Thank you Dana for all the excellent information......it will take me a while to absorb it all.
Another question: each of my bedroom exterior walls are about 12ft wide by 7ft 8" tall......both walls have a window about 6 ft wide and 3 1/2 feet tall with U factors of 0.33. Since the windows have an R value of about 3-4, (and also I cant really insulate the 6 ft 2x10 header above the windows ) don't the windows/headers severely limit the total wall R value capability anyway?.....making it seem like going to a lot of extra effort/spending a lot of money for a 15% improvement in R value on the 2x4 studded part of the wall kind of not really worth it in the overall walls R value?.
Thanks
Adding interior foam would insulate over the header framing too. If you can give up an inch of interior space and deal with the window sill/trim issues you can cut the losses of the framing portion by more than half with an inch of EPS, and still not create a moisture trap.
Is it "worth it"? Depends on what your goals are. From a net-present value on future energy savings for the next 3 years, maybe, maybe not, for the next 10 years probably, and over the next 15 years, CERTAINLY. And that's assuming you're heating with something cheap like a condensing gas furnace. If you're heating with propane, electric baseboards or some other more expensive source dial it back in. But there is the long term resilience issue too, which also has value.
The way you framed the question is sort of like asking, "Since the back door gets left open a lot, is it really worth insulating the house?"
Of course it is. It may also be worth retrofitting the back door with an automatic door closer too, but that's a separate project on a separate budget.
Heat loss is on a per-square foot basis, as is insulation cost. If it's worth it for 10 square feet it's worth it for 100 square feet, and conversely. Code minimums are VERY conservative on cost of construction vs. cost of heating/cooling that building. If you plan to live there for more than just another year or three, just do it, and know that the after-tax payback of the "extra" investment is going to beat your 401K (by a lot.) Any time you get into a project, it's worth bringing performance up as close to current code as you can, and it's ALWAYS worth making it as air tight as possible.
While it's rarely worth it to gut a wall simply for the purpose of raising the thermal performance, if it's already opened up, taking it to code min (or as close as possible) is a financial no brainer if you are using cheap high density fiber for the cavity fill, saving the foam-budget for where it is thermally breaking the more conductive framing.
On a lifecycle basis for new construction there is a good argument for hitting "whole-wall" R values in the R20-R25 range (that's whole-wall, not center-cavity R, such as , a 2x6/R20 plus 3" of continuous rigid EPS). In a retrofit that's not really an option unless you're going to re-side the place and re-mount /replace all of the windows & flashing, and put the fat foam on the exterior. See
Table 2 p.10 of this document, (but read the whole first chapter for the rationale.) Seattle is climate zone 4, so refer to the row for zone 4, bearing in mind that those numbers include the thermal bridging factored in, and using insulation materials that are lower $/R wherever practical & appropriate.
Closed cell foam runs about 17-18 cents per R per square foot.
R15 rock wool even at box store prices runs less than half that: A 12 batt bag is about 60 square feet, at R15 makes (R15 x 60'=) 900 R-feet, and costs $45, making it, ($45/900=) 5 cents per R-ft.
R15 fiberglass may be somewhat cheaper still (though my personal preference is to work with rock wool, for a number of reasons.).
Unfaced 1.5 lbs density EPS runs about 9-10 cents/R-ft^2, open cell spray polyurethane about 12-14 cents, XPS (except fan-fold) about 13-15 cents/R-ft^2
Fan-fold XPS is a bit more expensive per R- running about $45 at box stores for 4' x 50' ( 200 square feet) for the R1-ish 1/4" stuff slightly more for the R2-ish 3/8" stuff, or roughly the cost of closed cell polyurethane, but since it's it's thermally breaking the studs, you get the full-R out of it. Some box-stores carry it, most don't, so you may have to shop around. The most commonly available versions comes in blue (Dow), pink (Owens Corning) and green (Pactiv), as well as a few others, but they are all pretty similar in terms of R and vapor permeance.