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Frequently Asked Questions:
Steep Slope Ventilation

General Questions: Why Ventilate & How Vents Work

Frequently Asked Questions: Steep Slope Ventilation

Q: Why is proper attic ventilation so important?
A: Attic ventilation serves two main functions: to lower attic temperatures and remove excess moisture from the attic. 

Q: What are the benefits of proper attic ventilation?
A: Lowering the temperature in the attic can help reduce air-conditioning energy costs and roof deck temperature, optimize the service life of a roof covering, and minimize ice damming. Removing excess moisture reduces the possibility for mold and mildew growth and minimizing the potential for wood rot. 

Q: Why is Marco Industries launching a Steep Slope Ventilation Division?
A: To focus on issues of heat buildup and excess moisture in attics with poor ventilation. Marco is leveraging its considerable manufacturing, engineering and design capabilities to address the many costly issues plaguing the ventilation of shingle roofs.  The realities of an aging housing stock combined with newer highly insulated and sealed homes; means the challenges have never been greater. These issues have not been adequately addressed by an industry noted for low innovation.

Q: What are some of the effects of excess heat from inadequate or no ventilation on a shingle roof? 
A: Extreme to complete granule loss, blistering on the shingle, shingles curling up on the end, brittle shingles, premature dry-out, cracking and fracturing throughout the shingle.

Q: What are some of the effects of excess moisture from inadequate or no ventilation on a shingle roof system?
A: Condensation causing the deck to swell and waviness and buckling the deck and shingles. Water can rot the roof deck, destroying its ability to carry weight loads and its nail-holding capability. Condensation can drip onto the insulation, reducing its effectiveness and possibly seeping through to the ceiling below. Impact – Damp insulation causes wood rot, mold, and mildew, leading to poor indoor air quality.

Q: How can I prevent ice dams?
A: Ice dams are barriers to water runoff on the roof. They usually form at the roof edge, just above the gutter, in cold, snowy climates. They form when snow melts on warmer areas of the roof, due to inadequate ventilation, inadequate insulation and inadequate sealing from conditioned spaces below. So, provide adequate and balanced attic ventilation, properly insulate the attic and ensure that the ceiling below the attic is sealed. 

Q: What is the Net Free Area (NFA) also known as the Net Free Vent Area?
A: The total unobstructed area through which air can enter or exhaust a non-powered vent; typically measured in square inches. This is shown as the NFA for a box or slant back vent or the rating is square feet of NFA per lineal foot in the case of rolled vents.  The NFA required by code is determined by the total square footage of the floor space of the attic to be ventilated. The bigger the attic floor space the more vents that are required.

Q: How do I determine how many exhaust or intake vents are needed for a roof as required by code?
A: Use our online vent calculator at It’s all based on the size and square footage of the attic. Building Code IRC 2021 requires; 1 square foot of Net Free Area for every 150 square feet of attic floor space (1/150). Attic floor is defined as length x width floor of the attic. In certain Climate Zones 6-8 (more northerly regions of USA) the use of a vapor retarder reduces the requirement to 1/300. 

Q: What is the math for calculating the number of vents?
A: First divide square feet of attic floor space by 300 providing the total NFA between intake and exhaust required. The total NFA as calculated is then further divided in half each for determining exhaust required and intake required. That total is then divided by the NFA rating of each vent to determine the number of vents required.

Q: Is using various exhaust vent types such as roof louvers, wind turbines, gable louvers, and power fans on the same roof with a common attic recommend?
A: Python does not recommend mixing two or more types of exhaust vents on the same roof and common attic because it could result in short-circuiting the attic ventilation system. This application inevitably leads to installation of exhaust vents at more than one level on a roof allowing the upper exhaust vent to inadvertently pull air in from lower exhaust vents rather than from the intake vents properly located near or at the eave at the bottom of the roof.

Q: What are the two methods used to ventilate attics?
A: Static and mechanical. 

Q: How do static vents work?
A: Static vents rely on convection which is warmer air rising after it enters the bottom of the roof at the eave and exits vents at or near the top of the roof and on outside wind which creates a low pressure or vacuum above the ridge helping to draw warm moist air out of the attic. 

Q: How do mechanical vents work?
A: Mechanical vents use power whether hard wired or solar powered to move air. A power vent with an airflow rate of 1.0 cubic foot per minute per square foot of attic space measured at the attic floor is generally considered to be equivalent to 1 square foot of static venting for every 150 square feet of attic floor space.

Q: What is meant by a balanced ventilation system and why is it so important?
A: A properly designed ventilation system requires balance between the intake and exhaust vents to achieve the desired air flow capacity. In general, the net-free area of intake venting should be equal to or greater than the net-free area of exhaust venting. Net-free area is the total unobstructed area through which air can enter or exit a vent, measured in square inches.

Q: Can there be too much intake ventilation especially if it exceeds the total Net Free Area of exhaust?
A: The exception to the balance rule is at the soffit in passive systems. Since the air that enters at the soffit acts to push out moisture and warm air, having extra soffit ventilation will not create an off-balance system

Q: What is the pitch (also known as “slope”) of a roof and how does it impact the use of vents?
A: The pitch is the measure of how steep a roof is. If a roof is “4 in 12″, the roof rises 4 inches for every horizontal run of 12 inches. Vents are designed to operate within a range of minimum to maximum slopes, so make sure that the vents selected are suitable to the slope of your roof.

Q: What are the drawbacks of Turbine Vents aka Whirlybirds?
A: Roof turbines need winds of at least 5 to 6 miles per hour to activate and spin the interior blades, which means that they won’t be effective on days with a slight breeze or no breeze at all.

Q: What are the benefits and drawbacks to gable ventilation?
A: Gable vents provide intake that also functions as an exhaust system. Unlike vertical ventilation, gable vents utilize horizontal cross-ventilation to help keep air moving through the attic space. A gable vent is mostly used with a gable style roof with a vent placed on each side of the home. These vents are less effective on complex roof styles where the cross breeze may be impeded by rafter beams, peaks, valleys, dormers and other parts of the roof.

Q: What are some of the considerations for use of popular box style or slant back vents for attic exhaust on roofs?
A: Their small size is sometimes a drawback due to exhaust capacity. However, they don’t need to run across the entire peak of the roof, so box vents can be installed strategically in smaller areas that need venting that cannot utilize a ridge vent. 

Q: How effective are hardwired (mechanical) vents?
A: They are electric-propelled fans that pull hot air out of an attic space but cause higher electricity costs. Their power is either not powerful enough to make a difference or could actually be pulling cooler air up through the main floors of the home and out of the attic, thereby increasing energy costs by forcing the air conditioning unit work harder. Vent fans have the potential to reduce measured peak summer attic air temperatures by over 20 degrees. However, the impact over the air conditioning season is modest with well insulated attics. Weaker power vents tend to circulate air rather than expel air. Higher operational costs are a substantial reason why traditional hard-wired systems have transitioned to solar power over the last few years.

Q: Solar powered vents seem like a sensible green alternative. How well do they work?
A: Solar-powered vents removes the electricity costs associated with older hard-wired vents but does not eliminate the downside that come with powered attic vents. The fans are often either too powerful, or not powerful enough. When added on top of a proper vertical ventilation strategy, like a ridge vent exhaust and soffit vent intake, powered vents can have unintended damaging effects. For these reasons, it is best to use more passive, proven methods of exhaust for your roof. 

Product Specific Questions:

Q: If I am using one type of exhaust vent do, they need to be installed at the same distance from the ridge?
A: Ensure that when installing either our Python 65 Slant Back or Python 150 Round Vents that they are installed at the same distance from the ridge per the installation instructions to avoid short circuiting of the attic ventilation system.

Q: What type of vent do you recommend for exhaust?
A: If the roof line is uncomplicated and has a long ridge run, the Weather-Tite Rolled Ridge Vent or the FlexFit Ridge Vent will provide the greatest vertical height to aid the movement of air through the chimney effect. The NFA ratings of both ridge vents will allow large volumes of warm moist air to be expelled through the ridge. If the roof line is chopped up or has a hip design, we recommend the 65 Slant Back that provides 30% more ventilation than typical competitive vents while keeping water from infiltrating the attic.

Q: What widths and lengths are available for the Weather-Tite Rolled Ridge Vent?
A: The standard widths available are 8”, 10” and 11-1/2” and come in standard roll lengths of 20’ and 50’.

Q: What is the NFA rating on the Weather-Tite Rolled Ridge Vent?
A: The ¾” thick Weather-Tite Rolled Ridge Vent provides 17-1/2 Square Feet of Net Free Area per lineal foot.

Q: What pitch of roof can the Weather-Tite Rolled Ridge Vent be installed on?
A: It is suitable for any pitch from 2/12 to 20/12.

Q: What nails are supplied with a 20’ roll of Weather-Tite Rolled Ridge Vent?
A: Coil of 1-3/4” nails.

Q: What is the recycled content of the Weather-Tite Rolled Ridge Vent?
A: Made in North America from more than 90% recycled plastic with over 30% post-consumer content.

Q: What is the Net Free vent Area of the FlexFit Ridge Vent?
A: The Net Free Area is 21 square inches per linear foot (NFA) or 84 square inches per vent.

Q: Will the FlexFit Ridge Vent still function if it is covered by snow?
A: A snow-covered FlexFit Ridge Vent will allow warm air to escape through the vent as the snow is porous. 

Q: What colors are the FlexFit Ridge Vent available in?
A: Black.

Q: How many FlexFit Ridge Vents are in a carton?
A: Ten.

Q: How does the FlexFit Ridge Vent adapt to smaller shingle cap sizes all in one product?
A: Using a utility knife, score the length of the FlexFit Ridge Vent (on top) in the groove opposite of the cut/break line. For 10″ application score one groove; for 9″ application score both grooves. Once you have scored the topside, bend the external baffle at the groove and remove the external baffle. Overlap the removed external baffle (one baffle for 10″ and both baffles for 9″) onto the body of the RIDGE VENT by simply inserting the 12″ nail support into the 9″ receiving nail hole. Then install vent per the instructions.

Q: How does the FlexFit Ridge Vent stay centered over the ridge slot when only one baffle is removed for the 10” shingle cap size application?
A: The vent is designed and formed with a movable center ‘hinge’ design to allow the FlexFit Ridge Vent to be recentered after field adjustment to the 10” shingle cap size.

Q: How is the final piece of FlexFit Ridge Vent installed at the end of the roof if there is less than 4 feet of ridge left on the roof after the last ridge vent was installed?
A: Cut a final piece to a length long enough so that it is flush with the edge of the roof. Ensure that the self-closed end cap is facing out to prevent weather infiltration. The interlocking feature will not be used on final piece.

Q: Are nails included with the FlexFit Ridge Vent?
A: Yes, a bag of 30 3” nails.

Q: What pitch of roof can the FlexFit Ridge vent be installed on?
A: It is suitable for any pitch from 2/12 to 12/12.

Q: What function does Python’s Weather-Tite barrier provide on the FlexFit Ridge Vent?
A: Our industry unique Weather-Tite barrier utilizes our non-woven material which is centered to the inside of the vent so it’s always positioned over the ridge slot. That barrier provides a no-clog, no excuses weather barrier blocking dust, dirt, insects, water and snow from entering the attic. Snow infiltration rating; zero. Sadly, competition has an air conditioning type fiberglass matting that will and does clog, preventing the vent from well, venting air. 

Q: What about competitive ridge vents that tout the advantages of an external baffle?
A: The claim is that the external baffle, which is additional flap running the length of their ridge vent deflects wind up and over the vent, creating a low-pressure area helping to pull air out of the attic. The Python FlexFit Ridge Vent however is shaped like an aircraft wing without the full-length flap. These flaps inevitably catch leaves and other debris blocking the baffles while the FlexFit Ridge vent just keeps on venting, no clogs and no excuses anywhere with 84 Square Inches of Net Free Vent Area per piece.

Q: When is it appropriate to use the Python Slant Back 65 Vent vs the FlexFit Ridge Vent or the Weather-Tite Rolled Ridge Vent?
A: The Python 65 Slant Back is an excellent application for hip roofs and or roofs that are chopped up with very short ridge runs. The Python 65 Slant Back can be installed in smaller areas that need vents but cannot utilize a ridge vent and for more complicated roof lines that have lots of different sections. 

Q: What pitch of roof can the Python 65 Slant Back Vent be installed on?
A: It is suitable for any pitch from 2/12 to 16/12.

Q: When is it appropriate to use the Python 150 Round Vent vs the Slant Back 65 Vent, the FlexFit Ridge Vent or the Weather-Tite Rolled Ridge Vent?
A: The 150 Round Vent can vent three times the Net Free Area of a typical competitive 50 NFA vent. This is great for large roofs where fewer penetrations translate into reduced labor installation times.  

Q: What pitch of roof can the Python 150 Round Vent be installed on?
A: It is suitable for any pitch from 2/12 to 16/12.

Q: On a roof without an eave can the Python 65 Slant Back Vent be used as an intake vent?
A: Yes, it can as long as attic insulation does not block the flow of intake air all the way through expulsion to the ridge with no less than 1” space between the insulation and the roof sheathing starting at the location of the vent. 

Q; Can Python 65 Slant Back Vent be used for intake venting on one side of the house (the side that does not have an overhang, for example) and another type of intake vent on the opposite side of the house the side that does have an overhang?
A: Yes, it’s OK to use Python 65 Slant Back Vent on one side of the house and use a different type of intake vent on the other side of the house. This will not cause any problems for the overall attic.

Q: What colors are the 65 Slant Back and 150 Round Vents available in?
A: Weatherwood, Brown and Black.

Q: How many 65 Slant Back Vents are in a carton?
A: Six.

Q: How many 150 Round Vents are in a carton?
A: One.

Q: Static vents are completely exposed to hail and other impacts. What has Marco Industries done to ensure that the Python 65 Slant Back and the Python 150 Round will provide enduring performance?
A: The Python 65 Slant Back and the Python 150 Round Vents are both manufactured from a proprietary formulation of no-break polypropylene plastic which provides the strength, flexibility and resilience to successfully pass UL 2218 Class 4 impact testing.


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