It is important to note that before we get into the different types of insulation that we recognize the evolution of hot tub efficiency. It used to be that spas in the 1980s were built into decks or built in-ground and the design was such that the shells had little to no insulation around them and the equipment was separated in a dog house structure. This meant that the water would have to travel some distance from the hot tub through plumbing before it reached the dog house structure where the heater and filter were located. In some cases they could have been 20-40’ away which caused the water to cool as it traveled. This made for a very inefficient design and high operating costs. In the 1990s we started to see many self-contained, portable spas hit the market. These had energy efficiency advantages from the get go because all of the equipment was located within the four cabinet walls of the hot tub. This meant the water didn’t have to travel very far and having the equipment in the cabinet meant that all of the excess heat and friction created by the equipment was trapped inside the cabinet walls which helped keep the hot tub warm (recycled heat).
Directly related to energy efficiency or specifically R value is insulation. There are currently three major types of insulation methods used in hot tub manufacturing. Some manufacturers use different buzz words or marketing slang to describe their insulation, however, they generally boil down to one of these methods.
The first method is called dead air insulation. If an air cavity or space is left between two layers (hot tub cabinet and shell) then the air trapped between those two layers acts as a barrier to heat loss because it is a poor conductor. The issue with this method in hot tub manufacturing is that there is equipment (pumps, heaters, motors, control boxes, etc..) located inside the cabinet of the hot tub that needs to be temperature regulated and thus ventilated so it doesn’t overheat. Vents or ports are placed in the panels of the hot tub cabinet allowing the warm air inside the hot tub cabinet to escape which cools the inside of the cabinet and ultimately reduce the R value. A handful of manufacturers market their spas with the dead air concept because they save on manufacturing costs (no foam insulation). There is, however, one advantage to the dead air insulation method and it actually has nothing to do with energy efficiency but rather serviceability. Having a hot tub with no insulation inside makes it very easy to service or repair the plumbing and equipment since everything is visible and accessible. Many second time hot tub buyers go this route even though they compromise energy efficiency because they may have had leak issues in their first hot tub and didn’t like that they had to dig out a bunch of foam and spend many hours trying to find individual leaks. Leak repair can be very costly as hot tub service labor rates aren’t cheap.
The second method is partial foam insulation. Partial foam means that once the shell is built and the drilling and plumbing is completed then a light layer of foam is sprayed on the back of the shell and the plumbing. The plumbing is not completely covered but lightly coated whereby you can still see many of the major manifolds, fittings, and plumbing lines. In a partial foam spa the manufacturer may also insulate the back of the cabinet panels with a reflective foil type of insulation which helps trap the heat inside the hot tub cabinet and improve efficiency. This method allows for good overall energy efficiency and good serviceability.
The third type of insulation is full foam. Full foam is probably the most widely used method of insulation in the industry. The foam is sprayed onto the back of the shell and it completely covers all of the plumbing clear out to the cabinet walls. The clear advantage of full foam is superior R value. It also adds cost to the manufacturing process as it takes more time and material to make a full foam spa and thus you see full foam in higher end hot tubs. An indirect benefit of full foam is that is provides structural support to all of the plumbing lines, manifolds, and fittings in the hot tub which dramatically reduces the likelihood of leaks. The full foam also dampens the sounds and vibrations from the plumbing and equipment inside the cabinet of the hot tub and makes it run quieter.
There are many types of foam used by manufacturers and they come in different weights. The heavier the weight the better the insulation value. As a general rule you want to stay away from open cell foams as they absorb moisture and can lose R value over time. Closed cell spray foams are great early in the hot tub’s lifespan, however, they deteriorate over time because those closed cell gas pockets can breakdown over time and absorb moisture thereby reducing R value over time. A full foam spa can also become heavier over time as the foam absorbs moisture. There are some premium foams available that don’t lose insulation value or absorb moisture (ie. Icynene Foam).