Hot tub shells can be broken out into two parts. The first has to do with the design (look and the fit) of the shell. The second has to do with the construction or the way the shell is made.
Let’s start with the design of the spa shell. When you peer inside of a hot tub the first thing you notice is the layout or the seating design. This needs to be aesthetically and functionally pleasing to the buyer. You have to like the seat locations, ergonomics, and styling. For example, if your hot tub is going to sit in your backyard in a location with a view then you want to make sure that the model you are buying has a layout that is going maximize the view. Another consideration is a “cool down” seat. This is typically a raised flat area inside the spa that may have the appearance of a step so that you can use it to easily get in and out of the hot tub and also use it as an area to sit partially out of the water and cool off. Cool down seats are also great if you have small children as they provide a safe area for them to sit. The fit also needs to be right. If you are a 6’ 3” tall person who wants a lounger then you need to make sure that your legs aren’t in your chest when you are in it. Furthermore, if you are a 6’ 3” man and your girlfriend or wife is 5’3” then you need to make sure that the model you are considering has a range of seat depths with some being shallow and some deep to accommodate both of you. Much like shoes, you definitely want to sit in a spa and “try it on” before you buy since each seat configuration from spa to spa will vary a lot or a little bit.
The second part of the shell has to do with the construction. By construction I’m referring to how it’s reinforced and supported. This can vary greatly from one manufacturer to another. A shell almost always starts with a flat sheet of cast acrylic which represents the top layer of the shell (it’s approximately 1/8th inch thick). This top layer is impermeable to water and mostly serves an aesthetic purpose since it is available in many colors and textures (textures can be rough or smooth). Hot tub manufacturers do not fabricate these sheets themselves. They buy them from a handful of acrylic suppliers (Aristech Acrylics or Lucite Acrylics). The sheets can also vary in price because some of the colors require more time and materials to make than others. Many hot tub manufacturers offer standard finishes and upgraded finishes which distinguishes the more expensive colors from the less expensive colors. Basically, you pay more for an upgraded color because the manufacturer pays their supplier more for that color and that added cost is passed on to the consumer. The sheets come in precut, standard hot tub sizes like seven foot and eight foot. If you’ve shopped for hot tubs and gone from hot tub store to hot tub store then you would have taken notice that different brands may offer the same color and that’s because those brands are purchasing their acrylic sheets from the same supplier. This doesn’t imply that the quality is the same because the next shell layer(s) is what differentiates one brand from the next as it is the structural support layer. A premium manufacturer will form the acrylic by putting the flat sheet into an oven to heat it up and then it will vacuum form the acrylic (using a mold) into the desired size and seating configuration and then let it harden. Once the 1/8” acrylic sheet is vacuum formed it will then be turned upside down and moved onto the fiberglass shop where a bonding resin will be sprayed all over the acrylic and then layer upon layer of chopped fiberglass will be sprayed onto the back for structural support. The fiberglass reinforcement step is both material and labor intensive. It is a costly step in the hot tub manufacturing process but a critical one as it provides an extremely durable and strong shell that will last forever.
Another common way that a hot tub shell is produced is by using an ABS plastic backed acrylic. Basically those same seven foot and eight foot precut sheets of acrylic have a second 1/8th inch layer of ABS plastic glued to the back. The two layers (co-extruded) are put into the oven simultaneously and vacuum formed into the desired hot tub mold. Once the shell cools then it moves onto drilling and plumbing. In some cases, a manufacturer will apply high density spray foam directly to the back of the shell for additional structural support. Manufacturers save a great deal on time, labor, and materials since no resin or fiberglass steps are required.
Our recommendation would be to purchase a hot tub with a fiberglass backed shell or similar support material to ensure longevity.