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On an open beam ceiling roof, I need to add insulation to the roof on a house built in 1973. As I understand it, I need to rip off the old shingles, and lay 2″ foam board insulation sheets over the old roof sheathing. In doing this, is it best to use the 4×8 foam sheets with 2x4s between the sheets to support the new sheathing over the foam boards? It seems that with the spacing this way it will be difficult to achieve proper support for the new sheathing and to nail the new sheathing to something below. How structurally sound is the foam sheeting with sheathing laid over it? Any better ideas on how to improve insulation on such a roof where you have to add insulation from the top rather than the bottom?
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Chris,
For clarification:
Is you current roof, as is, vented?
Is there an air space between the top of the insulation in the rafter bays and the bottom of the current roof sheathing?
Do you have soffit vents? A ridge vent? Pot vents?
*Mongo,No vents exist on this roof. Makes you wonder where the building codes were, eh? I haven't pulled up the old sheathing to see if there is an air space but I do know there are no soffit, ridge or pot vents. I realize this may necessitate crafting an air space in the new construction. Should this space be between the new foam board and the new sheathing, with appropriate soffit and ridge vents to allow convection flow? Would this be standard practice with a new construction of such a roof insulation system?
*Chris,Forget the venting.Foam the sheets to each other when laying them down.Lay new sheathing over the foam sheets as you goBuy Olympic Insulated Panel Fasteners and screwright through the new sheathing into the old rafters.Foam all around the perimeter of the foam board/old sheathing/etc to make the foam board airtight to the tops of the walls.Install felt and shingles as usualThe next roof I do like this that also has metal roofing going on I will probably skip the new sheathing all together and instead use a bearing plate under the clips that hold the roof down.-Rob