The Attic Needs Ventilation but How Much Exactly?

Good news, roofing contractors: You do not have to be good with numbers nor do you have to enjoy math to be able to quickly—and accurately—calculate the amount of attic ventilation needed for residential attics. Here it is, a handy shortcut for quick calculations:

Intake exhaust airflow in a house

Intake exhaust airflow in a house

Attic square footage ÷ 2 = square inches of EXHAUST and square inches of INTAKE Net Free Area (NFA) needed. (NFA is the unobstructed area through which air can pass through a vent, usually measured in square inches. Ventilation manufacturers assign an NFA value to the non-motorized vents they make.)

This shortcut conveniently calculates the 2015 International Residential Building Code MINIMUM (IRC Section R806 – Roof Ventilation 1, which states, in part, 1 square foot of Net Free Area for every 150 square feet of attic floor space with the attic defined as length x width floor of the attic). The shortcut actually overestimates a bit but that’s OK. It puts the roofing contractor in the ballpark which is useful when estimating.

To calculate the allowable IRC EXCEPTION to the MINIUMUM (that is, 1/300 ratio) here’s the shortcut:

Attic square footage ÷ 4 = square inches of EXHAUST and square inches of INTAKE Net Free Area needed.

Here’s an example using the shortcut for the 1/150 Code Minimum.
Say the contractor is standing in front of a house that has an attic with 2,200 square feet.

    2,200 ÷ 2 =

  • 1,100 square inches of EXHAUST net free area needed
  • 1,100 square inches of INTAKE net free area needed
  • The next step is to select a suitable exhaust vent and intake vent that fits the roof design for best performance and best aesthetics. After that, find out the vent’s NFA as rated by the manufacturer. Divide the vent’s NFA into 1,100 to yield the number of vents needed (either in linear feet or units/pieces). That’s it. It’s time to install.

There is a longer “official” formula based on building code you can reference or point your clients to for reassurance that you know what you’re talking about. Most attic ventilation manufacturers list the longer formula on their websites and inside key product brochures. But the shortcut is just as good and faster!

Calculation Q & A

Here are the answers to the five most frequent questions pertaining to calculating attic ventilation.

1. “Why is it important that the amount of intake ventilation matches the amount of exhaust?”
The goal of an effective attic ventilation system is to help fight heat buildup inside the attic during the warmer months and moisture buildup in the colder months. Additionally, in climates where snow and ice are common, attic ventilation can help fight the formation of ice dams. To achieve these goals the attic needs cooler, dryer air entering low (near the eave or the roof’s lowest edge) so it can flush out any warm, moist air that may have built up inside, pushing it out through the roof’s exhaust vents positioned as close to the peak as possible. This balanced-airflow approach allows the air to “wash” the entire underside of the roof deck from low to high.

2. “What if it’s not possible to balance the attic ventilation system 50 percent intake/50 percent exhaust?”
If it cannot be balanced it’s better to have more intake than exhaust because it has been our experience most attics lack proper intake ventilation, which is the leading cause of venting callbacks. Additionally, any excess intake will become exhaust on the leeward side of the house because the intake vents on the windward side of the house will have “pressurized” the attic. As a result, the intake vents on the leeward side of the house will work “with” the exhaust vents to release air.

However, if the attic has more exhaust than intake it potentially can cause the extra exhaust to pull its missing intake from itself (if it’s a ridge vent) or from another nearby exhaust vent (from one wind turbine to another or one roof louver to another), which means possible weather ingestion.

3. “What if the roof has 40 feet of available ridge length but the math calls for only 30 feet of ridge vent needed?”
It is OK to install all 40 feet of ridge vent as long as it can be balanced with intake ventilation. If the amount of intake ventilation cannot match the entire 40 feet of ridge vent, consider reducing the width of the ridge vent slot (thereby reducing the vent’s NFA per linear foot) to accommodate the amount of intake NFA available. Doing this keeps the airflow continuous along the entire horizontal ridge and balanced high and low. As always, be sure the overall amount of ventilation meets code requirements.

4. “If attic access is not practical is there another way to measure the attic square footage?”
Ideally, the attic square footage would be measured at the attic floor length x width (regardless of roof pitch, by the way). If this is not possible, and the homeowner does not have any documentation on file listing attic square footage, you could use the footprint of the house (aerial view of the house) or the number of shingle squares (one shingle square equals 100 square feet) to estimate the attic square footage. Neither of the alternate measuring tactics, however, is as accurate as an attic floor measurement.

5. “How does roof pitch come into play when calculating attic ventilation?”
Current IRC requirements do not factor the role a roof’s pitch plays in the amount of attic ventilation needed, but ventilation manufacturers do. Generally, as the roof pitch increases the volume inside the attic also increases along with the amount of needed attic ventilation. Here’s a rule of thumb to follow:

  • Up to 6:12 roof pitch use the standard formula as explained in this article.
  • 7:12 to 10:12 roof pitches increase the amount of ventilation by 20 percent.
  • 11:12 roof pitch and higher increase the amount of ventilation by 30 percent.

For projects involving vents with motors, the calculation formula is different.

‘Z’ Closure Ventilates Peaks of Standing-seam Metal Roofs

Glick Metals LLC has announced its Snap-Z vented “Z” closure designed for standing-seam metal roof applications.

Glick Metals LLC has announced its Snap-Z vented “Z” closure designed for standing-seam metal roof applications.

Glick Metals LLC has announced its Snap-Z vented “Z” closure designed for standing-seam metal roof applications. The Snap-Z product allows installers to vent the peak of a standing-seam metal roof in such a way that air is allowed to flow but water and snow infiltration are restricted. The Snap-Z is designed to work with 1-inch, open-hem metal ridge caps. It fits between the ribs of standing-seam roof panels. Snap-Z, which is covered by a 20-year limited warranty, comes standard with a matte black finish and is available in 15 additional colors to coordinate with a variety of roof-color offerings. It is available in 1-inch, 1 1/2-inch and 1 3/4-inch heights. Custom lengths may be ordered.

Wind-damaged Roof Systems

Wind damage to roof systems is often catastrophic, placing the building users at a life-safety risk, resulting in interior and furnishing damage and suspension of interior operations, loss of revenues, legal ramifications and great costs to repair. Because of my 30 years of experience in the design of roof systems and forensic investigation, I’m often called upon as an expert witness after wind events. In this article, I’ll review a couple wind-event roof failures, the causes of the failures and how they could have been prevented. I’ll also provide recommendations for failure prevention in the design process for new roof systems, as well as for existing roof systems.

1. The concrete roof deck panels deflected more than 3/4 inch, which the design architect should have accounted for if a thorough field investigation was undertaken.

1. The concrete roof deck panels deflected more than 3/4 inch, which the design architect should have accounted for if a thorough field investigation
was undertaken.

The Perfect Storm

How can it be that when roof systems are to be designed for code-required wind-uplift resistance that so many fail in winds well below the design parameters and/or warranty coverage? The answer could be design-related, material or installation; typically, it involves all three.

Architects and some roof system designers are often not as knowledgeable about roof systems as they should be, have little empirical evidence in how all the components work together as a system, and move beyond their abilities (a violation of their standard of care) when designing roofs where specific detailing is required. In addition, manufacturers are all too often
bringing new products to the marketplace that have not been properly vetted in the field and their long-term performance is truly unknown. Unfortunately, the roofing contractor cannot escape any of this. The lack of proper specification and contract document review; failure to review product data, including installation guidelines for new products; poor project oversight and management; and pressure from general contractors often result in installations that are subpar. The result is a “perfect storm” of design, materials and installation that fail under stress.

Consider the following case studies that I have been involved in as a forensic or “expert” witness when litigation was involved.

Coastal Facility

A large aged warehouse along the eastern seaboard was in need of a new roof system. Because the interior was not conditioned, thermal insulation was not required. The existing roof was an asphalt built-up with aggregate surfacing on high-density fiberboard on precast concrete panels 24-inches wide on a steel structure. The northern portion of the building had overhead doors that were seldom closed. On the interior, an aedicule structure (a building within a building) was constructed approximately 65-feet south of the overhead door, which had a ceiling level 5-feet below the roof deck.

2. The thin, flexible 1/2-inchthick high-density board was found to have little, if any, contact with the full-coverage spray-foam adhesive, making uplift extremely easy.

2. The thin, flexible 1/2-inch-thick high-density board was found to have little, if any, contact with the full-coverage spray-foam adhesive, making uplift extremely easy.

The architect who designed the replacement roof system called for the existing BUR roof to be removed down to the precast concrete roof panels. Then a new 1/2-inch 4- by 8-foot high-density wood fiberboard was set in full-coverage spray polyurethane foam adhesive with a 60-mil EPDM membrane fully adhered to the high-density wood fiberboard.

Additionally, the architectural drawings called for rooftop relief vents to be removed and capped over.

Around June 2008, a Nor’easter (an intense rainstorm), coming in from the east off the ocean, swept into the city. This resulted in the new roof system being lifted off the roof deck. Mode of failure was the fiberboard detaching from the precast concrete roof deck.

Investigation revealed several acts and conditions that contributed to the wind damage.

PHOTOS: Hutchinson Design Group Ltd.

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Soft Roll Simplifies Venting

Quarrix Building Products introduces Flex Vent Soft Roll

Quarrix Building Products introduces Flex Vent Soft Roll

Quarrix Building Products introduces Flex Vent Soft Roll, a lightweight, low-profile rolled ridge vent that will simplify venting for roofing contractors. Manufactured from non-woven and non-wicking polyester matting, the product is easy to install with no complicated fittings, wrapping or connectors. The rolled ridge vent is well suited for metal and asphalt and virtually disappears on the roof when installed. Quarrix Flex Vent is 10 1/2-inches wide, 3/4-inch thick and 20-feet long and includes 1 3/4-inch coil nails.

New CertainTeed Webisode Highlights Attic Ventilation

CertainTeed’s fourth webisode in the 2013 Living Spaces Facebook Home Makeover Video Contest, featuring the Bielfelt family of Mentor, Ohio, dives deep into the importance of superior weather-resistance and proper ventilation as part of a healthy whole-home system.

CertainTeed 5-Star Green and Master Shingle Applicator contractor, RJK Construction Co., of Willoughby, Ohio, installs Winterguard and DiamondDeck roofing underlayment as well as CertaWrap, CertaTape andCertaFlash housewrap products in the webisode to create a water- and wind-tight barrier to protect the century-old home’s original structure.

Grand prize winner of the $75,000 exterior makeover contest, the Bielfelts are the subject of a series of CertainTeed Living Spaces webisodes documenting the renovation of their 100-year-old home. The series follows the family, along with local contractors, from the planning stages of the renovation through its completion.

In the first few webisodes, the homeowner selected multiple products, including Cedar Impressions and Monogram siding, Restoration Millwork trim, EverNew decking and railing, Bufftech fence and Landmark Pro roofing. The homeowner also utilized the CurbAppeal iPad app, ColorCoach virtual swatch book and ColorView design tool, which allows homeowners to visualize how products will look on their home.

To view all of the webisodes, music videos and video entries from the 2013 $75,000 Exterior Living Spaces Home Makeover, visit the CertainTeed Living Spaces Facebook page or YouTube channel.

Ridge Vent Requires Fewer Fasteners

With the new improved design of Metal-Era's Hi-Perf Ridge Vent, much of the product will come preassembled, reducing the number of fasteners needed by up to 67 percent and decreasing installation times by up to 50 percent.

With the new improved design of Metal-Era’s Hi-Perf Ridge Vent, much of the product will come preassembled, reducing the number of fasteners needed by up to 67 percent and decreasing installation times by up to 50 percent.

With the new improved design of Metal-Era‘s Hi-Perf Ridge Vent, much of the product will come preassembled, reducing the number of fasteners needed by up to 67 percent and decreasing installation times by up to 50 percent as compared to other multi-component products on the market.

Engineered to individual job requirements, the patented, unique design of the Hi-Perf Ridge Vent is built to withstand winds up to 120 mph, and it’s backed by the only ventilation and wind warranty in the industry. The all-metal construction will not compress under stress, making it weather-proof and bug-proof. It has also passed water infiltration test TAS 100(A), ensuring a weather-proof vent.

The Hi-Perf Wind and Ventilation Warranty is available on all Metal-Era manufactured Hi-Perf ventilation systems. It guarantees that Hi-Perf systems:

  • when designed according to the Metal-Era calculated NFA will provide proper ventilation for the project
  • will withstand winds of up to 120 miles per hour
  • will be free of defects in Metal-Era supplied materials

The warranty covers repair or replacement of the system for 20 years or the lifetime of the roof on which it was originally installed, whichever comes first.

The product’s unique design allows for covers to be manufactured in 24-gauge steel, 0.040-, 0.050- and 0.063-inch aluminum. Cover plates provide a cleaner, more aesthetically pleasing appearance. Guarantees required exhaust airflow at the ridge to help prevent heat buildup and moisture accumulation underneath the roof covering, thereby increasing the life of the roof. The Ridge Vent is engineered to individual job requirements; this ensures that it will perfectly fit each roof condition.

The all metal construction withstands heavy snow loads and will not compress under stress making for the perfect weather and bug proof system.

It is provided in 12-foot lengths with pre-punched holes for quicker installation and lower labor costs. The fastening holes are slotted to allow for proper thermal movement of the materials and ensure correct fastener placement and spacing.

Choose among a wide range of standard colors and finishes that meet your job requirements. A 30-year Kynar 500 finish warranty is included on coil-coated standard colors. Custom post-coated Kynar 500 colors are available with an included 10-year Kynar 500 finish warranty or a 20-year Kynar 500 finish warranty is available upon request.

Free Catalog about Natural Ventilation Products

The Bilco Co., a manufacturer of specialty access products, has released its new 2014 Colt Natural Ventilation Products Catalog, detailing Colt’s line of natural ventilation and smoke and fire control products for the commercial markets. The catalog is available free of charge by calling (800) 366-6530, emailing commercial@bilco.com or downloading it.

Bilco partnered with Colt, a United Kingdom-based manufacturer of smoke and natural ventilation products, earlier this year and now serves as the company’s distributor of natural ventilation and smoke control products in North America. The Natural Ventilation Products Catalog features a user-friendly layout, complete with information on the benefits of and the products and options available.

Natural ventilation is becoming an increasingly important design strategy for commercial buildings, particularly in GREEN building design. With careful design, buildings can be cheaper to maintain and operate and the addition of fresh air and natural daylight creates a more pleasant and productive working environment for building occupants. Colt’s products provide an innovative and attractive solution for design professionals and building owners.

CSI Specifications Available for Colt Natural Ventilation Products

New Construction Specifications Institute (CSI) three-part specifications have been developed for Colt natural ventilation products and are currently available online, making it much easier for building design professionals to specify these new products.

CSI is an organization that keeps and changes the standardization of construction language as it pertains to building specifications. CSI provides structured guidelines for specification writing and its MasterFormat serves as the organizational structure for building products in the construction to help architects, engineers, owners, contractors, and manufacturers classify how products are typically used.

CSI 3-Part Specifications are available for Colt natural ventilation products and can be found and downloaded from the Download Center on the Bilco/Colt website.

Ridge Vent Provides Continuous Airflow

Benjamin Obdyke Inc.'s Shark Vent XLP

Benjamin Obdyke Inc.’s Shark Vent XLP

Benjamin Obdyke Inc. has introduced its Shark Vent XLP, an externally baffled shingle-over-roof ridge vent available in 25-foot rolls. Shark Vent effectively extracts heat and moisture from residential attics by providing continuous airflow along the attic ridge, the hottest part of the attic. Its sine-wave-shaped internal and external baffles with 1/8-inchwide vent holes protect attics from rain, snow and insect infiltration. For extreme weather areas, Shark Vent also comes in a filtered option (Xtra).

Add a Glazed Natural Ventilator to the Roof

Colt Group</a>'s Firelight glazed natural ventilator

Colt Group’s Firelight glazed natural ventilator

Colt Group‘s Firelight glazed natural ventilator installs easily in a glass atrium or directly onto the roof of a building. This dual-purpose product provides buildings with day-to-day ventilation and smoke control to aid building evacuation in the event of a fire. Firelight is available in single- or double-leaf design and with control options for automated climate control in a building. This innovative product is now available from The Bilco Company in all North American markets.

Naturally ventilated buildings are generally less expensive to construct and maintain than those that are mechanically ventilated alone. They offer a lower environmental impact through reduced energy demand, which results in lower emissions of carbon dioxide and other combustion products. Further, the addition of fresh air and natural daylight creates a more pleasant, productive, and healthy working environment for building occupants.

Firelight glazed natural ventilators are designed for high aerodynamic, acoustic and thermal performance. The product provides ventilation for most kinds of industrial and commercial buildings and is particularly suited for installation into atriums and glazing systems.

The Firelight is available in several standard designs, each offering a unique appearance and performance to meet the design requirements of any building. Models are available in single or double-leaf construction that can be specified with a powder coat finish to complement or blend into any roof exterior. The product features a thermally broken design to minimize heat loss for reduced carbon emissions into the environment and maximum energy efficiency.

In addition to the standard manual operation, the Firelight can be specified for electric or pneumatic operation and set to open and close as required for proper mixed mode ventilation. With a small initial investment, these systems can provide a highly energy-efficient solution to maintain a good internal climate, low humidity levels and extremely low running and maintenance costs. In addition to climate control, the Firelight control options allow the product to be used as a smoke control system in an emergency situation. These systems save lives by providing smoke-free escape and access routes for building occupants and improving visibility to aid firefighting efforts. They also protect building contents from smoke damage and reduce the overall risk of damage to the building.

Firelight glazed natural ventilators are custom-fabricated to meet virtually any size or building ventilation requirements. Products feature aluminum construction and are available with a number of cover and finish options.