How to insulate a shed with metal roof?
I am re-habbing a 14x28 foot wood frame shed. I would like to install an insulated ceiling to hide the metal roofing. The shed has a wooden floor, T1-11 uninsulated walls, four windows and a sloping metal roof using Master Rib metal roofing. I think the rafters are 2x6 set on 24" centers. The roof has a layer of vapor barrier material installed right up against the inside of the metal (more for leak prevention I think). There is a screened ridge vent the length of the roofing. The lower ends are closed off with T1-11 to prevent insects from free entry into the shed. I understand I need to provide for some airflow between the metal and whatever I use for insulation, and that I should have a good vapor barrier between the finished room and the insulation. I am wondering if I can use fiberglass roll insulation with a vapor barrier on the inside, then finish off the ceiling with some sort of thin wood paneling?
I believe I will have to install some screened vents at the lower ends of the rafters to provide for some intake air, as I don't expect just the air flow from the ridge vent to be enough flow.
To also ensure the air flow needed to prevent condensation I was thinking of installing something like Durovent plastic spacers between each rafter for the full length of the run. The idea is they would ensure the fiberglass does not directly touch the metal roofing. The Durovent would go up against the metal roofing, with the big dimples holding off the platic from the metal and providing space for air to flow between the plastic and the metal roof. We live in the foothills of the Blue Ridge mountains and it is very humid in the summer. The winters are not too bad, with not much snow. Not sure yet how to heat this room, but for now it would be electric heaters when the weather is cold enough. Eventually I would also insulate and finish the walls.
From my reading it must be common to just install roll insulation against the metal roofing, but the reading also says that is not a good idea, as any water can collect in the insulation. Air flow and a vapor barrier is necessary to prevent the metal roof from condensing the plentiful moisture in the air on days when it is warmer inside the shed than outside.
OK folks, am I on the right track?
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