Every degree your hot tub loses, the heater has to put back, and you pay for it. The lid is the cheapest part of the fight. This guide covers where the heat goes, what the makers say about their covers and what you can do this week to keep more of it.
Where the heat goes
Hot water at 38 °C loses heat in three directions:
- Up, through the surface. Warm water evaporates and the steam carries heat away. With no lid, this happens all the time. With a lid, the air gap under it warms up and slows it down.
- Out, through the walls. A plain air wall lets heat through faster than a foam core or an insulated liner.
- Down, into the ground. Paving and soil stay cold all year and pull heat out through the floor.
Wind makes all three worse. A breeze across the lid or the wall carries warmth away faster than still air.
What the makers say about their covers
Here is how the listings describe the lids on some of our tubs. Note what is missing: no maker on our site publishes the thickness or the insulation value of its cover.
| Hot tub | What the maker lists |
|---|---|
| Lay-Z-Spa Miami | TriTech wall, insulating lid |
| Lay-Z-Spa Carrara | ThermaCore foam wall (no separate cover detail) |
| Intex PureSpa range | Fiber-Tech wall, insulated cover |
| Aoxun 208 cm | Insulated cover |
| Arebos Barcelona 6 | Thermal cover and floor mat included |
| Wave Atlantic 6 | Thermal cover and floor mat in the EnergySave bundle |
| Wave Tahoe 4 | EnergySave bundle, 6-inch foam core |
The Arebos and the Wave Atlantic 6 stand out because they include a floor mat as well as a cover. That tackles the heat that goes down, which a lid cannot help with.
Why a few degrees matter
Heating 1 litre of water by 1 °C takes about 1.16 Wh. So if a 1,000 litre tub drops 5 °C overnight, the heater needs about 5.8 kWh to win it back. At 25p per kWh (our assumption, check your own tariff), that is about £1.45. Do that every night for a month and it adds up to over £40, for heat that a better seal could have kept.
Ten ways to lose less heat
- Lid on whenever nobody is in the tub. Even for ten minutes while you fetch a towel.
- Strap it down. Clip every strap. A lid that lifts at one corner leaks warm air and lets the wind in.
- Check the fit. If you can see a gap between the lid and the wall, the seal is not doing its job.
- Keep the lid dry on top. Rain and snow sitting on a lid chill it, and a sagging lid touches the water. Brush snow off.
- Add a floating thermal blanket. A cheap bubble sheet cut to size and laid on the water under the lid cuts evaporation.
- Put a floor mat underneath. Foam floor mats or interlocking tiles insulate the base. Many tubs need one anyway to protect the liner.
- Block the wind. A fence, screen or hedge on the windward side helps more than you might expect.
- Keep a steady temperature. Holding 37 to 38 °C usually costs less than letting the tub cool and reheating it from cold.
- Replace a waterlogged lid. Old foam lids can soak up water, get heavy and stop insulating. If yours is much heavier than when new, it is time.
- Shorter lid-off time. Plan your soak. The longer the surface lies open, the more heat walks off as steam.
The wall matters as much as the lid
A good lid on a plain air wall still loses heat through the sides. The makers' own figures:
- Lay-Z-Spa claims its ThermaCore foam wall is up to 60 % more efficient, and its EnergySense liner up to 40 % more efficient, than its plain walls.
- Wave claims up to 60 % for its WaveEco foam and quotes about £2 a day to hold 40 °C in the Tahoe 4 and Garda 6.
These are the makers' claims, not our tests. We have not measured the heat loss of any tub. They still point the same way: foam beats air. Our inflatable, foam or frame guide explains why.
Should you buy a better cover?
If your lid is torn, waterlogged or does not seal, yes. Buy one made for your model so it fits the wall. If your lid is fine, put the money into a floor mat and a thermal blanket first, because those tackle the heat a lid cannot stop.
Quick answers
Should I leave the lid on while the tub heats up? Yes. Heating with the lid off wastes much of the energy as steam.
Is a thicker lid always better? Thicker foam insulates better, but only if it seals. A thick lid with a gap loses more than a thin one that fits.
Does a cover stop leaves and dirt too? Yes, and that keeps the filter cleaner. See how to clean a hot tub filter.
More ways to cut the bill: hot tub energy saving tips, hot tub running costs and using a hot tub in winter.
