A long time follower of This Old House, but a first time poster. I have a question on insulating the ceiling of a converted screen porch. This is on a 100+ year old lake front cottage in Maine that had little insulation anywhere, so even some stuffed newspapers would be an improvement.
The porch is made with 2x4 walls and then 2x6 rafters (attached to side of the house with deck hangers), all fully open. The roof is old 1x6 boards on top of the rafters with a full metal roof. To convert the porch, I have fully rebuilt the side walls with large double pane thermal windows. Kept the 2x4 walls to maximize space, with 3/4" plywood sheathing and also 1" foil poly iso on the outside. Planning to install R-13 pink inside for the walls.
My question is, How do I insulate the ceiling regarding airflow or not? Because it was a screen porch, no need to worry about moisture per se, because it was always vented. By adding the windows and then insulation, I obviously will be sealing the envelope.
The intent is not to make this a showplace of energy efficiency with R-345 ceiling and R172 walls so I can warm with an LED lightbulb. I would like to make this more comfortable so it could eventually be a year round house (on piers), but that is another chapter. Need to keep in mind moisture issues and making sure the insulation stays in proper shape.
I know I just cant staple a bunch of R-24 to the rafters as eventually the moisture would make it useless. Should the rafters be ventilated with air channels built in? How would I vent this at the top as the metal roof is sealed to side of the house? Maybe go a sealed route with 1.5" of foil poly iso on the underside of the rafters with R-?? stapled behind that? Woudl this also need a vapor barrier or tyvek on the inside?
Any help would be appreciated. I have seen a number of different solutions, but most are new, or newer, construction.
Thanks so much.