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I have a three season room attached to the house, built on a concrete pad. There is no heat or a/c out to the room.

The ceiling is currently open and I can see the underside of the planked roof.

I'm planning to install some kind of ceiling. Should the space between the ceiling and underside of the roof be insulated and should there be a moisture barrier?

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The insulation will probably let you get more use out of that room. Usually fiberglass between the rafters works pretty well.

You have to leave some space between the top of the fiberglass and the underside of the roof so that moisture can escape in that air space. They make plastic baffles that keep this space open even if the insulation is too high or gets disturbed - good insurance. There also has to be some ventilation at the top and bottom so air can flow in from outside at the bottom, flow up the space between the rafters, and out again from the top.

The vapor barrier depends on the local climate. There are some differences of opinion on this subject, but if you're calling it a three season room, I am going to guess you're in a climate with cold dry winters and warm humid summers, and suggest insulation with a vapor barrier, installed with the vapor barrier down.

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The insulator could prevent room from heating in the summer (when it's hot) and make it warmer in the spring/autumn. In the winter if you need the heating, the insulator could make this room and underlying rooms warmer and prevent temperature from going under 0 Celsius, when the plaster starts to come off. The moisture barrier could help when roof starts to leak.

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