Our verdict: Worth paying for
Hot tub insulation
Short answer
Insulation decides how much a covered tub costs to keep hot, and how well it is installed matters more than its name. On the median, Full foamInsulation that fills the whole space between the shell and the cabinet with foam, burying the pipes in it.In the glossary tubs drew about 9% less power than Perimeter insulationInsulation that lines the cabinet walls and floor and leaves an air space around the plumbing, where waste heat from the pumps collects.In the glossary designs in California's test, about $2 a month on a typical 6- to 7-seater. Same-size tubs can differ by $20 a month or more.
Worth paying for
Compare what tubs cost to keep hot, not insulation names: same-size tubs can differ by $20 a month or more. A foam label doesn't settle it. Nordic's sprayed-foam Encore LS and Artesian's full-foam Eagle Crest, the same size, are about $4 a month apart in California's test.
How does hot tub insulation work?
A hot tub holds 100°F water all year, so heat leaks out every hour through the shell, the cabinet walls, the floor and the cover. Insulation slows that leak. It does not change how fast the heater works; it changes how often the heater has to run.
Makers use two broad approaches. Full foam fills the space between the shell and the cabinet with expanding foam that also wraps the pipes; Nordic sprays rigid closed-cell foam onto the shell and Artesian fully foams its Elite line. Perimeter designs line the cabinet walls and floor instead and leave an air space around the plumbing, where waste heat from the pump motors collects. Cal Spas' Thermo-Shield and PDC's TemperLok work this way.
Does full foam cost less to run than perimeter insulation?
A little, on the median, but brands vary more than methods. Some perimeter brands beat some full-foam brands, because installation counts: California's Energy Commission reports that more than 99% of spas in its database are listed as fully insulated, yet same-size tubs differ by up to 150 watts, about $20 a month, because of gaps and missed spots. What any tub costs to keep hot is on hot tub running cost.
Is there a downside to full foam?
Service. When a joint leaks inside foam, a technician has to dig to find it. Hydropool, which uses a thermal-blanket design, argues that full foam holds parts steady longer but is harder to repair; read that as one maker's case for its own method. Ask the dealer how their technician reaches a leak in the model you like. If you live somewhere with hard winters, see the best hot tubs for cold climates.
What to ask the dealer
- Was it tested with the same cover I'm getting?
- If a pipe leaks inside, how does your tech reach it?
The details
Which brands offer it, the research behind the verdict, and the sources.
Which brands offer it
15 brands
Spec sheets with an insulation type
351 of 390
| Sundance Spas | Perimeter or panels5 of 19 spec sheets | 69% |
|---|---|---|
| Nordic Hot Tubs | Full foam26 of 26 spec sheets | 78% |
| Cal Spas | Perimeter or panels57 of 57 spec sheets | 81% |
| Jacuzzi | Multi-layer foam2 of 19 spec sheets | 82% |
| Hot Spring | FiberCor fiber fill11 of 24 spec sheets | 84% |
| Dimension One Spas | Full foam15 of 15 spec sheets | 84% |
| Bullfrog Spas | Full foam18 of 25 spec sheets | 87% |
| Artesian Spas | Perimeter or panels24 of 41 spec sheets | 88% |
| Caldera Spas | FiberCor fiber fill18 of 23 spec sheets | 89% |
| Marquis | Foam over plumbing only18 of 25 spec sheets | 94% |
| Master Spas | Full foam28 of 29 spec sheets | 94% |
| PDC Spas | Perimeter or panels14 of 14 spec sheets | 96% |
| Wellis | Perimeter or panels31 of 31 spec sheets | 99% |
| Vita Spa | Perimeter or panels20 of 20 spec sheets | 100% |
| American Whirlpool | Perimeter or panels22 of 22 spec sheets | 100% |
Standby power by insulation type
263 tested models
We sorted 351 spec sheets by insulation type and matched 263 of those tubs to California's appliance database, then divided each tub's tested standby watts by the most the state allows for its volume, so a small tub and a big one compare fairly. Full foam came out at a median 84% of the limit and perimeter designs at 92%.
| Full foam7 brands | 121 | 97 | 179 W | 84% |
|---|---|---|---|---|
| Multi-layer foam2 brands | 10 | 8 | 158 W | 81% |
| FiberCor fiber fill2 brands | 29 | 29 | 192 W | 87% |
| Foam over plumbing only1 brand | 18 | 10 | 215 W | 95% |
| Perimeter or panels7 brands | 173 | 119 | 218 W | 92% |
What is California's standby limit?
California tests portable spas with the cover on for at least 72 hours and caps standby power at 3.75 times the water volume to the two-thirds power, plus 40 watts. Oregon, Connecticut, Washington and Arizona use the same rule. Every tub sold there must stay under it, so 100% means a tub just passes. The Commission also found standby is typically about 75% of a spa's lifetime energy use. Every tested model is listed on the most energy-efficient hot tubs.
What the research says
9 findings
- Government The California Energy Commission found standby mode is typically about 75% of a spa's lifetime energy use, and more than half of standby energy goes to keeping the water hot.
- Government The CEC reports that more than 99% of spas in its database are listed as 'fully insulated', yet same-size spas differ by up to 150 W in standby power because of insulation method and materials, including gaps and missed spots.
- Government The CEC report estimates that applying insulation uniformly could cut energy use by up to 30% for average- to low-efficiency spas, and notes manufacturers use closed-cell foam with radiant barriers rather than fiberglass.
- Government California's Title 20 test measures normalized standby power with the cover on over at least 72 hours, and the label shows tested standby watts against a maximum of 3.75 × volume^(2/3) + 40 W for standard spas.
- Trade UL notes that Oregon, Connecticut, Washington and Arizona have adopted California's standby-power rules, based on the ANSI/APSP/ICC-14 test.
- Maker Hydropool, which uses a thermal-blanket design, argues that full foam holds parts in place longer but is harder to service; this is a maker's own trade-off claim.
- Government California listings put Nordic's 336-gallon Encore LS at 178 standby watts and Artesian's 335-gallon Eagle Crest at 211.
- Maker Nordic sprays rigid closed-cell foam onto the shell instead of filling the cabinet.
- Maker Artesian fully foams its Elite tubs and calls full foam the most efficient insulation.
Common questions
What is the best insulation for a hot tub?
Judge the result, not the name. On the median, full-foam tubs drew about 9% less power than perimeter designs in California's test, but individual brands overlap.
Is full foam insulation worth it in a hot tub?
Often, but ask what the tub costs to keep hot. More than 99% of spas in California's database are listed as fully insulated, yet same-size tubs differ by up to 150 watts, about $20 a month.
How much energy does a hot tub use sitting idle?
California's Energy Commission found standby is typically about 75% of a spa's lifetime energy use, and more than half of standby energy goes to keeping the water hot.
Sources
6
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