Hi, I'm living in '78 New England typical 2x4 walled colonial. I have to replace the clapboard siding and repaint. Frustrated that I can't access the interior walls without a huge cost in money and labor, I am considering adding a layer of foam board on the outside before I replace the clapboard with composite board. What is the risk of moisture problems in the intereior walls? Do I put the Tyvek over the plywood before applying the foam or cover the plywood/foam with the tyvek? Or do i just give up trying to make this a more efficient house and look for a one that was built better?

Thanks

Get as much insulation in the cewiling / attic as possible, and then carefully caulk and seal all windows and any other possible openings, and weatherstrip all doors. Do all that first, then upgrade the insulation in the floor / basement ceiling. After you've done all that, then worry about the walls, if ever.

If you do ass rigid foam to the walls, be sure it is of a type that will allow passage of moisture from the wall. You do not want a vapor retarder on that side of the wall.

Thanks for the reply. This is my first time at this site and really got educated last night reading the string. There are those who want something to work so badly that they become blind to contrary evidence. Attics been done for years, the last most important parts of the system left is wall and windows. Cost has been a factor for the windows. I won't do vinyl, but I see that fiberglass has come along and may be the best solution.
Do you know of an insulating board that has enough permeability to be suitable for the type of exterior project that I have in mind?

Thanks,
Larry