Does My Pole Barn Need a Vapor Barrier Above the Metal Ceiling?
Reader DAVID in BALDWINSVILLE writes:
“I have a new pole barn with bubble vapor barrier under roof metal. I am installing a metal ceiling with R-38 cellulose in the attic. Do I need a vapor barrier above the metal ceiling? The barn will be heated somewhat in winter.”
From Mike the Pole Barn Guru:
Kudos to you for recognizing bubble wrap products advertised as ‘insulation’ are, at best, a good vapor barrier if properly sealed!
My son Brent is working on his doctorate at UMass Amherst, so I utilized Building and Construction Technology information from their Department of Environmental Conservation in penning this response.
My concern is with blowing in cellulose above a steel ceiling. From UMass:
“Wet insulation of any stripe is bad. But cellulose is hygroscopic. It’s able to soak and hold liquid water. Undetected leaks can wet cellulose causing it to sag within framing cavities. Water leaks can compress the blanket of fiber and in extreme cases, can create a void space, degrading its thermal value. Another concern is that chemicals used to protect cellulose from fire make it potentially corrosive in wet environments. Tests conducted by the Oak Ridge National Laboratory show chemical treatments used to treat cellulose can cause metal fasteners, plumbing pipes and electrical wires to corrode if left in contact with wet, treated cellulose insulation for extended periods of time. “
I’d much prefer to see you blowing in rockwool as it remains unaffected by moisture. Last thing you want is to have damp cellulose insulation chemicals eating holes in your ceiling.
According to building scientist and founding principal of Building Science Corporation Joe Lstiburek, “Plastic vapor barriers should only be installed in vented attics in climates with more than 8,000 heating degree days.”
As your site has under 8000 heating degree days a year, you should not have a ceiling vapor barrier. You do need to ensure your newly created non-conditioned dead attic space is adequately ventilated at eave and ridge (https://www.hansenpolebuildings.com/2018/03/adequate-eave-ridge-ventilation/).
For extended reading on heating degree days, please see: https://www.hansenpolebuildings.com/2022/11/what-is-degree-day/
Cellulose is “green.” It’s made of 80% post-consumer recycled newsprint. Paper fiber is chemically treated with non-toxic borate compounds (20% by weight) to resist fire, insects and mold. Cellulose Insulation Manufacturers Association (CIMA) claims insulating a 1500 square foot house with cellulose will recycle as much newspaper as an individual will consume in 40 years. If all new homes were insulated with cellulose this would remove 3.2 million newsprint tons from our nation’s waste stream each year. There’s room to grow. Fewer than 10% of homes built today use cellulose. Cellulose earns “green” points because it requires less energy than fiberglass to manufacture. Disciples claim 200 times less petro-energy than fiberglass. More realistically, Environmental Building News reports fiberglass requires approximately eight times more energy to make when adjusted to reflect energy cost per installed R-value unit.
Lake Charles is in climate zone 2A. 2018’s International Energy Conservation Code prescriptively mandates (for your zone) a minimum R-38 value for ceilings and R-13 for wood framed walls. This would require 5-1/2″ in roof and 2″ in walls. You could go with 2″ of closed cell directly to underside of roof deck plus 6″ of open cell, or 2-1/4″ of closed cell with 5-1/2″ of Rockwool as alternatives.
Blown in cellulose weighs roughly 2.5 to 3 pcf (pounds per cubic foot). Blown in cellulose will settle as much as 20% and can take up to two years to do so. Cellulose gives R-3.5 per inch, so your R-38 would be 10.86 inches thick (after settle), so I would plan on no less than 13.5 inches blown in to achieve your R-38. To be safe, I would assume 3.4 psf (pounds per square foot). Besides settling, blown cellulose does have another downside – in order to be fire retardant it is treated with borax or boric acid. Borates can be corrosive to steel in humid or moist environments, so you may want to consider a different insulation type, such as fiberglass. Fiberglass is also lighter weight, coming in at about a pound per cubic foot.
