Rooftop Greenhouses: Unheated Greenhouse On The Roof
In cold or moderately cold climates the growing season can be extended by about 3 months (one and a half in spring and one and a half in the fall) by the provision of greenhouses, which need not be heated other than by heat escaping from the house.
I believe the greenhouse on the roof, the glass or the fiberglass secondary roof over the earth covered primary roof, to be technically sound, economically affordable and quite logical in terms of ecology
In late spring and early fall, when night temperatures are not lower than about -3° degrees Celsius, the solar heat gain generated within the glass of fiberglass roof, (when greenhouse windows are closed) can be circulated into the house to make it warm enough even during the day, without turning the heat on in the house. At this time plants will thrive without heat, even during cold nights.
In early spring and late fall, when outside temperatures are about -3 to -7 degrees Celsius and heat is turned on within the house, often this heat is produced to excess. This excess heat can then be allowed to escape into the roof greenhouse through roof stair hatches or other louvres.
In winter with all hatches to roof greenhouse shut tight and all greenhouse windows shut tight, the natural heat loss escaping through the ceiling below the greenhouse floor and the condensation forming and turning to ice on the inside of the greenhouse skin will form an additional wind barrier and some insulation against the very cold outside.
In my own house I experience a difference of approximately 6 to 10 degrees Celsius between the outside and the interior of my entirely unheated greenhouse. More scientifically-measured studies are in order to further substantiate and support my experience.
The smooth greenhouse roof keeps the snow load away from the primary house roof.
The structural floor of the roof greenhouse should be designed and constructed according to the guidelines shown in the previous chapter, i.e “The Earth-Covered Roof”. But, while there are no snow loads in the floor of the roof greenhouse, other problems such as slope of floor as well as flashing around chimney flues, vent pipes, and ducts must be solved correctly.
Slope of Roof Greenhouse Floor:
Greenhouse floors are messy: Plants in greenhouses need frequent thorough dousing by water to combat mites. An earth or gravel floor is best for hosing down. Water drips out of pot drain holes. Condensation forming on the underside of the greenhouse roof falls down onto the floor. The earth covering the house roof below my get water-sogged and excess water should be drained off. A small slope within the broken length of watertight floor panels will lead excess water to gutters, which may or may not be led to the outside.
The insulated greenhouse or insulated portion of the uninsulated greenhouse:
For the average family the cost of maintaining heat and artificial light within the greenhouse throughout the winter, the season of sparse light, is not warranted by the amount of food which can be raised during the months of low light and cold temperatures.
As pointed out above, the efficiency of the roof greenhouse lies in extending the normal growing season by taking advantage of
- natural solar heat gain within the greenhouse and
- the excess heat of the house at times when home heating cannot be controlled satisfactorily, i.e. when the heat is too great and fall or in spring
In special cases however, additional insulation can be installed on the inside of the exterior skin using double skinned translucent plastic or simply polyethylene sheet plastic.