Insulating a Cathedral Ceiling

queequeg92 asks,

We bought a house made in 1960 in southern California. It has vaulted ceilings and no attic. There was never any insulation put in the walls of the house or above the ceilings. During the summer it gets really hot upstairs. We have installed ceiling fans. There is no air conditioning. We have not experience a winter yet. Can we install/blow installation into the walls/ceilings? Would it be worth it?

Our Answer

This is an unusual situation and, unfortunately, quite the undertaking no matter which route you take:
There are two ways to insulate a cathedral ceiling: 1) Insulate the rafters themselves, which requires leaving an inch and a half between the top of the insulation and the underside of the roof sheathing to allow for ventilation. Air needs to move to take out heat in summer and moist in winter. 2) Add insulation on top of your roof. This would be done the next time you replace your roof shingles. You'd add 2-3 inches of foam board insulation on top of the roof, and then more plywood, and then the new roof.
Both of these are major projects in your scenario because your ceiling is already constructed. If you opt to insulate the rafters, consider removing drywall for this project, and then deepen the rafter by adding additional material so it can hold more insulation. Or better yet, use spray foam insulation to fill entire rafter bay, because it doesn’t need to be ventilated.
Good luck! If you tackle this project, post pics to Money Pit's Facebook page letting us know how it goes.