In last month's solar greenhouse article, I wrote about the need to store the sun's heat gathered during the day. Building materials such as brick, stone and adobe store heat well and should be used in the floor or can be used in the walls if the walls are then insulated on the outside.
Why insulate on the outside? We know to insulate in the wall, in a regular stud wall, and many have probably added some insulation underneath drywall on the inside of cement block or stabilized adobe walls. However, insulation blocks the transfer of heat (and cool) between the wall and air. If insulation is on the inside of a wall, heat or cool stored in the wall is blocked from moving into the house. It makes more sense to put insulation on the outside of the wall, keeping outside temperature extremes from affecting the wall and helping to stabilize interior temperature.
In a solar greenhouse, the sun will enter the south windows on a winter day warming the floor and walls. The absorbed heat will be released into the greenhouse at night. However, the walls and floor won't have enough mass to store all of the available day time heat, unless of course you have walls about three feet thick. So where to store the heat? In water!
Water stores more than twice the amount of heat or cool as rock, masonry, or adobe. This means it will stabilize the greenhouse temperature more effectively in both in winter and summer.
How does one use water and where in the world do you put it? Water can be contained in a variety of ways from gallon jugs to ponds. Enclosing water in Jugs or 55 gallon drums keeps the water contained. Ponds or large water storage tanks can contain large amounts of water and add humidity to the greenhouse.
Fifty-five gallon drums make wonderful storage containers, however know your drum! Don't use drums that have been used to ship hazardous or flammable materials! Around here it is fairly easy to find pvc drums that once held iodine.
Metal drums should be rustproofed on the inside and then filled. The outside, if painted a dark color and placed in direct winter sunlight, will be more effective heat gatherers than drums made of pvc.
Drums can be laced against the north wall of the green house, under a table or rack that holds growing beds. Seed lings raised here will never get cold toes!
Full gallon jugs, the water dyed a dark color to better absorb the heat, can be stacked on shelves against the north wall under planting beds. Gallon jugs, even if not in direct sunlight, can be placed under work tables or against the walls.
Or, you might think about adding an in-ground pond for semi-tropical fish. An aboveground water storage tank might not be as aesthetically pleasing, but would also store lots of heat. Maybe you'd rather go all out and put your hot tub in the greenhouse!
However you do it, plan to put some water storage in your greenhouse in addition to using lots of mass in the walls and floors. And, be sure to call me to personally inspect and try out your hot tub, should you choose that route!