I have some questions concerning solar greenhouse construction and operation. My sunspace is framed with 2-by-6 inch pieces and has six inches of fiberglass insulation in the walls, five large double-paned windows gracing its south side, and five more double-paned portals on the sloped top. Particleboard lines the interior of the structure. My question concerns the amount of moisture that collects on the inner walls. Can you advise me as to the best way to protect particleboard against moisture?
I could also use some input as to heat storage techniques. I don't have the room needed to accommodate barrels of water, and I'm finding it difficult to obtain smaller vessels for storing the liquid. Do you have any suggestions?
A good preservative for protecting particleboard in a humid greenhouse environment is clear Cuprinol. This product won't harm plants or fish, but it is a light-duty coating and will have to be reapplied annually. (The heavier Cuprinol preservative contains copper naphthenate, which is detrimental to the health of plants and fish and thus is not suitable for greenhouse use.) Another option is to plug the pores of the particleboard with a sealer and then brush on a “porch and deck” enamel. When dry, this coating will not harm plants or fish.
In answer to your second question, there are various ways to add thermal mass to a greenhouse that has insufficient heat storage. If you haven't done so already, install insulation under either the floor or the ground-level growing beds. Put down 1-to-2 inches of extruded polystyrene, top that layer with a vapor barrier, and finally, add 4-to-6 inches of concrete for thermal mass. (In addition, be sure to insulate the sides of the slab.)
If standard oil drums are too large for your greenhouse, try using narrow (12 inch or 18 inch) Kalwall tubes, which are fiberglass containers. These are available from Solar Components Corporation. For maximum heat absorption, you should dye the water a dark color (water dye is available from Solar Components Corporation). There are also a variety of plastic industrial containers that can easily be recycled as water receptacles. Do make sure that any scrounged container is airtight and keep in mind that it's going to be exceedingly difficult to repair a leaky tub that's located on the lowest tier of your “water wall,” if half a ton of liquid is balanced in vessels above it! Five-gallon containers are generally easier to stack than are smaller ones, though almost any receptacle will work.
— Bill Smith, energy-conservation specialist