Typically, vacuum insulation thermal resistance is 25 to 30 R/inch. Typical urethane foam insulation material is about 7 R/inch for comparison. The R value of an evacuated space rises quickly as pressure drops and then flattens out after a relatively low vacuum is achieved. In other words, increasing the vacuum above a nominal amount doesn't make that much of a difference. The cross section of the evacuated area should not make too much difference either. Fundamentally, vacuum is a thermal break for conductive and convective heat transfer. It's important for the vacuum insulation to have a mirror surface to also minimize radiative heat transfer. The typical level for a vacuum is