Tag Archives: attic venting

Gable Venting a Post Frame Attic

Gable Venting a Post Frame Attic

Reader ALLEN in KIRBY writes:

“30×50 13 foot walls roof is 4-12 pitch I need to vent the attic. What size vents do I need to order? Two, one for each gable end what size do I need?”

Mike the Pole Barn Guru says:
From the 2021 IBC (International Building Code)

1202.2.1 Ventilated attics and rafter spaces. Enclosed attic and enclosed rafter spaces formed where ceilings are applied directly to the underside of the roof framing members shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Blocking and bridging shall be arranged so as not to interfere with the movement of air. An airspace of not less than 1 inch shall be provided between the insulation and the roof sheathing. The net free ventilating area shall be not less than 1/150 of the area of the space ventilated. Ventilators shall be installed in accordance with manufacturer’s installation instructions.

Exception: The net free cross-ventilation area shall be permitted to be reduced to 1/300  provided both 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 venting area is provided by 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 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.

You are not in a Climate Zone 6 or higher. As long as 50-60% of your venting area will be in the lower portion of your attic and 40-50% in the upper three feet, you can meet Code with a NFVA (Net Free Ventilation Area) of five square feet (720 square inches), half of which should be at each end. To give you an idea, www.airvent.com offers 14″ x 24″ rectangular wall louvers with a NFVA of 92.4 square inches each. It would take four of these in each gable end to provide adequate NFVA.

Ventilation, planned in advance, with air intake from enclosed vented soffits and exhaust at ridge yield a better airflow, at lower investment, with far superior aesthetics.

Attic Venting, Moisture Reduction, and a Vapor Barrier

This week the Pole Barn Guru answers reader questions about ventilation for an attic space, what type of insulation to use for the reduction or elimination of moisture, and to place a vapor barrier under the concrete slab.

DEAR POLE BARN GURU: I know you have answered a few questions regarding attic venting so I apologize if this is a repeat. I am looking at finishing the ceiling in my 24′ x 48′ pole barn with thin gage interior steel and blowing in insulation. I currently only have soffit on one gable end and I can see daylight coming in from under the trim on the other end. According to some other threads I have read, I need 3.84 sq. ft. of venting area. The gable end soffit is approximately 25 sq ft but I don’t know how much of that is free venting area. Do you count the whole 25 sq. ft? Also, any recommendations for what to do on the other end? Is the daylight that I can see coming in under the trim good enough? BRYCE in ZIMMERMAN

DEAR BRYCE: By Code you are unable to count any contribution from gable end soffits and areas between overhanging end purlins should be blocked solid to prevent airflow (and to properly create a load path to transfer shear forces from roof to ground). A construction flaw (such as daylight coming in under trim, probably should be fixed, rather than counted. By Code requirement is 3.84 square feet or 552.96 square inches of net free ventilating area located in upper 1/2 of your building’s attic space. My recommendation would be to install gable vents sufficient to provide airflow requirements.

 

DEAR POLE BARN GURU: What type of insulation do I need to keep out moisture in my residential building? JACLYN in WHITEHALL

DEAR JACLYN: Here is my ultimate guide to post frame building insulation https://www.hansenpolebuildings.com/2019/11/post-frame-building-insulation/

More important is removing sources of moisture https://www.hansenpolebuildings.com/2018/10/pole-barn-moisture-issues/

 

DEAR POLE BARN GURU: I have nearly completed my DIY Hansen Building and am preparing to pour the slab. Is there a way to search the blog posts for my questions? I am wondering if I should install a vapor barrier under the concrete or wait and seal it after? Is 1/2″ rebar recommended? Should I use an 18in grid or can I go on the cheap and get away with 2ft? Planning on a 5 inch slab. NICK in GLIDDEN

DEAR NICK: Good to hear from you, we are looking forward to seeing photos of your new building!

You should install a well-sealed vapor barrier under your slab while Code requirement is 6mil, however 15mil is far less likely to be damaged during a pour. Run vapor barrier up sides and onto top of 2×8 splash planks Overlap seams by at least 6″ and tape. Most often we see 1/2″ rebar on a 16″ grid.