I recently had an energy audit done at my house, which is about 100 years old, in Southern Ontario, Canada. The main walls of the double brick house, and the area behind the second storey Mansard roof at the front of the house, are uninsulated. The auditor suggested building out the main walls with 2″ x 2″ and using rigid 2″ XPS R10 insulation and then drywalling over it. He also suggested accessing the small “attic” like space behind the sloped front and insulate using R12 batts. I have a few questions about this approach.

MAIN WALLS: If I use the XPS on the main walls and then drywall over it am I correct in thinking I DO NOT need vapour barrier? The rigid foam will prevent warm moist air escaping and condensing on cooler surfaces within the wall cavities.

MANSARD WALL: I can easily gain access (via a window seat) to the inside of the “attic” like space behind the Mansard and thus have access to both the backside of the room’s drywall and the backside of the roof sheathing on the sloped front. Could I use the XPS in this situation? I am thinking I would cut the foam board to fit tightly between the studs and then seal the boards to the sides of the studs using spray foam. This is similar to what was suggested for the underside of my cold closed in porch floor. Since the area to be insulated behind the Mansard is so tiny, the increased cost of rigid vs. batts is basically negligible.

In order to reduce thermal bridging via the studs, and make installation easier, could the XPS just be attached horizontally across the backside of the studs. This would result in a wall section, from the inside to the outside, of: drywall, lathe (see below), studs within an approximately 4″ air space, 2″ XPS, open “attic” space, Mansard sheathing, roof shingles. Does the introduction of the airspace cause new problems?

(Note re: lathe: The house was renovated 8 years ago by the previous owner. They stripped all the plaster out and then drywalled directly over the lathe. I do not know why they didn’t take the lathe down and add insulation, vapour barrier, and then drywall, but that is what I have to deal with.)

I do recall reading somewhere that installing a traditional vapour barrier from the backside of the studs is a VERY bad idea. I am thinking this is because it would be almost impossible to get a 100% seal between the plastic and studs, thus concentrating escaping warm moist air right beside the piece of wood you are trying to protect from condensate and thus rot etc. So I thought the rigid plus spray foam would work from the backside. You may be wondering why I don’t just do the same to the front wall as the main walls (i.e., just add 2″ x 2″‘s and rigid foam followed by new drywall) but the loss of 2-3″ of room space in that direction would be more of an issue. Also, all the widow openings, sloping wall due to the Mansard, and various other assorted corners, etc. would significantly complicate the surface application of XPS followed by new drywall.

Could you please let me know what you think about this.

Thanks very much.