If you decide to add insulation this fall, focus on your attic or roof line and then reducing air leakage in the basement and/or crawlspace. Especially, where your foundation meets your floor joist.
Access to your walls is not really a good option because you're not going to remove the drywall unless you're doing some remodeling. If you're planning to remove your siding and re-side the house, there are some measures you may want to consider at that time to improve the insulation.
REMINDER: R Value – Fact or Faulty Myth R-Value in the lab just doesn’t cut it in the field when it comes to the real-world. Most consumers are familiar with the term but have no concrete understanding what it really means other than it has to do with insulation and energy efficiency. The problem is the understanding stops there and crowds out the important information and science necessary to realize/achieve the results your hoope for. Unfortunately, in the construction world r-value is the “pink” standard.
The problem IS "R-values can be misleading! To use a quote from an ultra energy-efficiency engineer, “if both insulation materials have the same R-value, they should perform the same,” ask yourself this question, “would you rather pour hot coffee (which is served at around 180 degrees) over your lap into a thin foam cup or into 1″ of fiberglass insulation (which is about the thickness of your furnace filter)?” Does one inch of foam truly perform the same as one inch of fiberglass? No! You get the idea!
All insulation materials (except for urethane foam) are going to test out at between R-3 and R-4 per inch. In fact, if air can penetrate the building materials(like fiberglass) the effective R-Value is ZERO, no matter how deep you stack it.
So before you add rolls of fiberglass insulation or blow more cellulose in your attic, make sure you seal any air penetrations in your attic floor
To get the 'biggest bang for your buck' when adding insulation, focus on your attic or roof line and then reducing air leakage in the basement and/or crawlspace. Especially, where your foundation meets your floor joist.
Hope that helps!