With a conventional 2 x 6 wall, you get a nominal R-19 rating, but the effective R-value is usually only a little over ½ of that due to heat transfer through wood components, voids around the wiring, and real-world conditions.
Our minimum preferred framing system is a 2 x 4 wall with sprayed-in-place cellulose insulation backed with R-7 rigid foam insulation on the outside. The asphalt-impregnated paper facing on conventional fiberglass insulation is extremely flammable and difficult to extinguish once ignited. The cellulose insulation is a recycled product that is treated with borax and boric acid. Another example of green products having unexpected side benefits. Additional benefits are: reduced air infiltration, reduced cleaning, increased the usable area of the house and the overall measured external square footage, controlled insects and mold, and increased fire resistance.
We have also built frame walls with a nominal R-value of up to R-35, which is about what a straw bale wall is but requires much less floor space. This gives an effective R-value close to R-27.
Almost 20% of the heat loss in a conventional building is due to air infiltration under frame walls. About 15% is due to heat loss through the opaque wall areas. Air leakage under the walls is a more important factor than the nominal R-value of the wall, but it requires more research, more attention by the builder, and it is much harder to put in an advertisement.
A typical house with 2 x 6 walls might have 120 less usable square feet than if it were built with our minimum framing system. It would have an appraised value area of 60-120 feet more. At $100 per square foot, that would be an $18,000 to $24,000 difference in your taxed property.
More important are the long-term ownership benefits. As an example, if a $10,000 investment in energy conservation saves $800 the first year and energy costs rise at 10% per year while housing values increase at 8% a year, at the end of the second year the energy savings will be $1,680, while the $10,000 investment will have appreciated to $11,800. After ten years, that $10,000 investment will have appreciated to $52,338. It will have saved $12,750 in energy costs. Also, the initial investment is typically part of a mortgage and may be tax-sheltered.
This does not count intangibles such as protecting the environment, preserving indigenous cultures from exploitation, providing a stronger economy, and having lived in a healthier home. Experience has shown that over time intangibles are often the most valuable and the most profitable. Economy and ecology have the same root word: Oikos, which is Greek for house. What goes around comes around. We pay now or we pay later with interest, and the hidden charges are the ones that hurt the most.