The house was built in the early 70's prior to the energy saving efforts. I have 2x4 stick framing with craft faced f-glass insulation. There is a 3 mil vapor barrior on the interior side of the studs (although it appears to be sloppily applied with no taping of seams).
I'm planning on replacing windows & siding in the next year. Living in tropical Minnesota this house can see weather from -35 to +110 (with lots of wind at the lower extreme). The largest exposure of the house faces south which makes a great collector in the winter but makes summer cooling impossible. Presently HVAC estimates indicate that I should be able to cool this house with a 2 ton AC (Current 2 ton AC will not cool house to 85 on a 90 + sunny day -AC running constantly (AC is supposedly in good working order)). Heating has not been a problem except for drafts on windy days (bad windows & vapor barrior).
My question is: While I'm re-siding can I put expanded polystyrene sheeting (3/4" to 2" thick) on the outside without causing moisture problems. I have found very little information about this in my research. What I have found suggests that an extremely tight seal on the outside that prevents any leakage will keep the interior (inner wall) moisture level down so that the existing (crummy) vapor barrior won't be a problem.
The entire interior of the house is finished so that adding vents to the top or bottom of the 2x4 sill studs is not pratical.
I'm thinking that a 1" foil backed insulation board (rated R-3.9) will be a great improvment over the existing walls (even at the loss of the winter heat gain).
I know that some local siding companies add some 1/4" fanfold styrene, usually sloppily stapled on top of existing siding (I would think it would have little if any effect). I expect the sloppyness allows for the insulation to not count as a vapor barrior.
The only reason I'm not considering 2" styrene is the difficulty locating the studs through the styrene & fiber sheeting board to nail the new siding securely.
No, I haven't contacted local building inspector yet. I was hoping to get a general consensus before I attempted talking to him.
Thanks for any input.