Home >> NFVA

Tag Archives: NFVA

Attic Ventilation – Where Hindsight is Always 20-20

Attic Ventilation – Where Hindsight is Always 20-20

Reader CHRIS in TULSA writes:

“Hello! I have a question about attic venting that I’m just not experienced enough to answer properly. I have a new 24’x’40’x12′ post frame building I’m currently working on finishing out (fiberglass insulation, walls, ceiling, etc.). My hindsight was bad and I didn’t opt for getting an overhanging roof with soffit venting, etc. Instead I did have the builder install 2, 10′ ridge vent sections on both ends of the roof. The building will not have plumbing, but will be air conditioned with mini splits, etc. My question to you is regarding properly venting the “attic” space created once the ceiling is installed. Again, I have 2, 10′ ridge vent sections as the exhaust, but was wondering if I need to add some gable venting or something similar as the intake. I should also add the entire barn has been wrapped in Tyvek. Any help with this is greatly appreciated, or if you sell a product that would help I would be interested in more information on it. Thank you for your time!”

Sadly builders and building providers rarely take proper ventilation into consideration, leading to me getting lots of questions asked about solving later challenges.

Your ridge vents will only “work” when you have properly sized and placed air intakes. In your case, you will be limited to gable end vents. Gable vents are not nearly as effective as eave/soffit vents, so you are likely to experience poor air flow as you approach center of your building (and could have some condensation issues there as well). Tyvek would not have been my first choice in your roof, as it allows moisture to pass out and become trapped between it and your roof steel. This could potentially cause some premature degradation, over time.

At least 40% and no more than 50% of your ventilation can be located in upper portion of your attic space. You will need to know NFVA (Net Free Ventilating Area) of your ridge vents, then NFVA of your gable vents should be equal to, or slightly greater than your exhaust. Gable vents should be placed as low as possible, to achieve best airflow.

This entry was posted in Building Interior, Insulation, Pole Barn Questions, Building Overhangs, Barndominium, Pole Building How To Guides, Pole Barn Planning, Pole Barn Structure, Trusses, Ventilation and tagged gable vents, soffits, NFVA, net free ventilating area, Ventilation, ridge vents on July 11, 2024 by Rick Ochs.

How To Vent An Attic Below A Lean-To Porch

How to Vent Attic Below a Lean-To Porch

Long time reader JON in SPRINGDALE writes:

“Hi Mike, long time reader. Thanks for the info you provide. These questions come from your

home town area. I was talking to my local building department and attic ventilation came up. He

said that using a ridge vent and vented soffits isn’t enough, because the purlins restrict the

natural air flow between the two. So he suggested powered gable vents. I believe code calls for 1″ minimum clearance between insulation and sheeting what would you recommend as this affects the size of the heel on the trusses. Still on attic ventilation, so I want a porch covered by a lean to with a continuous roof line peak to eave. My question is if the underside of the lean-to is open, how do I vent the attic with no overhang to put vented soffit on? Thank you.”

Mike the Pole Barn Guru says:

I have been unable to find any published research to back up your inspector’s theory, nor would anything in written Building Codes support use of powered vents as a method of either intake or exhaust to meet Code requirements. Code does require a minimum one-inch space between top of blown or batt insulation and bottom of roof deck.

To ventilate eaves on side of building with continuous roof and covered porch, you could oversize eave strut (purlin at top of wall between enclosed portion and porch) to say 2×10. Cut notches into top edge of purlin an inch deep, by whatever length would be necessary collectively to meet proper ratio of eave air intake to exhaust. Notches would need to be covered with Code compliant screen to restrict entry of small, flying critters and wall steel stopped below bottom edge of notch.

In summary, ventilation requirements in IRC’s 2018 edition are as follows:

Provision of one square foot of NFVA for each 150 square feet of attic floor. One important note – attic floor area is just as it reads – area – not volume. This is a minimum requirement and does not stipulate required ventilation openings provide intake (low), exhaust (high), or both.

Provision of one square foot of NFVA for each 300 square feet of attic floor if both following conditions are applicable:

A Class 1 (≤ 0.1 Perm) or 2 (> 0.1 to ≤ 1.0 Perm) vapor retarder is installed on warm-in-winter side of ceiling when the structure is located in climate zone 6, 7, or 8.

At least 40%, but not more than 50% of NFVA is provided by vents located not more than 3 feet below roof’s highest point.

Provision for a minimum one-inch air space between roof sheathing and insulation in attic at vent location.

There are a few items I would suggest, after looking at your provided portion of plans.

Do away with all of expensive OSB sheathing. Order roof steel with an Integral Condensation Control factory applied.

Increase ceiling height to 10′ 1-1/8″ from top of slab to bottom of trusses. This will allow you to use 10-foot sheets of gypsum wallboard (sheetrock) run vertically without cutting.

Use bookshelf wall girts to create an insulation cavity and for ease of interior finish.

Have your engineer check purlin spacing on each side of ridge to account for drift loads. Purlins at high side of dropped right side porch also need to be checked for slide-off and drift loads.

Code requires a minimum 6mil vapor barrier under concrete slabs on grade in conditioned areas.

This entry was posted in Pole Building How To Guides, Pole Barn Planning, Trusses, Porches, Pole Barn Questions and tagged attic floor, purlins, osb sheathing, attic vents, lean-to, integral condensation control, NFVA, IRC 2018 on October 10, 2023 by admin.

2018 IRC Attic Ventilation Requirements

2018 IRC Attic Ventilation Requirements

Reader SCOTT in MINNESOTA writes:

“I read a couple of articles on your website and was hoping you could answer a question or give me some insight on venting my shop building,

It is pole barn construction with 24” vented soffits and a ridge vent. The original foam/screen closures on the ridge vent were along the entire length of the ridge but over the years have pretty much deteriorated and come down in pieces. This last Minnesota winter was hard on them. I think replacing them with an “LP2 like” option available from Midwest Manufacturing will be a good idea and probably last longer than the simple 1” wide foam style. My building is 40’ x 40’ with a 42’ ridge and it is finished inside with a level ceiling.

Questions:

Should I do the entire length of the ridge with vented closures or do I just need a percentage?

If I don’t do the entire length should the sections of venting match on both sides of the ridge?

Any thoughts on this matter would be appreciated.

Thanks for your time.”

Mike the Pole Barn Guru says:

Historically, IRC (International Residential Code) ventilation requirements are applicable to one and two family homes and have been based on a ratio of “net free ventilating area” (NFVA) being area of ventilation openings in attic to area of attic space. NFVA is the total unobstructed area air can pass through and it is calculated at the most restricted location through a vent’s cross section.

Ventilation requirements listed in Section R806 in IRC’s 2018 edition are listed in 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, perforated vinyl or similar material with openings having a least dimension of 1/16 inch minimum and ¼ inch maximum. Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air and shall be protected to prevent the entry of birds, rodents, snakes and other similar creatures.
R806.2 Minimum Vent Area. The minimum net free ventilating area shall be 1/150 of the area of the vented space. Exception: The minimum net free ventilating area shall be 1/300 of the vented space provided both of the following conditions are met:

In climate zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
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. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space. 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, blocking, bridging and insulation shall not block the free flow of air. Not less than 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.

In summary, ventilation requirements in IRC’s 2018 edition are:

Provision of 1 square foot of NFVA for each 150 square feet of attic floor. One important note – attic floor area is just as it reads – area – not volume. This is a minimum requirement and does not stipulate 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 following conditions are applicable:
• A Class 1 (≤ 0.1 Perm) or 2 (> 0.1 to ≤ 1.0 Perm) vapor retarder is installed on warm-in-winter side of ceiling when the structure is located in climate zone 6, 7, or 8.
• At least 40%, but not more than 50% of NFVA is provided by vents located not more than 3 feet below roof’s highest point.
• Provision for a minimum 1 inch air space between roof sheathing and insulation in attic at vent location.

Hopefully this 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 ridge, provided roof steel’s upper edges from each side are at least 1-9/16” apart.

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

You will need to determine NFVA of your vented soffits in order to calculate the correct ratio of intake to exhaust. If the entire ridge does not have to be vented, it would be prudent to have equal footage of vented closures on each side of the ridge.

This entry was posted in Pole Barn Questions, Pole Barn Structure, Ventilation and tagged Ventilation, IRC, vapor retarder, net free ventilation area, NFVA, screening on June 8, 2023 by admin.

Moisture Barrier for a North Idaho Hanger

Moisture Barrier for a North Idaho Hangar

Reader BILL in SANDPOINT writes:

“Hey guru! Question about insulation – moisture barrier. We are under construction on a 64×70 post frame aircraft hangar in North Idaho. The eave height is 19 feet, plus a two foot heel. There are no overhangs. Roof pitch is 3:12 and the ceiling is vaulted 1:12 there will be a 58 foot wide by 18 foot tall hanger door. Attic venting will be from gable end vents. The roof will be sheathed with 7/16 OSB and receive 30# felt under the 26 gauge PBR. The walls will be sheathed 7/16 OSB and house wrapped under the 29 gauge steel panels. The plan is to install metal liner panel on the ceiling and 5/8″ drywall on the walls. Insulation will be r49 blow in the attic, and r25 unfaced batts in the walls. The hanger door is yet to be determined, but it might also be unfaced batts under a metal liner. Two 125,000 BTU radiant tube heaters will be installed in the ceiling. They will be vented outside and will draw combustion air from outside. I am thinking that it makes sense to install a full vapor barrier for the walls and the ceiling, but would appreciate your thoughts on the matter.”

Mike the Pole Barn Guru writes:

Before we get to your question, I have some concerns….

First – you are putting a 58 foot wide door in a 64 foot endwall. This leaves only three feet of solid wall on each side of your door opening. These areas need to be shearwalls and maximum aspect ratio for a shearwall is 4:1 (four feet of height for every foot of width). With a three foot wide shearwall your maximum opening height would be 12 feet. You are likely to experience some tremendous racking problems (if not a failure) on your door endwall.

Second – trying to ventilate a 4480 square foot dead attic space with gable vents will require 1075.2 square inches of NFVA (Net Free Ventilating Area) in each endwall and this is providing half of NFVA is located in each upper and lower half of each endwall. You could accomplish this with say four 24″ x 36″ galvanized gable vents each end, however this might not be your most aesthetically pleasing design solution. Downside of gable vents is once you get past your first pair of trusses, airflow becomes negligible. I would be much happier with vents in your sidewalls (in area of truss heels) as an intake, combined with continuous ridge venting.

If it were me – I would use unfaced R-30 rockwool batts in sidewalls (they are not affected by moisture and R value does not degrade over time like fiberglass does) with a well-sealed interior vapor barrier before hanging drywall. This would allow your walls to dry to outside. Ceiling should not have a vapor barrier, as you want moist air to be able to escape into your well ventilated attic. I would blow in R-60 of rockwool.

Best of success with your build.

This entry was posted in Trusses, Ventilation, Insulation, Pole Barn Questions, Pole Barn Design, Pole Barn Planning and tagged gable vents, shearwalls, dead attic space, racking, net free ventilation area, NFVA, rock wool batts, ridge venting, continuous ridge venting on December 6, 2022 by admin.

Help! My Barndominium Roof is Dripping!

Help! My Barndominium Roof Is Dripping!

Reader TIMM in WHITEFISH writes:

“Thanks for taking my question. I recently built a barndominium in NW Montana. I tried to find someone to build it for me, but the demand and cost in the area had gone up so much that I had to do almost all the work on my own. I was not completely unfamiliar with building but not an expert by any means but I was able to get it built with helpful videos found online. I finished the home in late October and have moved in. The home is 28’x36′ with 10′ walls and is all living space, no garage. I had planned on doing spray foam insulation around the entire shell of the barn and had hired a company in August to come out and spray the barn but they were not going to be able to get to the building until December at the earliest but we were willing to do it and fight through the winter in our camper. Our plumber mentioned a product to us that he had seen some other clients use called Prodex that had similar characteristics of spray foam with a reflective surface on both sides and it was something I could do myself and much sooner. I did some research and the product looked good and the reviews looked good so I bought some and installed it. The steel was already on when I installed it so the Prodex was installed by stapling or screwing to the Purlins/Girts around the whole building which was an install method on their website. While we were mudding/painting/texturing I noticed some condensation in the attic in between the steel and the Prodex insulation (I could see where it was coming through a seam in the Prodex). I asked some people and they thought it was just because I was putting a lot of moisture in the air that was causing the condensation and it would dry out when we were done. On a recent trip up to the attic I noticed that the steel is still condensating when it is cold outside and the Prodex itself seems to be condensating as well. I emailed Prodex and they told me that it is caused by cold air moving across the inside surface of the steel and I should put foam around the ridge cap, eave edge of roof and tops of wall. I have foam around the ridge cap, but nothing on the ridge cap ends, I have foam on the eave edge of the roof, but only in the high ridge parts, and I have nothing on the walls. I am also concerned that this is happening inside of the walls which may lead to a bad mold problem next summer. My question is, how do I get it to stop condensating? I am ready to do whatever I need to do. I just don’t want to throw ideas at the house until something works. As far as ventilation goes, I am sure I do not have enough but was hoping to address that in the summer months. I do not have eaves on the building which I regret so my only real ventilation is the ridge cap and the little bit that may be coming through the ridges on the eave edge of the roof. I thought about gable vents, but I felt like that would let too much cold air in and would make the issue worse, but maybe that is what I need? If I put in gable vents, do I pull out the Prodex insulation and leave bare metal on the inside of the attic? I am trying to figure out a way to reduce the moisture right away (dehumidifier?) while I work on a long term solution but I don’t know which direction to go to solve this issue. I thought about pulling off the steel and putting in plywood sheeting, but we are in the middle of winter and that would have to wait until Spring at least and I am afraid I will end up with too much water damage by then. I have even considered putting sheeting under the roof and replacing the outside walls with wood siding but the cost would be high and I feel like there should be a solution to this issue. For heat we electric wall heaters (Cadet in-set wall units) occasionally and a pellet stove most of the time. We put the Prodex insulation as well as blown insulation in the attic to about 12 inches deep and we put Prodex as well as rolled insulation in the walls for a total of about an R30 value. Dryer and bathroom vents both go outside and nothing is venting into the attic. Any help would be appreciated! Thanks.”

Mike the Pole Barn Guru says:

Kudos to you for doing a D-I-Y. Sadly you were lead to a product (Prodex) claiming to be insulation, however in reality it is a condensation control, and only if totally sealed.

All of these issues could have been easily addressed at time of construction had your building kit provider given you proper advice.

First thing to do is to get your attic properly ventilated – you need to add at least 121 square inches of NFVA (Net Free Ventilating Area) to each gable end. This will give you an air intake and your vented ridge will then function as a proper exhaust. By itself, this should greatly minimize, if not totally cure your problems.

As time allows, remove roof Prodex, have two inches of closed cell spray foam applied to roof steel underside, and increase thickness of blown in attic insulation to R-60.

If you do not have a well-sealed vapor barrier under your concrete floor, if possible, seal top side of it (this is where moisture is coming from).

Heating as much as possible with your pellet stove will also help to dry your interior air out and provided your slab is sealed, should help greatly.

I do have some concerns about your walls, if you have faced insulation batts with Prodex on outside of batts, you are potentially trapping moisture between two vapor barriers. If this is indeed your case, come Spring, remove siding (one wall at a time) , remove Prodex (as much as possible) and add a Weather Resistant Barrier (Tyvek or similar) to the exterior of framing, properly seal all wall openings and reinstall wall steel.