Heavy Broken Hot Tub Covers Waste Energy. I saw this advertised on line the other day. When I went to check out the guys website, guess what he’s selling? Rigid foam spa covers that will end up heavy or broken, usually within two years. To me this is like a coat salesman telling you wet wind breakers won’t keep you warm, here you need to buy this new dry wind breaker and go play outside in the rain.
Instead of selling you something that won’t get heavy or broken he keeps selling the same thing, hoping you won’t notice. 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. Rigid foam board is used in lots of insulation applications. It can be used around refrigeration storage areas like in a super market. Layers of foam board can insulate the cool storage area while the customer area can be kept comfortably warm. But in this type of installation the foam is not subjected to hot moist air. 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 wanted to produce the perfect environment for a rigid foam board to become saturated it would be to put it over a source of warm, humid, steam. You couldn’t get water into it faster even if you tied it to the bottom of your swimming pool. 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. Load it up with snow so it will keep the spa cover frozen and stand back and watch the power meter spin. Instead of buying another rigid foam spa cover that will positively end up like the one you are replacing now, shop online for one that is designed better. Look for Spa Covers that will insulate the water from the water surface, without rigid foam.