Tag Archives: vented soffits

Venting an Attic

Saving Money When Venting An Attic?

While some of you may think I have been doing post frame buildings since dinosaurs roamed our planet, I can assure you this is not true. Now my youngest son, when he was pre-school aged, did ask me (in all seriousness) what was it like watching space aliens build Egypt’s pyramids!

When reader DOMINIC in FESTUS wrote his question to me, it got me thinking about when I first had a client ask for a building with insulation at ceiling level. While I truthfully do not remember, in my first 6000 or so post frame buildings (we are talking 1980s here) I doubt there were more than a handful.

Fast forwarding to today’s modern fully engineered post frame buildings and nearly every building – garage, shop, barndominium, etc., is going to be climate controlled to some extent and most of these have enclosed attic spaces with insulation to be placed at ceiling level.

Here is DOMINIC’s question:

“I will be building a 30×40 pole barn soon. I plan on putting a ceiling in with insulation. My question is on attic venting. It seems best practice is to use a ridge vent with vented soffits but are gable vents alone sufficient? It would be cheaper for me to just do gable vents.”

Your best practice is to have even airflow from eave intakes to ridge exhausts. If your building will have sidewall overhangs, you might as well take advantage of this. You COULD (as an alternative) utilize gable vents. Provided at least half of your gable venting is located in the upper half of the attic, you can get by with as little as 576 square inches of NFVA (Net Free Ventilation Area). To achieve this would require (3) three 20″ x 30″ gable vents in each endwall. This could prove unsightly, difficult to install and is unlikely to result in being less of an investment than ridge vents. NOTE: a 20″ x 30″ gable vent provides roughly 106 square inches of NFVA (not 20″ x 30″ for 600″).

Of course, me being me, I had to snoop our records to see if he had requested a quote from us – and indeed he had…..

In looking over your quote from us, you may also want to consider increasing your overhead door width from 14′ to 16 (or even 18′) as you cannot safely get two vehicles side-by-side through a 14 foot wide door. For sake of resale value, with a 14 foot door it will appraise as a single car garage, wider doors will nearly double your appraised value as it is a two-car garage then.

Venting an Attic Without Soffit Air Intake

Venting an Attic Without Soffit Air Intake

Loyal reader KEN has an attic space with only air exhaust points – a vented ridge, and no air intake. His dilemma, how to adequately ventilate his attic without vented soffits.

Ken writes:

“I finally was able to make contact with the manufacturer of the open foam like material used under my ridge as a vent. The following is cut and pasted from their technical material: Net Free Area 1”nom.Thickness 8.5 sq.in.per lin.Ft.per side (17 sq.in .per lin.Ft.ridge).  Since I have a 60 foot ridge, that would equal 1020 sq. in. or 7 sq. ft. 

Given the ridge vent, do you still recommend 3 sq. ft.  venting on each end?

You note that an 18×24 would provide 140 sq in (or 0.97 sq. in.), but 18×24 is 432 sq in.  Do the louvers and other components reduce that by a factor of 3, just wanting to confirm.  With a 3 sq. ft. requirement, I would need a vent nearly 3 times an 18×24.  Maybe it just comes down to not the ideal but what can be practically installed. 

In order to prevent rust at the cut line, should I caulk the outside seam where the louver comes through the wall?”

Mike the Pole Barn Guru writes:

Another way of thinking about ventilation – view it like a straw. Obviously with best (and most) straws each end has an opening. What happens when a hole exists somewhere between ends? A reduction occurs in your ability to draw up water or soda. A similar effect happens when different vent types are employed over a shared air space.

Air and water are similar, their flow follows a path of least resistance. In a properly designed and installed attic ventilation system air flow travels from intake vents to exhaust vents, flushing out warm, humid air along roof deck (think underside of roof steel). However, if two or more different types of exhaust vents (gable and ridge) are used above a shared attic air space, one exhaust vent will likely to interrupt air flow to the other. In this arrangement it becomes another intake vent for a primary exhaust vent – leaving large sections of attic space incorrectly vented.

I would be inclined to seal ridge vent off and utilize gable vents. Dimensions of gable vents are overall unit dimension, not net free area (reduced from overall size by both exterior framework and louvers). Each endwall may require multiple vents in order to achieve an adequate airflow.

Caulking cut edges of endwall steel would certainly not hurt. A suggested caulking would be TITEBOND Metal Roof Translucent Sealant.

 

 

 

How Not to do a Post Frame Sheer Wall

How Not to Do a Post Frame Shear Wall

Reader DAVID in MIDDLETON writes:

“Hi we are currently building a 40/72 pole barn. We are wrapping the bottom 4 feet in OSB for sheer strength along with sheeting the roof with osb. We want to insulate the walls and put a drop ceiling in the pole barn to fully insulate it later this year. What do we need to do now for ventilation while we are building it to make sure we don’t have condensation issues later.”

Whilst friend David is writing about one issue, he is throwing out a bone as to why self-engineered post frame buildings are not always the best route to go. David is well intentioned, however his design solution would result in added expense without added benefits.

Always (may I repeat Always) construct only post frame (pole) buildings which are designed by a RDP (Registered Design Professional – engineer or architect) specifically for your building upon your site. You will always get piece of mind and usually the RDP will save you more money than what you invested for their design work.  

Here is my response to David:

Before we get to discussing your question, a few words about your design.

Wrapping the bottom four feet in OSB is going to do little or nothing to improve wind sheer resistance. In order to be effective as a sheer wall, the OSB needs to run from the splash board up to either the eave girt on the sidewall or the bottom chord of the truss on the endwall. All panel edges need to be blocked with 2x material. The shear panels should be no less than a 1:4 ratio (one foot of shear panel width per four feet of height) and ideally 1:2. On the roof, make sure to use at least 30# felt between the OSB and steel roofing and locate the roof screws so as they go into the underlying purlins, not merely into OSB.

Back to the question at hand…..

Your building should have vented soffits, of at least 18 inches in width to provide an adequate air intake. Trusses should be fabricated with raised heels – ideally two inches higher than the thickness of the attic insulation. Take care not to block the airflow from the soffit with the attic insulation. Vent the ridge.

Pour the slab on grade only over a well sealed vapor barrier, otherwise excess moisture will enter the building from the ground beneath the building. Also, the slab will need to be sealed – not as good as the vapor barrier route, but it is better than doing nothing.

Completely fill the wall cavity with insulation. Unfaced batt insulation can be used, however BIBs will be a fair superior design solution. Read more about BIBs here: https://www.hansenpolebuildings.com/2011/11/bibs/.

Either choice of wall insulation requires a clear visqueen vapor barrier on the inside. Make certain to seal any seams, rips or tears. There should not be a vapor barrier between the ceiling framing and ceiling materials.