R-value is a measurement of the resistance to heat transfer for materials like the fiberglass insulation in your house or the insulating foam in our hot tub covers. In theory, the higher the R-value, the greater the heat retention and, heat retention is the primary function of any spa cover.
Most industries using R-values are regulated by FTC standards. Commercially sold insulation must pass independent tests created by American Standards and Testing Methods (ASTM) in order to be advertised or marked with their R-Value.
The Story Behind R-Value
As there is no FTC recognized independent test for hot tub covers, so any R-value stated by a spa cover dealer is actually an unregulated, unmonitored “interpretation” of the insulative value. It’s open to speculation and abuse, from adding the R-value of the air space between the water and the cover, the space between the vinyl and the cover insulation, or simply blatantly inflating the number. At HTW, we state only the known R-value of the insulation itself plus a small increment for the vinyl and plastic wrapping the closed cell foam core.
The R-value of the actual foam insulation plus covering in our Standard 1.5 lb foam covers is approximately 13.5 and for the denser 2 lb foam, it jumps to 15. This is based on an average spa cover thickness of 3″; our thicker hot tub covers will retain proportionally greater amounts of heat, as you can see in the image below.
R-value testing of materials is done at room temperature, and doesn’t take into account moisture and vapor. In the spa environment, we have hot water and lots of moisture. Both of these can facts will dramatically reduce the R-value of a material. A spa at 105 degrees will challenge the R-value of any material tested at 60 to 75 degrees.