I want to add insulation to the cathedral ceiling in my shop
About 20 years ago I built a workshop. It has a metal roof installed over 1X4 strips that are nailed directly to the 24″ oc 2X10 rafters. I had a local insulation installer spray cellulose insulation directly against the metal roof. The insulation began to fall off almost immediately and today approximately 1/2 of it is gone. I would like to put more insulation overhead and have considered either fiberglass batts or spray foam. I have always been taught to provide an air space above fiberglass but there is not an easy way to ventilate above this cathedral ceiling type area. The shop is usually unheated and has a concrete floor that was poured on a 6 mill plastic vapor barrier. If I use fiberglass, I would also like to install a product directly under the fiberglass to keep it and the old cellulose from shedding, house wrap maybe? I have looked at spray foam, but it is expensive and I have been told that I would need to remove all of the remaining cellulose so that the foam would “stick”. This would bee both time consuming and messy since there is still a good coat of the cellulose on the sides of the rafters. Thanks for your help
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You didn't mention whether the shop includes an attic. If it does, the best location for the insulation is on the attic floor.
Since you didn't mention an attic, I'll assume that you don't have one, and that you want the insulation to follow the sloping roof line.
If that's the case, and you want to improve the insulation in this shop, the first step is to remove all of the cellulose and throw it away.
Once that's done, you have a great many options. If you choose spray foam, you shouldn't spray the foam against the underside of the metal roofing. You have to install something between the rafters (plywood, OSB, or cardboard) to prevent the spray foam from sticking to the metal roofing.
If you choose to install fiberglass batts or cellulose, you need to include a ventilation gap between the underside of your 1x4 purlins and the top of the insulation. This can be accomplished by installing 1.5 inch by 1.5 inch "sticks" in the corners of your rafter bays, followed by a baffle made of plywood, OSB, cardboard, or rigid foam.