Lack of Adequate Attic Ventilation is Sadly Becoming a Recurrent Theme
If only providers of poorly designed pole barns could be keel hauled…..
Reader AARON in CASPER writes:
“Hi there, I have a 40×104 pole barn. It has 16 foot sidewalls with rafters every 4 foot on center for snow load. The entire interior of the building is spray foamed to about 1 inch thickness. I want to install a ceiling under the rafters and put in r 38 insulation on top. I know that there were ridge vents when the building was put together, however the guy who spray foamed sprayed over the vents. I plan to clean that out. My question is are just those ridge vents enough for ventilation or do I need additional intake vents. The building does not have an overhang or soffits so they would not be easy to install. Do you have any suggestions?”
Mike the Pole Barn Guru says:
You certainly have a challenge at hand.
Here are requirements for adequate ventilation: https://www.hansenpolebuildings.com/2018/03/adequate-eave-ridge-ventilation/
You could add 1000 square inches of NFVA (Net Free Ventilating Area) vents in each gable end and it would meet Code (along with cleaning out your ridge vents for an exhaust), however this would prove to be a very poor design solution as your attic air flow will be highly constricted once you get past first truss in from each end.
Short of a major rework to add enclosed ventilated soffits, your best design solution if going to be to have a conditioned attic – besides, one inch of closed cell spray foam is not adequate to control condensation (it usually takes no less than two inches).
To get to R-38, you could increase closed cell spray foam to a total of 5-1/2″ or add another inch of closed cell plus 6-1/2″ of open cell.
Before considering adding a ceiling, confirm your roof trusses are adequate to carry extra weight. They need to have a minimum BCDL (Bottom Chord Dead Load) of 5 psf to support ceiling joists 24 inches on center and 5/8” sheetrock.
DEAR ERIK: Yes, our structural plans, sealed by our independent third-party engineers are designed to meet or exceed loading requirements for each individual building’s jurisdiction. We have provided several buildings in Clark County, including the giraffe barn at your Lion Habitat Ranch in Henderson. We do always have our clients verify load conditions with their Building Department prior to ordering. This will give you an idea of what will be on form to be completed: 
DEAR DAVID: If your building’s roof trusses have been designed for at least a five psf (pounds per square foot) bottom chord load (often shown as BCDL on engineered truss drawings) then they could support weight of either 7/16″ OSB or 5/8″ sheetrock. Neither product is designed to span greater than two feet, so you would have to add framing between trusses to adequately support.
For walls – my preference would be two inches of closed cell sprayed directly to wall steel, then fill balance of wall cavity with unfaced insulation of your choice. Fiberglass would be least expensive for this, although open cell spray foam or BIBs are options (I have done BIBs personally and think it is great).
