The New Greenhouse on the Block -- Part 2

an interview with Farm Manager Carrie Hargrove, continued from Part 1

The whole idea of a greenhouse seems to be to gather as much solar exposure as possible; what is it about this design that makes the greenhouse a passive solar building?

So this building is more than just a big sunlit room where the seed trays get protected solar exposure.  There is also guidance in the plans on the positioning and proportions of the building, as I mentioned, as well as the angle of the roof, and very importantly, the presence of filled water barrels made of black plastic or painted black and stacked on the back wall.

The water barrels soak up the sun's energy during the day and release it later.  This helps to keep the greenhouse from getting too hot during the day (along with ventilation).  Then after dusk, when the building cools, that heat is slowly released; that is especially important in the late winter and early spring.  So overall, the presence of these filled water barrels minimizes the changes in temperature that the seedlings in the building would otherwise experience. We got our barrels from Civic Recycling. The Bradford plan provides a formula that calculates how many gallons of water should be placed in the building given the building's size and the area of glazing (glass or clear plastic) that will be used.  For the size of our greenhouse, the formula provided in the Bradford plan called for nine or ten barrels, but we decided to use eleven to compensate for the fact that the building's 18" elevation would create some additional exposure to cold.

The slant of the roof is determined by the latitude of the location where the building will be constructed.  For our latitude, we rounded slightly to a 45 degree angle.  The roof is 'glazed' with two layers of poly plastic.  So while the layers of plastic let the sunlight in, the layer of air between them -- about four inches -- insulates the greenhouse to keep the temperature from cooling off too radically in the late winter and early spring.

You mentioned ventilation.  How is that designed in this building?

We have a window in the west end of the building and the entry is in the east end.  Opening those can create a good cross breeze to keep the building cool.  And we built a fan into the east end at the peak of the roof to create draw on days where there is not much breeze.  Being on the east end, it doesn't have to fight against the predominant west wind, and being at the peak, it pulls the hottest air out of the building.

Were you able to get any of the materials as recycled or repurposed donations, or was all of it new?

We were very lucky that we were able to get seconds quality 2x4 lumber from Boone County Lumber for much of the construction, particularly the entire deck floor.  The 2x4 rafters needed to be new, but the window was repurposed from the Eastgate Shopping Center during a remodel.  The 6mm poly plastic 'glazing' also needed to be new, but it wasn't very expensive and it should last for several years.

So how many labor hours do you think were spent on building the greenhouse altogether?  And what kind of maintenance will it require?

I'm not really sure about the total, but I'm almost positive it was over 500 hours, with several of us working on it.  It's definitely going to require some careful upkeep: every year we'll need to completely empty the building, clean it up, recaulk, and repaint.

Are there any aspects of the project that are still in progress or features on a 'wish list'?

The building still needs siding to help it last as long as possible under the stress of the weather.  And there are a few aspects around the roof that need to be adjusted -- just small tasks.  One specific thing that will be upgraded when we can is that the ventilation fan will be automated to respond to the air temperature and pressure inside the greenhouse.  Right now the fan is activated manually, so it will be good for the fan to activate and deactivate automatically whenever conditions call for it.