Best Insulating for 2×4 and 2×6 walls in old construction (no exterior insulation) – GreenBuildingAdvisor

Friends:

Greatly appreciate all the good ideas shared here in GBA but still struggling to come to happy decision for renovating my house.  Been at it for long time and finally getting to finishing the interior and looking for ideas and/or reassurance on how to “best” insulate/finish the walls.  I am well aware of air sealing etc and am working on that throughout whole house. Ceiling/attic is fully taken care of (R38-60) – now just needing to deal with walls.

House in zone 6a in New Hampshire.  Eighteen hundred sq. ft (1800 sq.ft) house is half 2×4 and half is 2×6.  Exterior was rebuilt 15-18 years ago (sadly before the value of external insulation was really much understood) with Hardi board, Typar and 1/2″ ply.  Typar seams were “somewhat” taped (it is what it is).  The exterior shell is (has been) fully water tight for 15-18 years (I know because there is no interior finish just open walls with fiberglass batts and I have recently repaired/replaced many rotted window trim made with the disastrously bad Windsor 1 finger jointed wood – that’s another story!!:-) But now I am going to finish interior.

In my situation which I think represents many thousands of houses in the USA (maybe millions) it seems incredibly difficult to find a reasonable, economical balance between all the aspects of wall insulating, i.e., environment, cost, R value, potential for vapor/condensation problems, etc.   

What does anyone think of the following plan that seems to be my best idea:

(1) for 2×4 walls: 3″ xps board cut and installed in wall bays with can spray foam seal all around; covered with Intello Plus on studs (completely intact with only a couple penetrations – no elect.); covered with 1×3 horizontal  strapping (potentially with thermal break using pieces of sill seal etc.); covered with 2×3 vertical strapping (creating electrical space which can be reasonably filled with fiberglass bat); covered with 1/2″ drywall and paint.

(2) for 2×6 walls: 3″ xps (like above); covered with 2 1/2″ fiberglass bat (this zone is where electrical will be installed); covered with Intello Plus on studs with appropriate gaskets etc for penetrations (I am still considering the furring option, but since not needed for electrical it seems an unnecessary added cost/effort); covered with 1/2″ drywall and paint.

Now, I know that xps foam is not considered the friendliest environmental product, but balancing all the related issues, it seems to come out on top: meaning for me, that even with the Intello no other product (other than spray polyurethane) gives as much R while minimizing vapor getting to the sheathing. I should add that I have concluded that the design has to be capable of surviving/performing if and when interior Relative Humidity gets into high 40’s or around 50% in wintertime – and yes the house will have adequate venting to try to control RH but reality of occupants following proper RH control can not be counted upon.  Summer time cooling is not a design factor though ac is used on rare occasions. 

So, I would appreciate anyone’s thoughts and ideas on how I could save some money and still get a decent wall insulating job done that I can confidently feel that it won’t rot away for at least 50 years:-)