There are a lot of things to consider when purchasing a new or replacement hot tub cover for your home spa. You have to take into account size, need, and price that will accommodate your lifestyle and budget. Amongst all the noise of measurements and price, there is a term you may not be familiar which is often associated with covers: hot tub cover R-Value. What does R-Value mean? What does it measure? Is it a useful tool as I shop for a new hot tub cover?
R-Value is a term that is meant to equate for a material’s ability to resist the flow of heat through a solid. This is valuable in a hot tub cover because it helps your spa water maintain a constant temperature, allowing it to operate more efficiently because it isn’t constantly heating and adjusting the fluctuating water temperature. The industry utilized a standard that has been used in home insulation so that they could compare hot tub covers’ measurable ability to contain heat.
Here are some helpful facts about hot tub cover R-Values:
Most industries using R-Values are regulated by FTC standards. Commercially sold insulation must pass independent testing conducted by American Standards and Testing Methods in order to be advertised or marked with their R-Value. The problem is that there isn’t an FTC recognized independent test for spa covers as a whole and an R-Value attached to a hot tub cover is essentially an interpretation of the foam’s ability to insulate.
R-Value only truly applies to the foam core inside of the hot tub cover. While the surrounding polyurethane and vinyl that envelops the foam core contribute to its ability to keep heat contained inside of the hot tub, the term isn’t allowed to be applied to anything other than the foam.
The R-Value of insulation INCREASES as the air temperature on the cold side of the insulation DECREASES. To explain further, if you test insulation at 25° F and at 10° F, you get a HIGHER R-Value at the lower temperature. R-Values are relative and the air temperature is directly related to the R-Value number.
The following chart lists R-Values for different thicknesses and densities of expanded polystyrene foam (which is what 99% of hot tub covers are made using). For tapered covers, the R-Value is calculated using the average thickness of an 8 foot square cover. If your spa is smaller than 8’ x 8’, your over will be thicker because the outside edge of the foam is cut off to construct the cover shape. Therefore, your cover will have a slightly HIGHER R-Value than the table suggests.
As previously stated, the hot tub cover R-Value number doesn’t account for all the other elements that are often standard with spa covers. R-Value rated foam cores are typically heat sealed in polyurethane, encapsulated inside a high-grade vinyl, and have vapor skirts to help maintain water temperature. All of these increase the insulating value of the hot tub cover above what will be quantified in the above chart.
R-Value testing is tricky because it’s conducted at room temperature and doesn’t take moisture and/or vapor into consideration—major factors in hot tubs use. Both hot water and moisture reduce R-Value. So be careful when using R-Value as an absolute factor when shopping for a hot tub cover.
It’s important to find a cover that is well constructed and high quality. They will inherently have a high heat retention rate. Look for a great quality hot tub cover and don’t get too distracted by flashy numbers. If it’s not well constructed, that hot tub cover R-Value won’t mean much.