We want to encourage all of our customers to vent…

Filed in Home Improvement, Roofing by on January 4, 2016


…their roofs, that is!

Proper roof ventilation is an essential part of any healthy home and also helps with a home’s overall energy efficiency. Additionally, adequate roof ventilation, in conjunction with proper placement and quantities of insulation, goes hand in hand with ice dam prevention.
On average, a person loses 800 milliliters of water per day due to breathing and evaporation from their skin.  That’s just over 1 gallon of water per person every 5 days.  Cooking, showering, humidifiers, steam heating systems, indoor fountains, aquariums, damp basements and even toilet bowels also contribute their share of water vapor into our homes.
When moisture collects in a home it can be disastrous.  If left unattended, moisture is the perfect breeding ground for mold.  Airborne mold spores can be extremely toxic.  They can cause respiratory and skin related problems for household members and, if ignored over a long period of time, may result in damage to the home’s structure.

Under the direction of the Department of Housing and Urban Development (HUD), FHA approved loans require that a qualifying property must meet minimum airflow requirements of net free ventilation area in attic spaces.*

The minimum airflow requirements are expressed as a ratio of square feet of ventilation to square feet of attic space.  That ratio is 1 to 150 (expressed as the 1/150 rule).  State and local codes may impose their own requirements, but in no case can they be less than the Federal guidelines.*

Fortunately for us, this ventilation calculation is not as daunting as it may seem.  The example below should help to illustrate the requirements and show you how to apply it to your own home.

How do I calculate my roof ventilation requirements?


Measure your home to determine Net Free Air (NFA) needed.  The home illustrated to the left measures 40’ long by 20’ wide.  The area of the attic space would simply be the length multiplied by the width. 40 feet X 20 feet = 800 square feet. NFA is determined by dividing the total square footage by the 150 minimum requirement, per the 1/150 rule. 800 sq. ft. ÷ 150 = 5.33 sq. ft. of NFA required.


Balancing the system.  This amount of air flow must be equally divided between the total intake at the eaves and the total exhaust at the ridge vent. 5.33 ÷ 2 = 2.67 sq. ft. intake and 2.67 sq. ft. exhaust.

Convert NFA requirements to square inches.  Manufacturers of roofing shingles and accessories produce a variety of intake and exhaust mechanisms to satisfy airflow requirements.  These are rated in terms of net-free square inches of airflow.  Therefore, we need to convert our airflow requirements from square feet to square inches.  Since 1 square foot is equal to 144 square inches, we multiply the square footage by 144 to convert. 2.67 square feet X 144 square inches/1 square foot = 385 square inches (rounded up).

Select the system components.  In many cases your home’s architecture and construction will dictate what components can or should be used.For ridge exhaust venting, we typically use GAF’s Cobra Snow Country ridge vent, rated at 18 square inches of net-free airflow per linear foot.There are far more choices for intake vents, in many shapes, sizes and styles.  For this example we’ll go with a 4” x 16” rectangular vent, rated at 26 NFA.

Calculating quantity of vents needed. Now that we know the rated NFA for our intake and exhaust vents, it’s a simple matter to figure out how many we need of each. For the exhaust vent with a rated NFA of 18 per lineal foot: 385 square inches X 1 Linear foot/18 square inches = 22 linear feet (rounded up)

Ridge vents come in 4 foot sections and wherever possible, we always want to round up.  Therefore, we’ll install a minimum of six, 4’ sections, for a total of 24 linear feet.
This works out well, since our example house is 40 feet long.  We’ll have ample room to exhaust the house along the ridge.

For the intake vents with a rated NFA of 26 each:385 square inches X 1 vent/26 square inches = 15 vents. However, it’s important that the exhaust ventilation does not exceed the amount of intake ventilation.

Since our sample house is a symmetrical gable roof, we’ll want to even things off and put a minimum of 9 intake vents on the eaves on the front and rear of the house for a total of 18 vents.  Note that our end result provides 432 NFA at the exhaust vent (24’ x 18 NFA/L.F.) and 468 of NFA at the intake vents (18 vents X 26 NFA/vent).  Remember that the ventilation requirements are a MINUMUM and exceeding those minimums is perfectly acceptable.

Handy Shortcut:
Attic sq. ft. ÷ 2 = sq. in. of intake NFA and exhaust NFA needed @ 1/150 ratio.
Using our previous example:
800 ÷ 2 = 400 sq. in. of intake NFA and exhaust NFA needed @ 1/150 ratio.
The longhand method gave us 385 square inches of NFA for intake NFA and exhaust NFA needed.  This shortcut method overestimates the requirement slightly, but that is perfectly okay.
We’ve now met the net-free airflow requirements for our sample house.  In many cases calculating NFA may not be so straight forward.  Roof styles, architecture and construction will often play a critical part in determining how best to ventilate a home.
When considering hiring a roofing contractor there are many things you should ask them.  One of them should definitely be if they’ve taken into consideration NFA requirements and how they will address this in their estimate.   If they give you a blank stare it’s time to get another contractor.

For your free evaluation contact Ready Pros, Inc.

1 Refer to HUD Handbook 4910.1, sub-section 403-1.2, titled ‘Attic Spaces’ for an explanation of the rule and its exceptions that allow a 1/300 ratio.
2 Massachusetts building code requirements for ‘Attic Spaces’ are covered in 780 CMR, section 1203.2 and also require the ratio of 1/150 with an allowance of 1/300 under the specific exceptions noted at the end of the same section.

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