[DanielDudley]

I have done foam board insulation on the outside of houses, and it does have a lot to offer in that it is non invasive and you can live in the home while it is being done. I usually put up the board, tape and seal all of the joints, and then put up 3/4 inch wood furring strips that are tied into the studs to carry the siding. Often new energy efficient windows are installed at the same time, mounted over the 3/4 inch spacers. These spacers are also usually vented from top to bottom. This give longevity to your siding, but also creates a chase for heat to rise in. Two inches of rigid foam seems to be the sweet spot, but many people here choose to go to 4 inches, so you can see why you would retrofit windows flush to the outside. This is a super effective system when done correctly, but it is very labor intensive and requires a total remodel of your exterior.

I did not do this to my own house. Instead, I cleared the attic and had spray foam sprayed to rafter bays. Then I repaired my 8 inch timber frame sills down at the foundation, and sealed them to the stone. after that, I made sure that all wiring in my outside walls was up to modern standards, and I had blown in cellulose installed. Modern cellulose machines blow it in at a higher rate of power and pressure, so it tends to fill the cavities better and settle less. The company that did mine put holes every three to four feet vertically, so I am sure I got good coverage. The foam company also sprayed around the inside of my sills in the basement to seal that better. As an additional step, I also went around the inside walls of the house and made sure any cracks around windows or baseboards were caulked and sealed.

Originally, I was also planning on putting 1 1/2 inches of rigid foam board on the outside of the house and residing, and planning on retrimming the original windows to match that siding. However, I found that the insulation as installed was SO effective, and my energy usage so drastically reduced, that I never bothered to do that.

Obviously you need to keep up with the exterior of your house, but in general, your interior walls and particularly the paint will act as a vapor barrier. Also, you don't want the insulation to get wet, but guess what ? You don't want any insulation to get wet. That is just standard procedure. Also, open cell foam or blown in cellulose does breathe, so it will dry out if it had to, assuming that proper repairs are done to deal with any problems you might find in the future.

No system is perfect, but I can tell you that any improvement to the insulation of your house that you do now will add to your future comfort. Some people blow up to two feet of cellulose into thee attic space, but we wanted foam on the rafters because we use our space. Obviously the house is warmer in the winter, but you don't always notice that, as you would just turn up the heat. What we noticed is that the house is much more comfortable in the summer, particularly the second floor, which we used to refer to as the easy bake oven.

If I had up to date wiring inside my outer walls, I might consider spray in foam, and I think it is a no brainer for new construction. Otherwise, I would consider cellulose. The point is, it isn't always about what you use, or if it is the ''best''. The point is, you want to do something, because whatever you do, if it is done correctly, will make for a substantial and immediately noticeable improvement in your comfort and energy use.