I was working with a homeowner about adding a screened porch onto their house. The plan was to do a hybrid timber frame with a sloped ceiling with exposed rafters and T&G exposed decking. Easy enough. Now that we were about ready to start they’ve decided they want the ability to add some storm windows to turn the space into an extended 3 season porch and add a vented gas stove to the space so in the early spring & late fall the space can be heated up (not all the time, just when in use).

I was thinking that the best thing to do would be add some (maybe just 2″) XPS above the decking and screw OSB roof sheathing down through it into the rafters but I’m wondering if I’ll be creating a possible vapor/condensation nightmare in the future. They “want” insulation but doing a full R49 of rigid over the decking seems extreme (and honestly not really possible without making connection to the existing roof a mess) for a space only sometimes heated (and probably never heated when it’s really cold).

Am I overthinking this? Or should I tell them that it’s all or nothing on the insulation in order to prevent rot in the future?

Climate Zone: 6B

Thanks in advance.
JMS