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Hot Tub Insulation

Mar 17, 2017 | Building or Buying a Hot Tub

The value of a well-insulated spa! In a previous blog, I talked about electric heat vs. gas heat and how electric heaters for spas work. Whichever heating method you choose for your spa, you’re going to want to maximize the efficiency of your spa to reduce operating cost. Proper hot tub insulation will help achieve this goal.

I’m sure most of you realize that heat rises, therefore a good fitting spa cover with at least 4 or 5 inches of ridged board closed cell foam insulation is the recommend level of hot tub insulation for maximizing heat retention. Obviously, spa owners in our colder climates will want to pay closer attention to the specifications of the spa cover they will be using or that is supplied with the spa they buy. Spending a few dollars more for a cover with a greater “R” value is never a bad investment.

In addition to an insulated cover most portable spas will have some degree of insulation on the waters shell itself. This could be sprayed on foam insulation or possibly batts or foam board. Sprayed on foam is going to be the most common type you will find. Another important factor is the fit of the hot tub insulation on the spas surrounding outer walls. Heat given up to the spa cavity area can be captured and pumped back into the spa water via air controllers or the spas air pump if so equipped.

If a hot tubs outer surround wall structure is uninsulated, thinly insulated, or poor fitting, usable heat in the area outside the spa shell will be given up. Cold air infiltration will tend to cool down this area down and air pumped into your spa water will cool quicker and increase the duration the spa heater must run to maintain set temperature. In turn… this extra heating will increase your operational costs. So, if you’re buying a hot tub, check to see if the hot tub insulation is adequate, tight fitting and of the recommended thickness for your geographical area.

When shopping for a spa that will be used in normally colder climate areas you will want to pay close attention to these insulation details, it affects your pocketbook. Warmer climates will be more forgiving, but still, a well-insulated spa will cost you less to operate in any climate.

Over the years there has been a controversy over the merits of what they call a “full foam” spa or hot tub. This is a unit that has hot tub insulation sprayed on and completely fills the cavity between the spa shell and outer wall panels. Your first thought may be as most people might think, more is better. No so fast, there is this rule called the law of diminishing return you need to consider. After a certain thickness, adding more insulation doesn’t reduce heat loss any more. The heat loss has already been contained. Adding more hot tub insulation becomes a wasted expense. Below is a cutaway of a full foam spa.

Hot Tub Insulation Done Wrong, (in my opinion)


In the industry, these full foam spas have been tagged with the name “dog dish”. This is because they resemble an upside-down dog’s food dish. Manufacturers tout these types of spas and hot tubs the ultimate insulated units. However, there are a few inherent problems or concerns with spas and hot tubs with full hot tub insulation. (1) What if you have a leak? These units can be frustrating and very expensive to detect and repair a leak. (2) the equipment to operate a full foam spa or hot tub is typically located in one section of the surround cavity that has to be vented and accessed.

Venting an equipment access area of a spa or hot tub, sets up a situation where piping and equipment in this area may be vulnerable to freeze ups, because the area has little or no insulation, not good. For that reason alone, whenever anyone asks me of my opinion of a unit with full hot tub insulation, my answer is, Pass!

Here at Custom Built Spas, the DIY spas and hot tubs that we show you how to build, have taken advantage of several more efficient ways to utilize hot tub insulation by capturing the heat in the surround area given off by the piping and equipment. Everything is protected from cold conditions and if you should have a power loss in freeze conditions you’ll have plenty of time to address the situation before you ever have to worry about something freezing and or being damaged. Visit and get your package of hot tub building books and DVD. We’ll show you how to build a unit efficiently.

As a little side note… don’t overlook the value of possibly incorporating a solar heat set up for your spa. Done right, the extra cost can be offset by the heat advantage a solar panel can provide in reducing the run time of your spa heater. Even on an overcast day a solar panel can pick up a higher temperature than the set limit of your spa. So, by circulating spa water to a solar panel and back to the spa when the temperature differential is sufficient can be a real bonus in keeping operational costs down, especially in normally colder areas.

Thanks for the visit!

Gene Trumbull
Custom Built Spas

Call With Questions (860) 571-8440



Remember, my Mission is to help you, as my customer, build a beautiful hot tub of your own! I can show you how to build Spas, Hot Tubs, Swim Spas, Plunge Pools, Cocktail Pools or Exercise Pools at incredibly affordable prices! Everything you need to know is outlined in my series of easy-to-follow books and videos that cover a simple, step-by-step process that I’ve developed and refined over the years.

I will teach and help you with everything you need to know about building your own spa, hot tub, or swim spa yourself, regardless of what your DIY skill levels are and at a cost, YOU determine YOU can afford. As my customer, I’ll work with you at your own pace until your project is done and comes to life! For over 25 years now I’ve been helping people just like you, build their own hot tubs, spas, and swim spa projects. So, are you’re ready to let me help you build your project? Then what are you waiting for, let’s do this!

Custom Built Spas