I am now doing a project on an old building that I have not done before and need some advice.
I purchased an old commercial building that originally was a one story building. If you know of old Midwest commercial buildings in small towns you know they have about a 7 degree sloped tarred roof, sloping from front to back. These roofs have the roof rafters going perpendicular to the length of the building. The rafters are also perpendicular to the downward slope of the roof.
The roofs are usually so high in these old buildings there is room for living areas between a new first floor 8 foot ceiling and the original roof.
I managed to get a large amount of used 2 inch thick x 2 foot x 8 foot Corning Blue Polystyrene foam insulation free off of a large commercial building in a nearby city. It has an R value of 10 per 2 inch thick sheet. It was used on the outside of the roof on that building but is in really good shape without tar on it.
My problem and question is this. I would like to install 3 layers of this rigid foam in between the roof rafters for insulation. That would give me an R 30 for the roof. This would be like a cathedral ceiling with drywall attached to the rafters without a vented space. The problem is since the roof rafters run perpendicular to the slope, there is not a good way to vent the space.
New construction cathedral ceilings are suggested to be sprayed with foam on the underside of the roof to provide a sealed air space. Because of the shape and roughness of the underside of the roof this is also not an option. Additionally, this is an old building and this method does not seal between the rafters and the roof, just between the rafters.
I do not want to take any chances with this because these types of roofs are notorious for leaking if proper roof maintenance is not done regularly.
It would be easier to install the rigid foam on the outside of the roof but 3 layers on the roof would probably be out of the question. I do not believe it was designed for that purpose and fixing it in place good be cost prohibitive. A person would probably have to install the first layer with long ring shank nails with a flat washer next to the nails head so they would not pull out of the topside of the foam. Then use construction glue for the next two layers. On top of these layers the Corning mesh would have to laid over it and tarred. Then protected with a fine layer of rock.