Is it as effective to place foam board insulation on the interior of a 2x6 wall compared with placing it on the exterior? In Iowa, with flash and batt placed in the cavity, what would be the minimum thickness of foam board to help prevent cost efficient thermal bridging? What are the negatives to placing the foam board on the interior? Is there any value to using foam backed vinyl siding if the negatives for using foam board on the interior are substantial? Finally (), I have been led to believe that deciding to place 1" of foam board on the exterior of a home after the windows have been set is not possible? I am wondering if this is a "true" assertion, without a solution? Thanks a bunch.

dolphin,
Nothing is impossible. Just harder rather than easier.
I am not convinced that insulated vinyl siding adds any real insulation value, too much air movement around and behind.
As to adding insulation to the exterior, it would require some additional siding trim work around the windows, but is not that hard to do. Depending on the type of window you have, you may actually be able to see the results from inside the house.
I, personally, can see no reason not to place the insulation on the inside of the wall before drywalling. Some of the more informed insulation guys may have arguments that I am not aware of, so hopefully, they will respond here also.
You will have to deal with window and door jamb issues, again depending on the type of products you are using.

A thermal break works on either side of the stud (or in the middle, if you make it a double-studwall.) You have to pay attention to the vapor drive issues for your climate, and be sure not to build vapor traps. Flash foam isn't sufficiently vapor retardent to present an issue, and in the quad cities you want the most-retardent layers on the warm side of the layup. (You can use foil or vinyl faced foam board on the interior side of the studs. This wouldn't be advisable in hot humid climates though.)

If all of the windows have been set with no accomodation or flashing for an extra inch of thickness it's quite labor intensive to do it over. It's possible- anything is possible for a price- the question would be, "Is it worth it?". If you don't mind giving up an inch of interior space, putting the thermal break on the inside is ALMOST as good. You're losing a bit of that break at the band joists (at the sill, and between floors), but if they're flash-foamed on the interior it's really only lost a the floor/ceiling joist & band joist interface, and to a very slight level at electrical boxes & penetrations, etc. .

R5 is the minimum you'd want for a thermal break for a 2x6 (doubling the R-value at the stud). A inch of XPS will get you that, but an inch of foil-faced iso FSK-taped at the seams and caulked/foamed at the floor & ceiling interface will give you R6-6.5 as well as being excellent Class-I vapor retarder on the warm side of the structure. From a total building heat loss point of view this extra R1.5 might be enough to overcome the loss of the thermal bridging at the joist ends along the band joists. All else being equal, that's what I'd do at this point. An inch of interior space is less than a 1.5% reduction in all be the smallest of long-skinny rooms with the long side being an exterior wall. The worst-case would likely be the corner rooms of the house. A 13x15 corner bedroom only loses 2.3 square feet- a 195 ft^2 room becomes 192.7 ft^2, barely over 1% reduction.
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I would like to thank the responders to date. Great information/confirmation.

We have 1 inch on the inside. The biggest thing we didn't consider was the electrical. All the boxes had to be moved out (they didn't like us), luckily they found out before they put everything in. We also had to consider the in floor heating duct, and not have it right against the existing stud.

have 18 inch walls in sw bedroom with concrete block construction, furred out 12 inches with 1 inch sheeted foam, then drywall.  still pretty hot in summer, but am heating same with a very small propane lamp.

mild winters in az

nogottarancho in Gila Bend

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