Insulating an unvented shed/workshop

9 years ago

I have a barn-type shed/workshop and need to insulate it with an unvented roof section open to the lower level. The roof section is 2/4s on 24 in. centers, with 3 planes (angles) on each side. The roof is comprised of plywood sheathing nailed to the studs and covered with metal roofing panels. I don't know if there's anything between the metal roofing and roof sheathing. Venting from the factory is via one under-gable vent at each end. No ridge vent. No soffit vents. The plan is to cover the two provided vents.

I'm insulating the lower section (vertical walls) with R-13 paper-faced insulation. Walls and interior roof will be covered with 1/2" finished drywall.

My main question is how to most economically insulate the roof section and protect the roof sheathing from water vapor.

The shed/workshop will be heated by a thermostat-controlled electric heater -- minimal heat needed in order to protect a refrigerator (supplemental to the house's refrigerator) from freezing solid during weeks of sub-freezing weather.

Wondering whether R-13 paper-faced insulation (paper toward living space) could be applied between the 2x4 rafters, and in contact with the roof sheathing, if 6-mil visqueen is applied prior to the drywall, and all seams caulked and/or taped.

Would this not prevent moisture from condensing onto the roof sheathing?

Thank you for helping me understand the problems with this approach and, if this approach is not advisable, what's the next-best economical means to achieve my goals.

Mark

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Comments (3)

Bruce in Northern Virginia
9 years ago
With traditional insulation methods its hard to get much out of installing batts between 2x4s and still allowing for any air circulation.

Its becoming more common to insulate houses so that the attic is conditioned space. The green building folks have decided that the old rules about keeping air circulation under the roof sheathing are not applicable, if its done correctly. I would search online at articles that describe how it is done.

The only catch you may find is that most of them do it with spray foam insulation in the cavities under the roof sheathing, and that is something you will have to pay a contractor for. Getting the spray foam mix correct requires some skill, and even commercial companies have had issues with employees doing it wrong and getting a mix that does not work correctly.

Bruce
Mark Davis
Original Author
9 years ago
Thank you, Bruce.

Right. The traditional way seems to be with soffit vents, air baffles and a roof vent. Many sources recommend sprayed foam insulation. The latter, however, would be at least a couple thousand dollars, plus there are a number of "horror" stories out there about incorrectly mixed foam giving off toxic fumes.

A neighbor is a 30-year builder and now supervisor for major construction projects in VA, WVA and MD. He advised me that for this building I should be okay installing the 3-1/2" faced insulation in contact with the roof sheathing (faced side down) and not worry about air flow between the insulation and roof sheathing. The building is a storage shed / workshop. There is no bathroom or cooking done in the building. Thus, moisture and water vapor problems should be at a minimum and of little concern. He suggested blocking off the two gable vents during the cold months and unblocking them in the Spring/Summer months. There are also 3 windows available for additional ventilation as needed. We're proceeding per his advice and hoping it works out okay.

Thanks, Bruce, for your input.

Mark
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islnddog
5 years ago
Mark I have the exact same situation that you had in 2018, I am in southeast Virginia. How has 3 1/2" faced insulation in the 2x4 rafters work out for you, three years later?? Did you drywall the ceiling?? Thanks very much for any feedback!