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 nearly impossible. For more information, click over to the Full Foam Insulation section.

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 hot tub 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 uses 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.