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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, more than likely the entire spa will have to be replaced.

There are two components that, when sandwiched together, make up the spa shell: the shell surface and its understructure. These two components provide completely different benefits to the spa shell. The shell surface determines the aesthetic look and feel of the spa shell, as well as its resistance against UV rays, chemical sanitizers, and customer abuse. Other factors include stain resistance and ease of cleaning.

The shell understructure provides the structural integrity to the spa shell, determining both the spa's longevity and its ability to support the weight of the water. The shell understructure also decides the spa shell's resistance to deterioration such as delamination, bubbling, and cracking. The sections below explain each component in detail. of materials in constructing hot tub cabinets.


There are two major types of material used in manufacturing spa shell surfaces: OAS and acrylic. Both have good impact resistance and will take a fair amount of abuse. However, there are some significant differences between the two:

  • OAS. Many manufacturers utilize OAS (Olefin Acrylonitrile-Styrene) as the shell surface material under different names (such as Rovel or Ultralife), however, they are all essentially the same material. The proper nomenclature for OAS (be prepared for a mouthful) is ASA/AES terpolymer, which is an acrylic-styrene-acrylonitrile blend with olefin modified acrylonitrile-styrene co-extruded over acrylonitrile-butadiene-styrene (ABS) backing. OAS is a good alternative to acrylic because it is a less expensive material and may reduce the total cost of your spa. In addition, some people have found that they prefer the simplistic appearance of this plastic material over the luster that is typical of acrylic.
  • Acrylic. From two and three color marble patterns to simulated granite, acrylic offers a wide variety of colors and styles from which to choose. A major advantage of acrylic is its smooth, high-gloss finish that gives the spa shell surface a luxurious sheen and is easy to clean. Its hard, non-porous surface prevents dirt from accumulating and resists stains, allowing you to maintain your hot tub simply and easily. Most manufacturers have chosen to use acrylic as their shell surface material.

There are various types of acrylic used in the forming of a spa shell. Check with the spa manufacturer for details about the type of acrylic that they use, and ask about the inherent advantages that it may offer. 

There are various types of acrylic used in the forming of a spa shell. Check with the spa manufacturer for details about the type of acrylic that they use, and ask about the inherent advantages that it may offer.


As mentioned earlier, there are two different types of materials used for the shell surface: OAS and Acrylic. Neither material can withhold the weight of the water by itself; the hot tub’s primary support and structural integrity come from its understructure. Each material utilizes a different method of rigidizing with a variety of bonding agents in order to support the spa. Understanding the different types of shell understructures is an important step in choosing the perfect hot tub.Manufacturers that use OAS as the shell surface material usually use a thick layer of solid foam for its structural support.

This hardened, rigid foam works just like fiberglass and provides a structurally sound support system needed for your spa. However, this thick layer of foam necessitates covering the plumbing lines and fixtures of the spa, which can make servicing more difficult. 

The majority of manufacturers that use acrylic as the shell surface material utilize a combination of fiberglass and additives for its understructure. There are three factors to consider when determining a quality acrylic understructure: the thickness of the fiberglass, the quality of the fiberglass, and the quality of the manufacturing process. The thickness is a key factor in determining the spa’s structural integrity and longevity. Manufacturers can save hundreds of dollars by using only a thin layer of fiberglass. In order to verify that the spa you are purchasing has a thick layer of fiberglass, you may want to ask your retailer to see a cross section of the spa shell.

The quality of the manufacturing process relates to the application of the fiberglass to the acrylic shell. Look for manufacturers that hand roll each layer of fiberglass to the acrylic shell. Although this process is expensive and labor intensive, it is the only way to prevent deterioration and delamination of the acrylic shell. Delamination occurs when two bonded materials (the shell surface material and the bonding agent) expand and contract at different rates, usually due to differences in temperature and moisture content. This results in blisters, cracks, and bubbles in the shell. The use of cheap and inferior materials, along with improper application, are the primary causes of this deterioration. While delamination primarily occurs in acrylic shells, OAS shells are also subject to crazing, or spider cracking. A good manufacturer will design products that minimize and virtually prevent your spa from any type of deterioration.


Since the spa shell is the most important part of your hot tub, it should be considered a top priority to research the manufacturer’s shell warranties. Always understand that a warranty is only as good as its manufacturer. You should take into consideration both the longevity and service history of the manufacturer. It is also important to understand that often there are two types of protection in shell warranties; structural and surface. The structural warranty covers the shell’s ability to hold water, while the surface warranty protects against delamination, discoloring, and cracking. We recommend looking for a manufacturer that provides a substantial shell warranty and purchasing your spa from a retailer with a proven record of supporting the warranty.


Comfort is a top priority in choosing a hot tub, so choosing the proper seating design is crucial. Bench seating is the traditional seating design that is often found in octagonal or circular shaped hot tubs that are usually located at most hotels, resorts, and health clubs. They are popular because this seating design offers the maximum amount of seating, while providing equal seating options. These sparse, open spaced spas are great for conversations and for entertaining; but bear in mind, when looking for a spa for home use, comfort and contour should be a priority. One benefit of working with acrylic spa manufacturers is their ability to provide unique and intricate contours to the shell design, offering more comfort and better accessibility. The vacuum forming process that shapes the acrylic into the shell surface naturally lends itself to more intricate shell designs.

We recommend to look for a manufacturer that carries a variety of innovative seating designs, such as lounge seating, therapy seats, and ample footwells to meet your unique preferences and style.

  • Lounge Seating. Although lounge seating can restrict the number of seating areas in a spa, most people prefer a lounge because of the comfort it provides. It offers the luxury to lay back and recline the entire body for complete relaxation. Many people that decide to have a lounge in their spa believe it is well worth the extra space. Today, there are many unique design features to lounge seating. Some manufacturers have created double wide lounges and/or reversible lounges. Many of these spas even have pillows built into each seat for added comfort. Some people enjoy the double wide lounge because it offers the opportunity to either relax alone in the spa with ample space, or to lie side by side with a loved one. In addition, a reversible lounge offers the opportunity for bathers to lie facing each other, which promotes the ease of conversation.
  • Therapy Seats. Therapy seats are designed to offer a full body massage. In one seat, jets can massage your neck, back, shoulders, arms, thighs, knees, calves, and feet. For added comfort, look for a molded therapy seat, such as a captain’s chair. This type of seating provides great support for the entire body. Additionally, some seats include wrap around arm rests. This provides extra support for your body by giving your arms and hands a place to rest. When considering a spa for purchase, we recommend looking for a spa that has at least one therapy seat. Those who purchase spas today are not just using them for entertainment, but are seeking a lot of "much needed" therapy for different parts of their body. A therapy seat is considered to be a top priority and one of the most used features in hot tubs today. You should look for a seat that focuses attention on the neck and shoulders in particular, because this is the area most commonly affected by stress. Some manufacturers have even designed features that allow you to put your head on a pillow for a complete neck and shoulder massage using separate jets above the waterline.
  • Footwells. It is important to consider the size and the depth of the footwell in relation to the size of the spa. For example, if you are looking at a spa that can accommodate four to five people, it is beneficial if the footwell can hold four to five pairs of feet. Also look for areas with jets concentrating on your feet and calves. Jets focusing on this area can provide a thorough massage to these often neglected areas. It can be a major benefit to people that are on their feet all day.


Many spa manufacturers have incorporated design features into their spa shells that contribute to safety and comfort, while making the spa more appealing. Here is a list of some of the features available and the benefits they bring to your spa:

  • Regular and Lighted Grab Bars. Grab bars can be strategically positioned to provide added safety when maneuvering in and out of your spa. Lighted grab bars add safety and ambiance when using your spa in the evening. Some manufacturers offer lighted grab bars that come in three settings: dim, medium, and bright.
  • Lighting. Underwater lighting should be considered a necessity on all hot tub purchases. It aids in the safe entry and exit of your spa when using it in the dark, as well as creating "moods" for special occasions. Spa lighting is usually available in a variety of colors. L.E.D. lighting is now available by some manufacturers. This type of lighting not only provides a beautifully lit spa, but also is very energy efficient and long lasting.
  • Pillows. Built in pillows that are form fitted into the spa's shell provide a measure of comfort when relaxing in your spa. Look for pillows that are not easily detachable, as they can become annoying when and if they detach during spa use.
  • Aquassage Pillows. This moving massage feature provides a truly penetrating neck massage, similar to the massage pillows found in many mail order catalogs. This non-electric pillow is activated by water pressure and fully adjustable.
  • Ice Buckets. An ice bucket is the perfect place to keep your refreshments cold and within reach while soaking in your hot tub.
  • Snack Areas. Snack areas not only provide a convenient area for drinks and snacks while soaking in your spa, but it is also a great place for towels, books, and eyeglasses.
  • Independent Seating Controls. These controls are located within reach at each seat and provide the luxury and added comfort of being able to control the type of massage you choose without having to move.
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