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/
I have a pole barn/garage that I want to finish on the inside. I will not be heating it on a regular basis (at this time), but that could change in the future. I live in central Ohio and I’m pretty sure we have less than 8000 degree days (per one of your posts). My building has a shingle roof and vinyl siding. I want to install a metal ceiling with approximately 10 inches of fiberglass insulation. Two questions: 1. Do I need a vapor barrier between the metal ceiling and insulation? 2. Do I need a vapor barrier on the side walls between the OSB and insulation?
You do not need a vapor barrier in ceiling, however you do need to have adequate ventilation from eave to ridge. You do need a vapor barrier on warm side of wall. Kraft paper facing would qualify. I would recommend using Rockwool batts for walls, with a clear plastic interior vapor barrier.