What are some options to improve heat loss through a house underside that's raised above ground?
Our house is located in Central California, within a redwood forest just outside of town. Built in 1984, it leaks lots of heat, and its underside is a problem. The house is not on level ground, and most of its underside is raised between 1-3 feet off the ground. The structure is standard concrete perimeter foundation, with joists and subfloor about 24" O.C. R-19 fiberglass batts were stapled between the joists, but not installed particularly well, and the rats have been at them. We've installed wooden floors up top, and wonder how to avoid ripping those up and improve efficiency by focusing on the house's underside. Preliminary research indicates that some spray-on insulators suited for external, exposed application may offer a partial answer. We're wondering if any other options or hybrid combinations might offer higher-R insulation, while avoiding trouble with vermin and remaining installable in this challenging location.
A conditioned crawlspace may be the answer. It would entail enclosing, insulating, and conditioning the space, but would minimize heat loss and deal with the issue of vermin.
Unvented crawlspaces are addressed in the building codes and work well. You will need to discuss it with your contractor and with the local building department.
Building Science Information has a white paper on the construction, performance, and code requirements of conditioned crawlspaces. You can download it here.