provided by the federal government, so you might have more gains than initially expected with this variant. 
  2. In air-conditioned homes, power attic ventilators can be used if exhaust fans are mounted through the roof. Make sure that there are inlets in place for the ventilating air at opposite ends of the attic, or low-mounted roof vents if you don’t have an overhang.

Calculation formula

There is a generally agreed upon formula that you can apply for balanced attic ventilation, known as the 1/150 rule:

  1. It states that for every 150 sq. ft. of attic space, you need 1 sq. ft. of ventilation. Thus, you first have to calculate the square footage of the attic if you don’t already know it, and to do it you need to measure three values – the length, width, and height of the space.
  2. For the intake and the exhaust ventilation, every square foot of ventilation is divided by 2.
  3. Multiply the resulting number by 144 to have the result in square inches and you are done.

Here is an applied example for 3000 square feet of attic space:

3,000 / 150 = 20 square feet total

20 / 2 = 10 square feet/intake and 10 square feet/exhaust

10 x 144 = 1,440 square inches/intake and 1,440 square inches/exhaust

After you make the needed calculations, you can head on and confidently acquire attic fan ventilators without worrying about underperformance.

Rules and specifications

  • Make sure that the attic does not communicate with the conditioned space if you intend to add ventilation for this space as you diminish the effect of air conditioning indoors, which ultimately leads to the AC working harder and more in vain as the air mixes with the vented attic air.
  • Check and make sure that the ceiling air barrier is continuous and that there is no leakage. Evidently, if there are any leaks, fix them immediately.
  • Make sure that the vents are placed low and high on the roof.
  • Make sure that all mechanical ventilation ducts and plumbing stacks are venter outdoors.