Tag Archives: attic condensation

Dear Guru: Why Vapor Barrier?

DEAR POLE BARN GURU: I constructed a pole building with the help of Iowa based Amish group. They put up the main structure including metal roof. Due to city codes, I enclosed the 40x60x12 structure using 1/2 osb, house wrap and then vinyl siding. I want to use paper faced 4x8x4″ Styrofoam sheets on the walls, and roll insulation for the ceiling. My question is, do I use a vapor barrier on the walls after putting in the Styrofoam or none at all? And for the ceiling I would assume I would attach a vapor barrier to the bottom side of the trusses and lay the R-25 unfaced insulation on top of that. I have ridge vent and soffit vents. Thanks for your help! Curt in Center Point, IA DEAR CURT: For a properly performing system, your building should have a vapor barrier on the inside of all walls. The paper facing on the Styrofoam™ panels should be a vapor barrier. In order to perform properly, you need to make sure all edges and joints are tightly sealed, to prevent moisture from entering the wall cavity.

A vapor barrier should NOT be placed across the bottom of the roof trusses. If your building has steel roofing, I am hoping some sort of thermal break (like a reflective radiant barrier or similar) has been installed between the roof purlins and the roof steel, otherwise you are in for a plethora of problems. Warm moist air from your building needs to be able to pass through the ceiling and into the non-conditioned dead attic space, where it can be properly vented out of the ridge vent. You also should consider a greater R value in the attic. According to the North American Insulation Manufacturers Association https://www.naima.org/insulation-knowledge-base/residential-home-insulation/how-much-insulation-should-be-installed.html a minimum of R-38 should be installed in Iowa.

Mike the Pole Barn Guru

Dear Pole Barn Guru: We had hail damage to a post frame office building last Summer. Several months prior to the storm we had the side walls spray foam insulated (closed cell) and then framed and dry-walled. We have finally settled up with the insurance company and are ready to “re-skin” the building. The spray foam insulation was a significant expense and if we take off the metal siding the insulation will come off too. Here is my question: Can we simply install another layer of 29 gauge metal siding over the existing siding? Or can we fur out and install a different type of siding? Your input would be greatly appreciated!   KEN in Ft. Collins, CO

DEAR KEN: Although hail damage to steel siding and roofing is unusual, you have now found the downside to spray foam insulation applied to the inside face of it. If you place furring strips on the outside of the existing siding, you are most likely going to end up with the siding on the eave sides extending past the typical steel roof overhangs provided with most pole buildings. Plus, anything other than pre-painted steel siding is likely to come along with a lifetime of having to maintain it. In all probability, your best solution may very well be to install siding of the exact same profile over the existing steel. Screws will need to removed from each panel as you work your way down the wall, and replaced with screws of a larger diameter, as well as longer – in order to properly hold both layers of siding in place. With some patience, the results should turn out satisfactory