Ventilation Articles

A key point to remember when considering ice dams – the temperature should be the same inside the attic as it is on the exterior of the home if the attic is insulated. 

Ice damming occurs in the following situations: 

1. There is not enough insulation and poor air infiltration between the attic and the living space. This causes warm air to rise and go into the attic. 
2. The warm air in the attic melts the snow on the roof deck. 
3. The melted snow begins to run off the roof. 
4. Once it hits the soffit area it begins to freeze. It freezes over the soffit because the warm air that was under the roof is no longer there and the cold air under the soffit begins to freeze the melted snow. 
5. As the snow accumulates on the roof time-and-time again after each snow fall, the ice dam gets bigger. 

A common misconception heard from homeowners every year is, “I have ice and water shield on my roof – so I shouldn’t get ice dams.” 

Ice and water shield isn’t made to prevent ice dams, it is used to help stop ice and water from infiltration into the roof once ice dams form. It does not prevent ice dams, but rather protects you from ice dams once they form. 

How are Ice Dams prevented? 

Stopping ice dams is simple, in principle: keep the entire roof the same temperature as the eaves. You do that by increasing ventilation, adding insulation, and sealing off every possible air leak that might warm the underside of the roof. Using the Deck-Air, Model DA-4, is a solution for homes with inadequate intake ventilation.

Balance is the key to having an efficient attic ventilation system.  When balanced, a properly designed ventilation system will assure a continuous supply of air moves through the attic space, fighting off heat and moisture. Calculating the correct number of exhaust and intake vents is simple and will ensure you have an effective, balanced system.  

The first step in determining how many vents you need is to calculate the Net Free Area (NFA) required. Most codes use the 1/300 rule for minimum residential attic ventilation recommendations. This means that for every 300 square feet of enclosed attic space, 1 square foot of ventilation is required – with half at the upper portion (exhaust vents) and half in the lower portion (intake vents). This formula is traditionally used for static roof vents which are rated for Net Free Area in terms of square inches. 

Let’s go through an example… For a home with 2000 square feet of attic floor space, you’ll first divide 2000 by 300 (2000 ÷ 300 = 6.66). You need 6.66 square feet of attic ventilation. Since you want a balanced system, you divide by 2 so that half of the ventilation is intake and half is exhaust. Thus, 6.66 ÷ 2 = 3.33 square feet of attic ventilation for intake and 3.33 square feet of attic ventilation for exhaust. Because vents are rated in square inches, you need to convert the square feet required to square inches. This is accomplished by taking the square feet recommended and multiplying by 144(The number of square inches in a square foot). Thus, 3.33 X 144 = 480 square inches of attic ventilation is required for intake and 480 square inches for exhaust.

Once the recommended amount of Net Free Area is known and the type of vents have been selected, you can determine how many vents you will need. The next step is to divide the NFA required by the NFA rating of the vent. In our 2000 square feet example, we determined we needed 480 square inches for intake & 480 square inches for exhaust. For this example, let’s use the Lomanco 750 Slant Back Vent (50 square inches NFA) for the exhaust vents & the Deck-Air DA-4 (36 square inches NFA) for the intake vents. To calculate the number of 750 vents needed, divide 480 by 50 to get 9.6 vents. Rounding up, you would need ten (10) 750 Vents. Because you always want your intake NFA to meet or exceed the amount of exhaust NFA, we will take the amount of exhaust NFA provided and divide by the NFA rating of the Deck-Air. To calculate the number of Deck-Air Vents needed, divide 500 (10 X 50) by 36 to get 13.9 vents. Thus, you would need 14 Deck-Air Vents.  

If your head is now spinning from the calculations, don’t worry Lomanco has the tools needed to determine the number of vents needed (no calculator required!) Use the online ventilation calculator or download the Vent Selector App and skip the math.

You may be curious as to why ventilation is important – ventilation is very important for many reasons.

In order to fight heat and moisture, homes in all climates must be ventilated year-round. 

Heat in unventilated attics may cause temperatures to exceed 150 degrees. This can cause damage to shingles, roof sheathing, and also radiate down into the living area.

Moisture is a home’s #1 enemy, and can cause rot, mildew, mold, paint blister and ineffective insulation. 

It is estimated that over 50% of homes show visible signs of improper ventilation.

Proper ventilation reduces energy bills, winter ice build up, and eliminates mold and mildew, which can lead to major health problems. Ventilating the attic space extends the life of shingles, insulation, and other building components.

Lomanco offers a full line of products to properly ventilate homes. Why choose Lomanco vents? Read our article, Why Lomanco?, to learn the benefits of using Lomanco ventilation products.