The spa shell is one of the most important parts to consider when purchasing a hot tub. Other elements, such as pumps or heaters, are fairly easy to repair or replace if something goes wrong. However, if the spa shell malfunctions, a repair could be extremely costly, or even require complete replacing.
Most spa shells are made up of two components, sandwiched together. The shell surface and some type of understructure. The shell surface determines the aesthetic look and feel as well as resistant to chemicals and abuse. The understructure provides the structural integrity and strength.
There are a number of different shell materials available. Below is a brief description of each.
Vinyl is typically used in inexpensive soft sided tubs. As a shell material vinyl is easy to damage, especially if the chemical balance of the water remains out of ideal range for very long. In most markets where this type of tub is available, it will be, if not the lowest, one of the lowest priced tubs. The thing to keep in mind here is that "You get what you pay for."
Acrylic backed by Fiberglass:
Acrylic is the most widely used shell material and offers the greatest selection of colors and patterns. For the most part acrylic spas are backed by a fiberglass compound, which is adhered to the acrylic with bonding resins. While these bonding resins (think of them like a glue) have greatly improved in recent years, acrylic spas backed by fiberglass in the past were prone to "delaminating", where upon the acrylic and the fiberglass separate and cause unsightly bubbles and bubbles. The quality of this type of design is directly related to a number of things.
The Thickness of the Acrylic:
Thicker is obviously better, but it can be difficult to determine how thick a particular spa shell is. Many retailers will have a small cross-section of a spa to show you, but know that in the manufacturing process a flat acrylic sheet is heated and vacuum formed (sucked down into) into a mold. This can leave very thin areas,especially on vertical walls, if done improperly.
Ideally the each layer of the fiberglass would be hand rolled. This assures there are no air bubbles remaining which can lead to delamination.
It is important when purchasing a hot tub like this to look at the "Surface Warranty" rather than the structural shell warranty. These two are usually separated issues in the warranty coverage. It is also a good idea to purchase a spa from a manufacturer that has been around a while. If a company goes out of business, in most places their warranty cannot be enforced. So a 100-year warranty that was issued by a company that is only around 6 months is not much use. A wise "rule of thumb" is to be sure the manufacturer is at least twice as old as the surface warranty is long.
Acrylic backed by ABS plastic:
This material on the surface looks much like the acrylic that is backed by fiberglass, but rather than adding a stiffening agent such as fiberglass, the surface material is backed by (co-extruded with) a sheet of ABS plastic. ABS plastic is impact resistant and when co-extruded properly, rarely separates from the acrylic surface. This type of spa shell is not as strong as fiberglass backed acrylic and typically the space between the shell and cabinet needs to be filled with foam for additional support.
In many cases spas that are foamed this way will have cabinets that cannot be removed. This can possibly complicate future repairs involving plumbing that is buried in the foam. Also if the tub cracks, it can be virtually impossible to repair. If you go with this type of shell make sure it has a long structural warranty. The reason most manufacturers use this material is that it lowers labor costs, with nobody needed in production to fiberglass, and as EPA and state regulations on the discharge of air pollutants gets stronger there may be fewer manufacturers using the old method of fiberglassing.
Other Plastics Backed by ABS:
There are a few other plastics that are used in the same manner as acrylic backed by ABS. They are marketed with too many names to list here but they are usually a white or cream color with a slight texture. They have all the same up-and-downs as acrylic / ABS. Plus some can stain with high concentrations of chemical, for instance if a bromine tablet sits on the floor for an extended period of time.
Thermal Plastics: These are relatively new in their use in the spa industry, yet this type of shell seems to have great merit. In many cases the manufacturer will put a lifetime surface and structural warranty on this material (be wary of "lifetime" warranties). This product is thicker than acrylic, which can be both good and bad. Being thicker tends to make it more durable, yet it's thickness can make it more difficult form into the shape of the spa. This difficulty can cause areas to become very thin during manufacturing. It should be noted that the shell can and will turn a brownish color if low pH, high disinfectant levels and the presence of Iron oxide (rust) are allowed for an extended period of time. Because fiberglass does not adhere well to this product, the understructure typically consists of dense sprayed in insulating foam.
With all of these types of shells, it is worth noting, that the smooth shiny acrylics should be waxed occasionally. It will help keep water line buildup to a minimum, reduce the possibility of staining and will help to erase some of the fine scratches that can form during normal use. The textured materials can usually be cleaned with a plastic scrub brush. Because they hide scratches, the long-term appearance of these materials is generally better.
Also worth noting about shell materials, the acrylic material is generally less effected by improper water chemistry. Some of the softer plastic materials can be yellowed or faded by high disinfectant levels and prolonged exposure to sunlight in some instances. Damage like this is usually not covered under manufacturers warranties.
All "plastic type" shell materials can be damaged by direct sun exposure. When your spa is empty be keep it covered.
There are a variety of different materials used for spa shells. Each had their own problems when initially used, but in a short time were perfected. Any material that's been around successfully for at least twice the length of it's warranty can generally be considered a good choice. If any one material was better than the rest, you can be assured that every spa manufacturer would be using it.
One other material used in the construction of spa shells is worth noting, Stainless Steel.
Although not as common as the various plastics, stainless steel offers many advantages, which make it an excellent, though expensive choice for many spa applications.
Due to its unique properties and aesthetic value, the use of stainless steel is increasing throughout many industries. The bright, easily cleaned surface looks and feels like no other spa, giving a modern appearance to any home. A non-directional polish can create a distinctive luster finish that will maintain its appearance for the life of the spa.
Stainless steel won't crack, blister or delaminate as materials can. Direct sunlight doesn't effect it and it's also completely non-porous, so it can't absorb any stains, germs or odors.
Architects, engineers, designers have long been advocates of Stainless Steel because of its appearance, strength and high corrosion resistance. It is one of the most durable materials used in architecture and construction today. Made up of low carbon steel, which contains chromium, Stainless steel does not deteriorate. Because stainless steels are inherently corrosion resistant, no protective coatings are needed, and the adverse environmental impact associated with coatings is eliminated.
Unlike traditional acrylic where a vacuum mold must be used to create the spa shape, stainless steel spas are individually made by joining (welding) flat sheets together. This eliminates any thin or weak-spots that can sometimes occur with acrylic spas.
Also important to consider, is the interior shape and style of the shell. Some have molded individual seating, and some an "open bench" style. Either can be equally enjoyable depending on your personal preferences. An "open bench" style can be easier to move around in, less restrictive, give a feeling of more room, and make "cuddling" easier. Individual molded seats can make for more varied seating positions, allow jets to wrap around your body and more firmly hold you in place. And don't forget to check the footwell and if there's going to be enough room for everyone's feet and legs.