I’m trying to make final decisions about how to reside my house. I’ve done a lot of research but I’m hoping to get some final advice before it becomes permanent. There is no way to blow anything into the existing wall cavities. I am looking to add two or three inches of either XPS or ISO rigid insulation to the existing wall sheathing (ripping off the two layers of existing siding) and finish it with cement fiber siding. My original research led me to believe that XPS would be more forgiving of moisture problems and more environmentally friendly but I’ve seen some things recently that make me think that I should just go ahead and use an ISO like Dow’s super TuffR board. I have made ample use of HVAC-calc 4.0. This program does not have an option for rigid foam only, but my predictions are that my load with 2†of XPS would be about 8,000 BTU of heat loss for the walls, 2†of ISO would be about 6500 BTU and 3†of ISO would be about 4500â€. Currently with no insulation the load is estimated to be about 20,000 BTU, so anything I do is a huge improvement. Obviously from those numbers this is a small house. The house is roughly 20’ x 30’, although the first floor has enclosed and heated porches in front and rear.
At this point I am leaning towards 2†of ISO and use furring strips to create a “rain screen†wall system. Three inch just seems like fasteners would be getting too long and awkward, not to mention the loss of another inch of overhang and added depth to all the windows.

So the first question is which insulation and how thick. Assuming the majority agrees with 2†of ISO what exactly would be the assembly process? House wrap or no? What type of fasteners? My other concern is how to safely attach that heavy siding off the side of 2†or 3†of foam board. Cement siding was the “family†compromise between vinyl or cedar, however vinyl obviously is light and has it’s own drainage system so the furring strips and “rain screen†type construction wouldn’t be necessary.

Think that covers the first round of ideas. Thank you for any insight or suggestions about how this will all work out. Since I joined last week I have read a lot on here that’s helped quite a bit, but still left me a little confused as every project is slightly different. Hopefully a short discussion here will be the end of it. Thanks again and as this is my first post here I’m sorry if I have broken any posting policies.