Understanding R-Values

One of the biggest points of confusion customers seem to face is choosing the best foam core density for their cover. With three different thicknesses, and multiple foam core densities, it’s understandable why. People from all over the country buy replacement spa covers, and it’s important to choose one that works best for your climate. Let’s see if we can clear up the confusion.

The R-value is a measure of thermal resistance used in the building and construction industry.

From Wikipedia. Or, as another dictionary describes it: A measure of the capacity of a material, such as insulation, to impede heat flow, with increasing values indicating a greater capacity. So in essence, it’s a value that determines how well a material can insulate against heat loss.
Which one to choose?

That really depends on the conditions your cover is exposed to. If you live in the mountains, at 7000 feet elevation, and get about ten feet of snow every week, you want to choose the highest R-Value possible. That means you want 2.0# foam cores. Whereas, if you like to vacation in Maui, and your spa never sees the likes of snow/sleet with minimal rain, a 1.0# core will suffice. See the chart to the left for a comparison of the R-values in our covers at different thicknesses.

For a more complete view of how well our cores perform at various temperatures, refer to the image below. It shows the insulation details for one of our expanded polystyrene foam manufacturers, or EPS for short. Look at the areas outlined to see how a cover’s thickness will perform at 40 degrees vs. 75 degrees fahrenheight.


EPS values for spa cover r value
Comparison of R-values (click to enlarge)

Some observations from the sheet:

4″ Thick Cover
1.0 pound foam R-factor is 4.17 at 40 degrees F
R-value of the foam only would be 4″ X 4.17 = 16.68

6″ Thick cover with 2.0# core
Average foam thickness = 5″ (once you account for the vinyl exterior wrap)
1.0 pound foam R-factor is 4.17 at 40 degrees F
R-value of the foam only would be 5″ X 4.17 = 20.85

Now, if the same 5″ to 3″ slope cover is upgraded with a 2.0# foam core:

5″ to 3″ slope spa cover with 2.0 pound foam
Average foam thickness = 4″
2.0 pound foam R-factor is 4.76 at 40 degrees F

R-value of the foam only would be 4″ X 4.76 = 19.04

The 5″ to 3″ cover 2.0# core has a LOWER R-Value than the 6″ to 4″ with 1.0# core!