EAT UP“lettuce” in – it’s cold out here!
High tunnel veggies || Photo by Lauren Mandel
As your asparagus lies low waiting for spring, it has plenty of time to fantasize about warmer soil. Jump up to a rooftop, and this fantasy could be closer to reality than your vegetables have ever dreamed.
Buildings emit heat. Since hot air rises, local building codes specify how much insulation must be installed inside the roof to keep this heat from escaping. But what if there were a way to let some of this heat enter a rooftop greenhouse?
Some ground-level greenhouses rely upon radiant heating to regulate winter temperatures. This method involves heating the floor so that hot air can warm the plants and soil. Perhaps this same principle can be applied to rooftop greenhouses, whereby heat from the building is harnessed and re-used. With new construction, could municipalities be convinced to allow for thinner insulation below greenhouses?
In my travels, I have yet to see this practice in action. Do you think that it’s feasible? Would municipalities allow for such an unusual variance?
I would guess that since the R factors are the composite of the roofing structure, a garden could be counted as part of the insulation even though you would occasionally vent it’s heat so as not to cook the plants too much. But in our house the attic air is typically very cold in the winter — the insulation is under the attic, not at the roofline. Perhaps in taller buildings which lack attic spaces this idea would work! Good thinking!
Thanks for your comment, Gregg! To tell you the truth, I’m not sure if insulation can be reduced with the use of a green roof – I’ll ask at work today. Recently, I’ve seen about 4″ of insulation required by code in PA when a green roof is in the picture (for new construction). I’m not sure how much is required without this vegetated topdressing.
You’re inclination about taller buildings is correct! In large municipal, institutional, and even multi-unit residential, the insulation lies within the roofing buildup. Sometimes it goes directly below the waterproofing membrane, and sometimes over top. Single-family homes are quite
different.
The key you are touching on is lengthening the growing season. Greenhouses naturally extend this season much as the same concept of espaliered trees on a castle wall. Yes, the challenge is to create a symbiotic relationship between the rooftop greenhouse with the building. It seems that in this climate where you have cold winters and warm humid summers, the spring and fall would be where you may be able to use the heated air to supplement the heating required by heat demand of the building. Natural convective currents within the greenhouse will draw cold air from the lowest wall openings and be drawn upwards toward the roof peak. Maybe incorporating evacuated tubes for hot water or some other thermal loops into the greenhouse structures would help. You would want to orient the long axix of your building on the east west axis.