Sorry for the technical question, but I am an engineer, and not happy unless I can calculate stuff. What are the approximate R values for uninsulated walls and ceilings? If I have that, I can calculate savings for insulation, at least on paper.

Oops - guess I should have mentioned that the ceiling is sheet rock, the walls are plaster/lathe with clapboard siding, about 3.5" between them.

Since you need a starting point, assume an R1 or R2 for the minimal insualting value you have. - Being an engineer, I understand you need a starting point even if it is an assumption.

Since you are trying the impossible of trying to calculate real insulation values, you will have a real challenge. R13 fiberglass in a wall will give you less than R13 for the wall. - You have to recognize the difference between parallel heat flow, series heat flow and the concept if series-parallel heat flow calculation. That does not include the correction for mass, which depends on the local climate.

For simplicity, look at some of the accepted wall assembly R-values. Avoid the advertised "pink panther" type values.

I googled "R Values" and discovered a website where they are published. I don't seem to be able to cut and paste web addresses here but it is Coloradoenergy.org. I am not trying to calculate absolute heat losses, but rather to get a relative measure of where the heat is leaving the house, and where to focus my efforts. From the information on the website, an uninsulated wall has an R value of a litttle over 4, and an uninsulated ceiling of a little over 1. Looks like the attic is the place to start! Storm windows, on the other hand, would take years to recover their cost - they add about 1 to the R value of a window. My parents replaced some of the single pane windows that had storm windows with double pane windows and screens. According to the information on the website, this did nothing to improve the R value.

When using published R-values, keep mind that there is a big difference between the R-value of a material and the R-value for a wall assembly. As an example, 6" fiberglass has an R-value of R19. When you put this material in a wall assembly with steel studs, the wall R-value could be as low as R11. For wood there is a reduction, but it is not quite as much.

When you are looking at codes and the prescriptive wall assemblies, keep in mind this is a prescriptive, and not a performance value that you would use for comparisons since different assemblies have different properties.

As an example the the 6" steel stud with R19 fiberglass wall mentioned above, would not perform as well as a steel stud wall with no insualtion in the cavity and only 2" of good extruded foam applied over either side of the studs/rafters. The reduced infiltration from rigid foam is an uncalculated bonus.

If the same layer of foam was applied over a concrete masonry or another "heavy" wall, the annual heat loss could be reduced by possibly another 25% compared to a lightweight wall (mass factor adjustment).

There are some numbers here, but they don't look quite right to me for some reason: http://www.p1m.com/rv.htm
Here as well: http://www.e-star.com/ecalcs/table_rvalues.html

Many of those figures look like others I have seen in documents like ASHRAE.

They are just numbers for individual materials/items. If they are not used correctly, you can be off by 100% to 200%.

Advertising people (like those that wear pink and have feline pets) can use them in many different ways. Most people do not know how to use them. - That is the reason codes tend to be written in a prescriptive form that can be very political in the writing process. It leaves little lattitude for professionalism unless you can prove that some other assembly is equivalent, which is not hard if the building official can think.