building and insulating solar water tanks cheaply

I am wanting to build some solar water storage tanks for two purposes:
- to store solar-heated hot water to pull back into the house, seedling beds and mushroom-growing boxes when needed. These are going to get as hot as the solar water can heat them. These would be built into the greenhouse.

- to build some water tanks to be passive thermal collectors in the green house, and place my mushroom growing chambers and seedling trays on top them, and use these to try and maintain a temperature of 17 or 22C (for the different stages of mushroom growth or seedlings being grown).

I have found a few designs which use 3/4" ply board, but this runs at about NZ$72 for a 4 x 8 ft board, when I can get 1 x6 timber for just over a dollar a metre (approx. one yard) This makes the ply over twice the cost of the straight board per square metre of coverage.

These heavily insulated boxes are also generally built in climates with feet of snow. We are in coastal New Zealand, with severe wind-chill, but relatively mild climate. We have winter average min of 4C and average max of 12C. We have frequent -2C frosts, but never past -4C on record, as we are 5miles / 8km from the sea. Our summer temperatures are average max 21C and average min 13C with an absolute max of 29C
(Even in summer some greenhouse is needed to grow eggplant and capsicum etc.)
Here in they are in F : http://www.meoweather.com/history/New%20Zealand/na/-40.633333/175.275/Levin.html?units=f#

Given our mild range of temperatures, I am wondering if rather than put a lot of money into insulating a fairly small amount of water, it would be better to build a rather large water-storage facility that would be enough to almost self-regulate day/night variations without needing insulation. We have a source of plastic pallets that we thought could be used as a base to insulate from the ground heat-loss. Is there a formula that can be used to calculate heat gain and loss of stored water in relation to the water volume and temperature and the outside temperature?

Does anyone know a link to a waterstorage facility made with 1 x 6 ( I have put them two high and just put plastic in them and half-filled them with some water to grow duckweed, but I am not sure what would happen if I went higher than this).

Hi Annie,

I don't get a really clear sense of how big your greenhouse or home are, or how much water you're talking. That might help someone more mathematically inclined to help with numbers, though I suspect the calculations would get complex fairly fast.

For the in-greenhouse tanks, which are presumably more modest in size, scrounged stuff might work. Barrels, bathtubs, watering troughs, pools, hot-tubs, maybe an old fibreglass boat? Anything freeish that you can make watertight, really...

As far as building a tank goes, I'd think 3/4" ply would be notably stronger than your average 1x6 for this purpose, due to the laminate construction. You'd need about 22M of 1x6 to create a layer of 1x6 the size of a sheet of plywood, but I would expect you'd use up that savings by needing 2 layers, plus glue/screws, plus labour...

The cheapest way to store a lot of water is pretty well always in the ground; could you excavate a pond, and line with some form of insulation before installing a pond liner? Perhaps then install glazing over the pond?


I wouldn't think you'd want to have more water storage than your solar heating can heat effectively. 10 units of water at 1 degree over ambient temp are not as useful for achieving a warm stable temperature as 1 unit at 10 degrees over ambient. With the latter you can raise the temperature of a small area significantly, or a large area very slightly; with the former, you are more limited.


I am not sure how large a water storage facility you would need to maintain a stable temp though day/night cycles, but we have a cement reservoir, perhaps 12'x20' with a sloped bottom from 3-5' high. Mostly above ground. Open top, little shade. It goes through very noticeable temperature swings, and is near bathtub temperatures late in a hot day, but quite chill in the morning.

The other issue with a large reservoir would be that in the winter it would settle at some empeature between 4C and 12C, which doesn't do much to provide your mushrooms with 17-22C...