I am specifically considering the re-roofing of a an older home where rafters were made with 2x4 or 2x6 and the rafter space (and possibly the attic space) does not allow sufficient space for insulation and proper ventilation behind the insulation. The old layers of roofing have to be removed in any event. At this point I am considering applying insulation boards on the existing sheathing and then a new layer of sheathing before the new roofing material. (similar to a SIP: structural insulated panel) The two issues I am concerned with is that the insulation can act as a vapor barrier. Would condensation potentially rot the lower sheathing. How is this avoided in SIP roofing panels? The second issue may be with fire regulations.
Spray in foam could be an option.. you would need to talk to someone who knows more than I do.. but Ive seen great results
Good question and I do not know the answers. For the fire issue, call your local fire department and chat them up. Then call your building inspector and talk with him. If he's been around awhile he may have seen it done before and have some information on how it has worked out or what are must do's.
The sip panels are vented depending on what thickness over all that you want. The last ones I used had 4 inches of rigid foam board then on top of that had 3" strips every 16" an inch thick which created an airspace. On top was 5/8 Osb. To create an affective air flow the fascia cannot plane through with the plywood at the bottom but fall just short of the 1" space. To cap it a oversized vented drip edge is used. You must also have a ridge vent for proper flow. These panels can near 100$. You can do it your own way like I do when building log cabins by laying 2" foam down first then lay 2x4 16 on center on top then finally your plywood. If you use CDX instead of Osb it will outlast it by 100%. Rigid foam insulation is not like batt-faced. Rfboard ins. Is used in boat hulls and even after being submerged for long periods of time will return to its original density and r-value. For obvious reasons batt ins. Will not. You won't have a moisture problem as long as you don't have a venting problem. If you are using high hat lighting don't let the cans touch your rigid ins. Tuck batt in softly around them TAKE THE PAPER OFF! By code if you have an interior wall 10or more ft. It must have a fireblock, simply a 2x4 turned flat between the studs. On your roof the ceiling you see inside, (most likely 7/8 tongue and groove boards) passes as the same. Good luck!
