Roof & Attic Ventilation

In today’s world with advanced building standards and products keeping homes sealed, airtight and energy efficient, what more could you do to improve and save on energy? The answer may lie within your attic.

What effect does improper ventilation have on your home and roof?

The left side of the picture above shows improper ventilation

  • The excessive heat may enter your living space forcing your air conditioning to run more often and at a higher level
  • Moisture/humidity buildup in northern climates during the winter months can contribute to rot, mold, mildew and poor air quality
  • In addition, the freeze thaw cycle may cause water stains to show up on your ceiling
  • It may cut the life of your shingles by 10% due to the excessive heat
  • May cause the decking to prematurely deteriorate due to heat and/or moisture buildup
  • Increases the chances of ice dams
  • Two different types of exhaust can “short-circuit” the ventilation system meaning, one may try to act as an intake and weather filtration issues may arise
  • In addition, two different types of exhaust will not properly ventilate the attic space

Now that we have looked at some of the ill-effects of improper ventilation, here’s what it means to have proper ventilation:

  • Effective and proper ventilation occurs when you have intake vents located at the eave (low part) of your roof and have exhaust vents located near or at the ridge (top part) of your roof

The intake vents allow cool, fresh air to flow into the attic at the lowest point. The exhaust vents help to flush out the hot/moist air located up high along the underside of the deck. This constant flow of cool air coming in at the low point as hot air is being forced out at the high point is commonly referred to as the “stack effect”.

  • The most effective ventilation system provides a balanced system of intake and exhaust

What do we mean by “balanced”?

Constant flow – providing enough intake as well as exhaust so that air is always moving

Calculating the proper amount of intake and exhaust to provide a balanced solution:

There are formulas available to calculate the exact amount of intake and exhaust needed for any given home/attic.  However, the easiest way to do it which will provide you a balanced solution is to divide the square footage of your attic by 2. The solution is referred to NFA (Net Free Area). See below:

Attic Dimension: 60 ft. * 25 ft. = 1,500 sq. ft. … 1,500 sq. ft. / 2 = 750 sq. in. of NFA

Meaning, you will need 750 square inches of net free area at the low point (eave) as well as at the high point (ridge).

Air Vent’s picture below indicates a way to achieve the balance by having 18 NFA/ft. at the ridge and have 9 NFA/ft. on each side of the eave (18 NFA/ft. total).

Effectiveness of different types of exhaust ventilation:

5 Wind Turbines = 15 Roof Louvers = 42 ft. of Ridge Vent

Do’s & Don’ts of Ventilation:

DO: Make sure there is an intake as well as an exhaust
DO: Use the correct amount of net free area needed in ventilating your home
DO: Be sure the intake and exhaust are installed properly (example: make sure the holes are cut for the intake vents)
DON’T: Install two different types of exhaust (such as a ridge vent with a power fan, it can “short-circuit” the system)
DON’T: Have the intake vents blocked (example: painting over the vents)
DON’T: Place an exhaust system at the eave (low point of roof) to try and act as an intake, it is not designed for this use
DON’T: Just assume that every house needs a ridge vent for ventilation, each situation is different, do your due diligence in finding an experienced, reputable contractor

For questions, concerns or further information about ventilation please give John T. Morgan Roofing in Roanoke, VA  a call at (540) 342.7498 or contact us here.