Engineering Question #1164

DeeCee, a 46 year old male from Indianapolis asks on January 8, 2003,

How does vacuum insulation work and can an R Value be assigned to it? Does the depth of the vacuum change the R Value? Does the cross sectional area affect the value? Finally, what is the typical vacuum depth for insulation, (torr, ect.)?

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The answer

Barry Shell answered on January 8, 2003

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

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