How Should You Insulate Your Storage Tank

Keeping heat in a hot water storage tank is like trying to keep a leaky inner tube pumped up. No matter how hard you try, you're constantly fighting a losing battle. Some heat will always escape.

How much insulation is enough? It's theoretically impossible to over-insulate a solar hot water system, but here are some minimums: There should be at least 3-1/2 inches of fiberglass insulation — or its equivalent — behind the absorber plate in the solar collector, as we already know. There should be at least 1-1/2 inches of insulation on all pipes in the system, and there should be at least 6 inches of insulation around the storage tank.

If you can afford to do more than this, so much the better. Protecting your solar-heated water with more-than-adequate insulation (that has been carefully installed) will reward you directly because your back-up system will come into play far less often. You should also have a basic understanding of how much heat leaks through various types of insulation. Here's how those things are computed:

Every material has a measurable "C" value and an "R" value. The "C" value describes the conductivity of an insulation. It tells us how many BTG's of heat will pass through a square foot of the material in an hour's time when the temperature on one side of the material is 1 degree F. colder than on the other. You have no convenient way to measure C value, and you won't know what it is because it's rarely printed on the label of insulation packaging.

But the "R" value — which will appear on the package — is something you can know about. The R value is dependent on the C value. R tells you the resistance of the insulating material to heat passing through it. In mathematical terms, R is the reciprocal of C. In other words,

For example, if you read that the R value of a particular insulation is 20, that means that l/20th (0.05) of a BTG of heat will pass through 1 square foot of the material in 1 hour if the temperature differential is 1 degree Fahrenheit. If the package says R-ll, that means that 1/11th (0.09) of a BTG will pass through in the same amount of time, and so on.

To put it another way, the greater the R value, the greater resistance the material has to the passage of heat. So an insulation layer with a high number — 20 for instance — is better protection than one with a low number like 4. Unfortunately, doubling or tripling the thickness at a material doesn't necessarily produce twice or three times the R factor.

R value usually accounts for an inch thickness of whatever material we're talking about. One inch of fiberglass building insulation has an R value of 4. But 3-1/2 inches of the same material has a value of only R-ll. Six inches jumps to R-19, and 12 inches all the way to R-38. Styrofoam is rated at R-4.2 per inch — a little better than fiberglass. But urethane foam is better — R-6.3 per inch. Wood (pine specifically) has an R value of only 1.2, but concrete, metal and stone are much worse insulators — R-0.08 (Figure 70).

There's a third value to keep in the back of your mind. The "G" value is the total of all the heat resistances of all the materials surrounding the hot water — or any other heat source that's being insulated, for that matter. This includes the tank's metal sides, the insulation and the airspace if there is any. Again, (J is a reciprocal of all the R's,

Say the total R factor for one square foot of insulated tank comes to 20.2 (including the metal, airspace, 6 inches of fiberglass and some wood). The CI value will be 1/20.2 or 0.049. (This means, to say it one more time, that 0.049 BTG's per square foot can escape from the tank when the temperature differential is 1 degree F.)

If the water in the tank is 140 degrees and the temperature in your basement is, say, 60 degrees, the temperature differential is 80 degrees (140 - 60 = 80). Multiplying the G value times

Figure 70. Heat loss from a storage tank is continuous, and it can happen in any direction. To insulate a tank as effectively as possible it's good to be familiar with the conductivity (C value) and the resistance (R value) of the various materials surrounding the water. To calculate total heat loss, you should also know the U value, which is the total of all the resistance in the insulating materials.

Figure 70. Heat loss from a storage tank is continuous, and it can happen in any direction. To insulate a tank as effectively as possible it's good to be familiar with the conductivity (C value) and the resistance (R value) of the various materials surrounding the water. To calculate total heat loss, you should also know the U value, which is the total of all the resistance in the insulating materials.

the temperature difference (0.049 x 80) we get a heat loss per square foot of 3.9 BTG's per hour.

If the total square footage of a 120-gallon hot water tank is around 50 square feet, then the total heat loss from the tank is 50 times 3.9 BTG's or 195 BTG's per hour. This is a very round-about way of saying what was said originally, that no matter how well you insulate your tank, there is still going to be a certain amount of stored solar energy lost.

Because heat tends to rise, insulation does the most good above the heat source. Any extra material should be put at the top of the tank. Another way to insulate a tank very well is to build a

Styrofoam Bead or Cellulose Insulation

Styrofoam Bead or Cellulose Insulation

Figure 71. One of the best ways to insulate a solar hot water tank is to build a plywood insulation bin around it. This can be filled with styrofoam beads, sheets of urethane foam, blown cellulose insulation, orfoamed-in-place insulation.

plywood box around the tank and fill it with styrofoam beads (R-3.6 per inch) or the kind of cellulose insulation that gets blown into the walls of old houses. Blown cellulose has an R value of 3.7 (Figure 71).

Foamed-in-place insulation (R-5.2 per inch) is better still. It fills all the nooks and crannies around the tank, and it's just a matter of mixing two liquids together and pouring them into the box. The mixture will expand, foam and bubble like a witch's brew, and work its way into all the spaces between the tank and the plywood. A good source for this stuff is your local insulation contractor.

These last two methods although they're quite good, have certain disadvantages too — not the least of which is that the tank is awfully hard to get at once the insulation is in place. In any case, take some of these thoughts and wage your own war with heat loss as best you can.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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