Spa covers come in different thicknesses. They are also tapered so that water and melting snow can run off of them. When a hot tub cover is specified as a 4 to 2 taper it means that the centre of the cover near the fold is 4” thick. The foam will taper down to 2” at the edge of the spa cover. The 4 to 2 taper foam is usually a “starter” spa cover and is best suited for indoors or warmer climates.
The 5 to 3 tapered foam is for hot tub covers that are in moderate to cold climates where the hot tub cover has to work a little harder at keeping the heat in the spa. This hot tub cover will also have to be stronger to hold the snow loads that could cover the spa cover.
The 6 to 4 tapered spa cover is designed for use in very cold and snowy areas. This spa cover will have the most insulation quality and strength of all 3 sized hot tub covers. It is also the heaviest weight spa cover of the group.
While we are on the subject of weight, the other option with tub covers is the foam density. The standard hot tub cover uses 1 pound density foam. If you were to upgrade the density of your hot tub cover to 2 pound foam you would increase its strength, insulation value and also increase its ability to not absorb water. Since the foam particles are packed closer together there is less room for water.