Adding ceiling insulation to vaulted ceiling

Old 08-22-13, 10:27 AM
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Adding ceiling insulation to vaulted ceiling

My home was built in the mid 50's. We have a 3 season room that I believe was part of the original structure - there is a continuous roof line from the main roof and the windows are jalousie type that were popular in that period. I would like to add insulation above the ceiling. It is a vaulted ceiling with painted plywood nailed directly to the roof joists, so there is very little space between the ceiling and the roof deck. I planned to remove the plywood and add insulation and sheet rock, but I don't want to create a ventilation problem. Outside ventilation could be added through the fascia boards on one side, but there is no way to provide any reasonable air flow because there is just not a lot of space.
I've already installed a pellet stove in this room and even without the ceiling insulation, I was able to use the room throughout the 2012/2013 winter and still make a significant impact on my heating bills.
Any thoughts?
Old 08-22-13, 05:05 PM
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You didn't include the depth of your rafters, which is an important piece of information, but I'd probably either want to install styrofoam baffles, or if they take up too much room, put a 1" cleat on the side of each joist (to space the Styrofoam away from the bottom of the roof), and fit a sheet of rigid Styrofoam into the space between each rafter. It takes up some space, but will provide better r-value per inch than fiberglass. If you have room, you could install fiberglass under it.

The problem many people will have when they do this, heating a room like that, with cathedral ceilings, is that you will get condensation against the cold side of the roof- or even the foam, if there is a lot of air leakage. Cold air and warm air, when in close proximity to one another, is a recipe for condensation.

Adding sleepers onto the rafters is one way you could add depth to the rafters so as to get more insulation in there. The more the better. Fiberglass doesn't do a good job of slowing radiant heat loss.

Your best solution is probably no airspace and spray foam against the roof sheathing, to totally fill the joist space.

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