It's possible to retrofit insulation into an existing home, even without the access provided by remodeling. The choices, however, are much more limited. There are some retrofit insulation products that are designed for minimal disruption to finished wall or ceiling surfaces. Significant gains in R-value, however, may mean hiring a professional to assess your needs and devise an energy savings plan. Small or large, insulation projects can yield substantial gains in R-value and in comfort.
Wallpapering can involve a steep learning curve and most professionals advise starting in large rooms. Save bathrooms for last, or at least until you’ve got your confidence.