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What is Adequate Eave and Ridge Ventilation?

CODE REQUIREMENTS FOR ATTIC VENTILATION
Historically, ventilation requirements in the International Residential Code (IRC) are applicable to one and two family homes, and have been based on the ratio of “net free ventilating area” (NFVA) that is the area of the ventilation openings in the attic to the area of attic space. The NFVA is the total unobstructed area through which the air can pass and it is calculated at the most restricted location through the vent’s cross section.
Ventilation requirements listed in Section R806 in the 2012 edition of the IRC are listed in the excerpts below:

• R806.1 Ventilation Required. Enclosed attic and enclosed rafter spaces formed where ceilings are applied directly to the underside of the roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of 1/16 inch minimum and ¼ inch maximum. Ventilation openings having a least dimension larger than ¼ inch shall be provided with corrosion-resistant wire cloth screening, hardware cloth, or similar material with openings having a least dimension of 1/16 inch minimum and ¼ inch maximum.
• R806.2 Minimum Vent Area. The minimum net free ventilating area shall be 1/150 of the area of the vented space. o Exception: The minimum net free ventilating area shall be 1/300 of the vented space provided one or more of the following conditions are met:
1. In climate zones 6, 7 and 8, a Class 1 or 2 vapor retarder is installed on the warm-in-winter side of the ceiling.
2. At least 40 percent and not more than 50 percent of the required ventilating area is provided by the ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located no more than 3 feet below the ridge or highest point of the space, measured vertically, with the balance of the required ventilation provided by the eave or cornice vents.
Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet below the ridge or highest point of the space shall be permitted.
• R806.3 Vent and Insulation Clearance. Where eave or cornice vents are installed, insulation shall not block the free flow of air. A minimum of a 1 inch space shall be provided between the insulation and the roof sheathing and at the location of the vent.
• R806.4 Installation and Weather Protection. Ventilators shall be installed in accordance with manufacturer’s installation instructions. Installation of ventilators in roof systems shall be in accordance with the requirements of Section R903.

enclosed overhangsIn summary, the ventilation requirements in the 2012 edition of the IRC are:
• Provision of 1 square foot of NFVA for each 150 square feet of attic floor. One important note – the attic floor area is just that – area – not volume. This is the minimum requirement and does not stipulate that the required ventilation openings provide intake (low), exhaust (high), or both.
• Provision of 1 square foot of NFVA for each 300 square feet of attic floor if both or either of the following conditions are applicable:
• A Class 1 (≤ 0.1 Perm) or 2 (> 0.1 to ≤ 1.0 Perm) vapor retarder is installed on the warm-in-winter side of the ceiling when the structure is located in climate zone 6, 7, or 8.
• At least 40%, but not more than 50% of the NFVA is provided by vents located not more than 3 feet below the highest point of the roof.
• Provision for a minimum 1 inch air space between the roof sheathing and insulation in the attic at the location of the vent.

Hopefully the Code lingo didn’t dull your senses too badly!

A Marco LP-2™ ridge vent (read more here: https://www.hansenpolebuildings.com/2014/12/ridge-vent/ provides 18.4 square inches of net free ventilation per lineal foot of ridge when placed on each side of the ridge, provided the upper edges of the two sides of the roof steel are at least 1-9/16” apart.

As a maximum of 50% of the required ventilation can be at the ridge, 18.4 X 2 X 300 / 144 = 76’8” as the maximum building width which these vents can handle on a gabled roof.

For air intake, Kaycan brand full vented vinyl soffit provides 4.44 square inches of net free ventilation per square foot. This means with vented soffit on both eaves, the maximum building width with 12 inch wide soffits would be 37 feet; for 18 inch, 55-1/2 feet and 24 inch 74 feet.
When planning for adequate ventilation of dead attic areas, eave overhangs should be adjusted appropriately to achieve the required intake.

This entry was posted in Pole Barn Planning, Ventilation, Pole Barn Design, Building Overhangs and tagged , , , , on by .

Dear Pole Barn Guru: I Have a Roof Mold Problem

ADDED BLOG DAY!  Due to a large volume of questions to the Pole
Barn Guru, we are adding another day for answering questions – so look for more Pole Barn wisdom from The guru on Saturdays – starting – today! 

Welcome to Ask the Pole Barn Guru – where you can ask questions about building topics, with answers posted on Mondays & Saturdays.  With many questions to answer, please be patient to watch for yours to come up on a future Monday or Saturday segment.  If you would like a quick answer, please be sure to answer with a “reply-able” email address.

Email all questions to: PoleBarnGuru@HansenPoleBuildings.com

DEAR POLE BARN GURU: Have a 45′ wide x 120′ long by 20′ tall pole building.  All walls are insulated and covered with plywood sheathing.  Bottom cord of all trusses also sheathed with 1/2″ plywood.  Metal roofing is installed over felt paper that is installed over 1/2″ plywood roof sheathing.  Building has continuous ridge vent and multiple 10″x20″ soffit vents.  None the less we still have mold growing on the bottom side of the roof plywood. Here in Oregon we get ALOT of rain, so frequent moisture issues.  Should I add gable end fans?  Roof fans?  Thoughts. RAINING IN EUGENE

DEAR RAINING: Provided the underside of the ceiling plywood does not also have mold growth, then the moisture is an issue from the enclosed attic space. By Code, the requirement for attic ventilation is for 1/150th of the area of the attic to be in vents, provided at least 50% is in the upper ½ of the attic and at least three feet above the eave vents. This net free area is to be equally divided between the eave and the ridge.

In your case, the minimum requirement would be 18 square feet at both the eave and the ridge. My guess is your ridge is inadequately ventilated, thus the mold issue.

You should begin by replacing your current ridge with a product such as the Klauer Roofline Ridgolator 4 (www.https://klauer.com/metalcomponents.html). With 220 net square inches of free air per ten foot section – your 120 foot long ridge would provide just over the bare minimum required for proper exhaust.

Powered exhaust fans in each gable would certainly help, provided they are located towards the peaks of the gables, and have sufficient capacity to be able to pull air from 65 feet away.

DEAR POLE BARN GURU: What is the depth i should place my poles in the ground in concrete 24×48 10 feet tall not in city. SOMEWHERE IN SNOHOMISH

DEAR SOMEWHERE: As the county you are building in requires Building Permits for all structures the size of yours, and they do a structural plan check prior to issuing permits – you will need to go by whatever is on the approved plans.

 

Not knowing your design wind speed and exposure, dead loads on the building, roof slope and soil bearing properties, I can only say typically I would expect holes 40 inches deep, with the bottom 18” backfilled with premix. In the event you are constructing a “pavilion” style (all walls open), then it would be best to entirely backfill the holes with

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Pole Building Ventilation

Ridge vent without soffit vent

One of the most overlooked areas of pole building construction is proper ventilation. Lack of proper pole building ventilation becomes even a greater issue when an enclosed attic space is present. The International Building Codes require any dead attic space to be ventilated. Without adequate ventilation, moisture from condensation will begin to accumulate on top of the ceiling. Mold and mildew can form on the underside of the roof sheathing and on the roof trusses.

One of the least expensive options for a new pole barn, especially with steel roofing, is to have a vented ridge. Very easily installed at time of construction, if there is ever a possibility of a flat, level ceiling being installed in the building, a vented ridge is a must.

A ridge vent without a soffit vent doesn’t work, and here’s why. By virtue of their design and location on the roof, ridge vents are predominantly exhaust devices. Warm moist air from inside the building rises, passes through the ceiling material and attic insulation and out through the highest point – the ridge.

The attic space will get makeup air to replace the air the ridge vent has exhausted along the path of least resistance. If there is plenty of soffit venting and if you have a relatively tight ceiling, then the makeup air will come from outside, which is desirable, summer and winter. However, without soffit vents, the makeup air comes from indoors, a situation which is not desirable in any season.

So what to do if you have a building with an attic space, and little or no ventilation?

attic ventilationIf mold is already a problem, scrub the affected areas with a diluted bleach and soap solution. Once clean and dry, a mold resistant paint can be applied.

I’ve heard others suggest a roof design without ventilation, an issue that is volatile and multifaceted. Basically – doing away with the dead air space in the attic. As I see it, the choice to go or not to go with attic ventilation does not in itself ensure good performance. The bottom line with attic assemblies, whether vented or not, is that they be done properly.

Filling the attic space with cellulose insulation may be an option, although expensive and is not a 100% guarantee to solve the problem. Cellulose insulation is dense, blocks airflow and contain salts which inhibit mold growth. In my opinion, many of the innovative uses I’ve seen for cellulose are experimental, but those experiments seem to be working well. If conditions permit, you could find a way to blow in cellulose at the gable ends of the trusses. Better yet, you could fill the truss cavity from the ridge. In any case, with cellulose insulation as a “total fill”, I recommend the use of a vapor barrier, such as plastic sheeting on the underside of the trusses and/or ceiling joists or a vapor-barrier paint applied over the drywall.

Only without a dead air space should a ceiling vapor barrier be utilized.

Maintaining low indoor humidity may be also effective, but it may require wintertime humidity below 25%, which could be uncomfortable, as well as near impossible to achieve.

Add Pole Building Ventilation.

If your pole building does not have enclosed vented overhangs, it may be possible to add ventilation along the top of the building sidewalls. However, each individual case should be examined, as drilling holes or cutting into a structural member could compromise the building’s integrity.

If a continuous ridge vent is not present, install one. Make certain there is a clear air flow from the attic space through the ridge – which may entail the removal of any sheathing (oriented strand board – OSB or plywood), vapor barriers, or other insulations directly beneath the ridge.

Although the newer versions of the Code prohibit the combination of gable vents and ridge vents, gable vents may be installed in each end of the building – to provide an intake for outside air.

The real solution – is to build it right in the beginning. Prudent design with vented soffit overhangs and a vented ridge may involve some initial investment, but prove to be an insignificant cost over the life of the building. And trust me, no one I’ve ever talked to had a “fun” time dealing with mold.

This entry was posted in Ventilation and tagged , , , on by .
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