Hot Tub Insulation 101

When it comes to reducing the running costs of a hot tub, nothing has a bigger impact than the hot tub’s insulation. Today, we will look at the most common insulation systems to see which does the best job at insulating your hot tub, as well as looking at a couple of ways of improving any hot tub’s insulating ability.

How Does Hot Tub Insulation Work?

Hot tub insulation, like all insulation, works by trapping warm air and then using that warm air as a barrier against colder outside temperatures. Air is a great insulator as it resists changes in temperature. This slows the transfer of heat out of the hot tub as the heat now needs to travel between hundreds or thousands of small air pockets, instead of simply radiating out through the cabinet. The slower the heat dissipates out of the hot tub, the more is retained in the hot tub for use in heating in the water.

What Are The Different Types Of Hot Tub Insulation?

There are three major types of insulation used in hot tubs; spray foam, fibre board and thermal shield insulation.

Spray Foam

Spray foam insulation is a widely used insulation that is also commonly used to insulate homes. It is a special liquid that when sprayed on a surface will expand and trap air as it dries out. There are two ways that spray foam insulation is utilized in hot tubs, “full foam” and perimeter foam.

Full Foam Insulation

“Full foam” insulation is the most common way that hot tubs are insulated. In this system the foam is applied directly to the shell and plumbing of the hot tub, fully insulating the water and the majority of the plumbing. While this system is very effective at insulating the water, it does have a couple of potential drawbacks.

1. If the foam gets wet it needs to be removed and replaced. Wet foam is completely useless as an insulator. Spray foam is also notorious for retaining water once it is absorbed. For this reason, even small leaks can lead to large repair bills as large areas of foam have to be removed and replaced.
2. The heaters and pumps create a lot of waste heat. In a full foam system these components are outside of the insulated area meaning that all this waste heat is, well, wasted.

Perimeter Foam

In an effort to solve the issues of full foam insulation, some manufacturers have switched to a perimeter foam system. In this system, the outer cabinet of the hot tub is covered in insulation rather than the shell. This allows for the waste heat from the pumps and heater to be trapped and used to further heat the hot tub water, while also leaving the plumbing exposed for easier repairs.

Perimeter foam isn’t without its own drawbacks, however. The first is a lack of support for the plumbing. Hot tub plumbing is heavy, especially when filled with water. By encasing the plumbing in spray foam, full foam systems not only insulates the plumbing, it also helps to support its weight, leading to fewer potential problems over time.

Perimeter foam also requires that vents be added to the cabinet of the hot tub so that the pumps don’t overheat in the warmer months. In the winter, though, these vents mean that some heat can easily escape the hot tub, reducing the overall effectiveness of the insulation.

Fibre Board Insulation

Fibre board insulation is used by a few hot tub manufacturers. Comprised of solid sheets of interwoven fibres, this insulation is used as an alternative to spray foam in some perimeter foam systems. While it is cheaper and easier to replace if it gets wet, it also leaves small gaps around each piece, reducing its overall effectiveness.

Thermal Shield Insulation

Thermal shield insulation is a thick “bubble wrap” style insulation that is commonly used in a variety of applications where high amounts of insulating value is needed in a relatively small space (like water heaters or astronaut’s space suits). This insulation typically has two sides, one that is reflective to reflect heat back into the airspace on the inside of the system (to reduce the amount of heat transferred by the insulation), and the other which is black to absorb any ambient heat coming from outside the system.

In hot tubs, thermal shield insulation is used in both perimeter foam and specialized two layer systems. In these two layer systems one layer of insulation is wrapped around the shell and plumbing of the hot tub, while the second is wrapped around the outer cabinet and floor of the hot tub. These systems offer the best aspects of both the full foam and perimeter foam systems. They also offer the added benefit that the insulation won’t become saturated if it gets exposed to water; ensuring that your insulation is always running at peak efficiency.

The only drawback to thermal shield insulation is its R value. Each sheet of thermal shield provides the same insulating value as around 3″ of spray foam. This means that standard full foam hot tubs generally provide better insulating value if there is only one layer of thermal shield insulation in the hot tub. For that reason, most hot tub manufacturers that utilize thermal shield insulation include at least 2-3 layers.

The Importance Of Insulating The Floor In A Perimeter Foam Insulation System

No matter which type foam is used to insulate the hot tub, it is very important that the floor of the hot tub is insulated, not just the outer walls. If the floor of the hot tub isn’t insulated, the outer layer of insulation around the cabinet loses quite a bit of efficiency as heat is allowed to dissipate into the cold floor rather than being trapped in by the insulation.

Not insulating the floor also means that the air inside cabinet of the hot tub will cool down more quickly when the hot tub isn’t running; meaning that the water in the plumbing runs the risk of freezing in the case of a winter power outage or other issue that affects the hot tub’s ability to run its normal cycles.

So Which Insulation Is Best?

While each insulation system has it own pro and cons, on the whole thermal shield insulation provides the best energy efficiency; assuming that the hot tub has at least two layers of thermal shield.

How Can You Improve Your Hot Tub Insulation?

Check The Cover

Worn-out hot tub covers can dramatically increase your hot tub’s energy costs. In fact, continuing to use a worn-out hot tub cover can cost you $100s in added energy costs over the course of a single winter. One of the best ways to improve an older hot tub’s insulation, therefore, is to ensure that your cover is working at peak efficiency. There are two things you should be paying attention to:

If the cover is still sitting flat around the edge of the hot tub. To do their jobs effectively, hot tub covers need to form a tight seal around the edge of the hot tub. Without this seal, there will be significant heat loss around the edge of the cover.
If the cover has started to take on water. If the air pockets in the cover’s insulating foam start filling up with water, the efficiency of the cover will drop significantly; increasing your monthly energy costs.

How can you tell if your hot tub cover has started to take on water? Not only is air a much better insulator than water, it is also significantly lighter! If your hot tub cover is significantly heavier than when you bought it, chances are that it has become waterlogged. If placed on a cover lifter, you should be able to lift your hot tub cover with one arm fairly easily. If your cover is more difficult to lift than it is full of water and should be replaced.

Add A Floating Thermal Blanket

Buying a floating thermal blanket is another great way to improve your hot tub’s insulation over the winter months. These are simply thin sheets of insulation that float on the surface of the water.

Most of the heat loss in a hot tub is lost through the cover (even if the cover is relatively new). By adding a 2nd layer of insulation to between the water and the cover, you can dramatically reduce this heat loss and decrease your monthly energy bill.