Tag Archives: eave ventilation

Texas Post Frame Barndominium Insulation

Reader KIMBERLY in LINDEN writes:

“We are building a 52x40x10 post frame home in East Texas.  The entire thing will be living space.  I have been researching as much as possible on the best way to insulate a post frame home with metal siding and roof.  The information is overwhelming and you get a completely different answer depending on who you talk to.  I know not to skimp on insulation, but the consensus on most “barndominium” FaceBook groups is that spray foam is the only way to go.  I have reservations about that, because it may be a superior way to insulate, but it depends almost exclusively on who is doing the actual foam application.  On top of that you need to spend more money on your HVAC system to add the proper ventilation/air exchange.

I want a well insulated home that is specific to the type of building material and location we live in.  To me, “not skimping” on insulation doesn’t mean that it has to be the most expensive insulation either.  

I also know the insulation world is constantly changing and evolving, but what would your recommendation be to insulate our home in East Texas?

Thank you so much for your time!”

Mike the Pole Barn Guru responds:

Your insulation requirements will vary depending upon where you are in East Texas. Climates zone 1 (closest to Gulf) require R-30 ceilings, R-13 walls. Zone 2 requires ceilings to be R-49 and zone 3 (farthest north) goes to R-20 walls. You can look up you county’s climate zone here: https://codes.iccsafe.org/content/IECC2021P1/chapter-3-re-general-requirements#IECC2021P1_RE_Ch03_SecR301. I will cheat for you and tell you Cass County is Climate zone 3A.

For sake of discussion we will assume you have a dead attic space and will be insulating directly above a finished ceiling.  I would ventilate your dead attic space at the eave (air intake) and the ridge (air exhaust). Make provisions for preventing condensation on the underside of roof steel by having some sort of a thermal break. My personal preference is by using an Integral Condensation Control (https://www.hansenpolebuildings.com/2020/09/integral-condensation-control-2/).  You will want to order roof trusses with raised heels (https://www.hansenpolebuildings.com/2012/07/raised-heel-trusses/), so you can get full insulation depth from wall-to-wall with blown in fiberglass. Heel height should be R value of insulation divided by three and add two inches so you can achieve adequate airflow above insulation.

Should you want to condition your attic – delete ventilation, raised heels and the Integral Condensation Control. I would apply closed cell spray foam two inches to the underside of roof steel, then add open cell spray foam to desired R value.

For walls – best results will be from two inches of closed cell sprayed to inside of wall steel, then fill balance of wall cavity with either open cell spray foam, or unfaced batts (ideally stone wool https://www.hansenpolebuildings.com/2013/03/roxul-insulation/). You could also use BIBs to fill (https://www.hansenpolebuildings.com/2011/11/bibs/). Do not place a Weather Resistant Barrier (WRB) under wall steel or a vapor barrier on inside of wall.

As an alternative to spray foam, you can use a WRB between framing and wall steel, then BIBs with an interior vapor barrier or faced batt insulation.

Energy costs are not going to go down, so I would encourage you to err towards more insulation rather than less – and (since most heat loss is upward) invest more into added ceiling insulation than walls.

In warmer, humid climates like yours, your HVAC system should include an Energy Recovery Ventilator (ERV) regardless of what your choice of insulation systems ends up being.

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.