There is a book "From the Walls In" by Charles Wing that would be perfect for answering your question. Unfortunately it is out of print but you might have on in your local library of you might find a used book ******. It explains how to calculate the true R-value of your walls and how much they are costing you each year. It also gives an excellent explanation as to why most people are very disappointed in the savings they actually get from an insulation job.

An uninsulated wall actually has an R-value close to R-5. Filling the wall cavity with R-11 does not bring the whole wall up to R-11. More than 10% of the wall is studs which are unchanged by the insulation. The studs will actually "short circuit" the insulation by conduction heat around the insulation. The end result is a wall with about an R9 to R10.

In a typical house, about 20% of the total heat loss is through the walls. Windows and doors account for about 20% as does infiltration. The floors are good for 10% and the ceiling 30%. Your losses may be different, an actual survey would determine that. If your house is typical, then you would be looking at a possible 10% energy savings by insulating your walls. The 15% figure (even more) is possible if you have a two story house and you have already put a lot of insulation into the attic and have upgraded the windows, doors and the weatherstripping.

Something you should consider is having someone do a thermal image of your house this winter. Often very small heat leaks account for very large losses. For example, you might have a lot of heat loss through your plumbed walls. Heat and warm air can enter those walls through conduction and the holes for the supply and drain lines, rise up to the attic through holes in the top plates for the vent pipes and out through the attic vents. This can account for a large percentage of heat loss in a house that has had the insulation upgraded. It is often overlooked. And that is just one example.