I saw this ad the other day. Heavy Broken Spa Covers Waste Energy. When I checked out what the guy was selling it was more of the same. Just a new rigid foam filled spa cover that will end up the same. That's like going to the doctor and telling him it hurts when I do this, and the doctor says well just keep doing it, maybe it will stop hurting.

He's hoping you won't notice that he's selling you the same thing you're replacing now. If you have had to replace a spa cover because it got heavy, maybe the question you should ask before you buy the next one is, Why?

What causes the foam spa cover to get heavy is that it traps moisture inside. Foam board is used in many insulation applications. It can be used around refrigeration storage areas like in a super market. Layers of rigid foam board is what keeps the refrigerated section cold while the customer area is still warm. When used in this application the foam is not subject to warm moist steam. As long as the foam stays dry it has a predictable insulation value. But if the foam were to have moisture in it instead of the little air spaces it uses to insulate, it would have no insulation value at all.

If you were trying to invent a way to get foam to saturate as quickly as possible, the design would involve lots of steam under it and a cooler temperature on top of it. Totally submerging the foam in water wouldn't get it saturated any faster. Why? Because water molecules are bigger than steam molecules. Steam can get into smaller spaces faster than water. And once the steam cools, it condenses back into water, displacing air in the foam as it does.

Long before you notice the spa cover getting heavy, moisture has begun to replace the air spaces in your cover. When it does, the little insulation value that cover might have had, goes down dramatically. From whatever it may have been when you first put it on your spa it has gone down to as much insulation as a wet piece of plywood by the time you actually notice it got heavy.

You might get fooled into thinking that it is still insulating well because snow won't melt off it. Unfortunately, you would be wrong. Snow won't melt off a frozen pond either but it doesn't mean the ice is insulating the water. When snow falls on a saturated foam spa cover, it freezes the moisture in the cover because it is laying directly on the cover. The water of your spa is never in contact with the foam since the foam is resting way up on top of the acrylic of the spa, usually about a foot above the water surface. What's happening is the warm spa water is evaporating into steam. That steam is rising, because that is what heat does, until it hits the bottom of the frozen spa cover. Then the steam cools and turns back into water. The water, now cooled, falls, because that's what cold does, back into the warm spa water, cooling it off.

So if you wanted to invent a radiator to cool off your warm spa water this would be the perfect design. Put a block of frozen foam over the water. Pile a bunch of snow on it to keep the hot tub cover frozen and go over and listen to the power meter buzz. Instead of just getting another of the same type of spa covers that will end up in the same condition, shop around for one that is designed differently. Look for a spa cover that will insulate the water from the water surface, without rigid foam.