Spa Cover Heat Loss Calculator

Ever get in a hot tub and feel like you’re taking a lukewarm bath? It’s not normaly because of the heater itself.

A hot tub isn’t very efficient at retain heat. And the #1 obstacle between you and cold weather is your cover.

Why Hot Tub Covers Are Important for Saving Heat

If you think of it as simply something to keep the tub clean, you’re wrong. It’s a thermal seal; if the seal fails, you’re paying to heat air over your patio.

It’s all very simple science, but it doesn’t take long before you get punished by it. Energy flows from hot to cold. Water has far more energy than air. And as soon as you open up that surface area, evaporation will steal your heat much quicker then any conduction across the walls.

Just think how many thousands of BTUs escapes out of a standard sized hot tub over a few hours.

That’s why we run the numbers for you: The calculator above takes insulation values, temperature differences, and surface area, and converts them to a real daily energy cost.

R-value is the key input that throws people off their game. R-value is a measure of effectiveness of the cover at blocking heat flow. A really nice, thick, dry foam cover might be in the R-12 range, which isn’t bad. An older one that has sagged from its own weight or taken on water will probably be R-6 or less. Soggy foam conducts heat practically like water do. If you find the cover heavy and dense, there is a good chance it are taking on water. That water creates a bridge through the insulating gap and what was a shield becomes a heat radiator. Believing the manufacturer’s original spec is not as important than knowing the actual state of the foam.

The wind has a trickier effect than most believe. It not only blows the cold out but also rips the very thin cushion of warm, moist air that hugs the water surface. That is a natural insulator called the boundary layer. If it is disrupted then evaporation increases dramaticly. Wind can also increase heat loss even with a closed cover. This happens because of gaps in seams, where the hinge enters the cover, and small openings like light holes or jet holes. By allowing you to dial down for exposure, the tool factors that in and explains why a tub hidden behind a fence will lose less heat than one on a windy deck.

The silent killer of efficiency are gaps. Few covers fits perfectly. Even if they did initially, cabinets settle and most lids has a bit of lift so you can gain access for maintenance. The gap only measures inches around, yet it is large enough to let steam through easy. Evaporative cooling is very energy intensive. A gap an inch wide can offset the insulating benefit of multiple square feet of solid lid material. The percentage of uncovered area shows this.

Measure these gaps honest. A moderately tight seal with lower R-value may actualy outperform a sloppy seal with higher R-value.

The whole equation relies on temperature difference. The steeper the temperature differential between your water temp and the outside air, the quicker the heat will flee from your tub. On a 40 degree night, running your spa at 104 creates a large difference in temperature. Heat flees out to try and equalize itself. Lowering the thermostat when you are away reduces the driving force. This delta is what the calculator shows you the true cost of keeping the water hot all day long vs. You can just turn it down and cool the water off.

In short: How much you lose depends on how much you expose.

There is only so much you can do to reduce the loss. You can make sure the tub is covered as soon as possible after use, keep the lid dry, and seal any air gap. If you leave the door open, you’re fighting physics (and losing). With a good seal and reasonable expectations, though, you’ll get the best of both worlds by keeping the heat in and the bills low.

You should of kept it covered. The cover isn’t just a lid, it’s your first line of defense when it comes to wasting energy.