I have a 3-season room that only needs to be finished on the inside. It is built over the basement but I will not be running HVAC in it. My roof has a 2/12 pitch and I’m curious about insulating and ventilating it. I live in the Midwest so we have extreme temps. There is also the possibility that we may put a gas fireplace in there some day, but only for Fall evenings. My plan was to use rafter vents and placing the insulation on top of the ceiling, same as an attic. Will this be sufficient? Is there a need for a plastic barrier? Thanks in advance.
Once you enclose the room you will most likely create a sealed room with a variance between the indoor and outside temperatures – with or without mechanically heating the space. Once this occurs you have the possibility of condensation occurring as the warm air goes to the cold air, which can lead to humidity issues. Once humidity occurs you should have some sort of ventilation or air exchange available.
Adding batt insulation will require some sort of ventilation.
You could consider using a closed cell type foam insulation, like Dimelec, www.demilecusa.com, which doesn’t require ventilation. This type of insulation is very handy when used in vaulted roofs that are difficult to vent. The R value per inch is much higher with foam.
Install some nailing strips on the ceiling and then spray on the foam insulation, followed by a layer of drywall that can be finished and painted.
In the long run, you will have much more comfortable room.