I'd welcome any suggestions about my project to create a frost free winter lean to greenhouse which should have little or no heating input but keep tender crops and plants going all winter. We already have a 'conservatory' constructed around a south faced building which has the rear and side walls buried in a bank of earth. I've topped this with polycarbonate panels and fronted it with old single glazed doors and windows reclaimed from the tip. This works fine especially at the back where it never drops below freezing despite minus 20C during winter fairly often. My future project is to use the south facing bank in our garden to construct a 6 or 7ft wall to which I will attach a further structure in polycarbonate roof and glass windows, probably six foot to eight feet wide. To make this as near frost proof as possible, what thickness of wall should I aim for. I am planning a double layer of blocks with the gap between filled with local stone and rubble. Is this the best material? What might be more effective in creating a thermal mass? The whole thing might be as long as 30 feet, but what kind of considerations should there be for insulating the sides, roof and front? Or floor? The main purpose of the building would be as a place to put all our tender trees and shrubs...like guavas and avocados...to protect them in winter. At present I just about get these plants through winter with minimal damage and no added heating. Will my proposed greenhouse ever work or should I go back to the design board?
Last Edit: Nov 18, 2012 8:08:12 GMT -5 by bertiefox
Post by circumspice on Nov 18, 2012 8:20:24 GMT -5
You seem to have all the basics covered. The only thing I might suggest would be to say that you need to consider additional thermal mass with the flooring. You can go totally passive with the flooring or you could install a heated floor.
As far as insulation goes, remember that double-pane glass has an R factor of around 2. Anything beyond that is just a bonus. My greenhouse roof is insulated to R 19 (6 inches of fiberglass batts) and walls are R 11 (4 inches of fiberglass batts). That is overkill, considering that most of the structure is double-paned glass. Any form of insulation ought to be enough. I'm not sure of the R factor of polycarbonate, but I'd guess it's around R 1.
I might have made a mistake by not insulating the floor, especially under the water tank for thermal mass. If I get the chance, I'll correct that. I would recommend using foam insulation board under your thermal mass floor (with sand underneath and above it to help prevent damage to the foam board). I believe that will keep it warmer than the earth temperature. I used dark colored bricks for the floor.
In a nutshell: cheapest available foam board for insulation, dark colored brick or stone for the floor, water tank for thermal mass. Good luck!