Thermal bridging on unvented roof
I know roof/attic insulation is a popular topic on the forums so I do hate to throw out another ‘how about my situation’ question — but I’m looking for some guidance as I try to do as best as realistically possible with a small room in a 100-year old house.
The area is essentially a 10×15 attic space that is part of the 2nd floor of the home but is currently uninsulated – see attached picture. Climate Zone 4a. I would like some input regarding my plan; fully aware that the combined R-value may not be up to code.
My plan is to install 2” of closed cell foam on the underside of the roof, followed by R-23 mineral wool for a theoretical R-value of 37 in the bays themselves. The rafters are a true 6”, so this would bring my insulation depth to 7.5”, so would it be permissible to add 1” strips of XPS foam to the underside of the rafters to gain the added depth necessary & also help with thermal bridging?
Re-roofing with xps and/or furring out the rafters to gain more insulation space are two options that are not really on the table right now given the scope of the project & the use of the room.
Any thoughts appreciated.