THE NOT-SO-HOT PASSIVE-SOLAR GREENHOUSE

GUEST POST BY DOUG HANSMANN

This month marks the first plantings in the greenhouse Denise and I built on the south wall of our barn.  The first, we hope, of many productive growing seasons.

The concrete footings for the greenhouse were poured in the summer of 2007 when the barn was built.    An 8 foot high, 22 foot long section of the south barn wall was cast in concrete in order to provide a good heat sink backdrop for the sun drenched greenhouse.

...Easter end of greenhouse still covered in plastic sheeting.

In 2008 we added the white oak frame,  but time ran short, and we scrambled to cover it in plastic sheeting during an early blizzard.

Last spring we added more permanent twin-walled polycarbonate roof panels purchased from the Greenhouse Megastore,

Last fall, we built two big growing boxes filled with a mixture of some marginally friable top soil along with healthy measures of horse manure and peat moss (see previous post Greenhouse Ready for Winter).

On March 14, we planted rows of kale, spinach, chard, carrots and beets.  Now our challenge is to keep those plants alive and healthy.

With a typical frost-free date of mid May in Southern Wisconsin, you might think our primary concern is a killer frost, but the temperature hasn’t dipped below freezing in the greenhouse since the first of March.  We could have planted two weeks earlier if the planting boxes had been ready.

Sunny days have been pushing heat deep into the dirt and concrete since mid February, so it’s not the cold that worries us — It’s too much heat.

On a sunny March day, even with a chilly breeze and high temperatures in the forties, the greenhouse has reached nearly 90°F.  When outside temperatures bob into the 50s, the greenhouse has shot up to over a hundred!

...Closeup of the autovent when the weather is cold.

Our goal is to extend the growing season in the greenhouse without using any fossil fuel for heating, lighting or venting.  Right now ventilation is our main issue, so we’re adding autovent window openers , to two homemade casement windows on the ends of the greenhouse.   One window is positioned low in the west wall to sweep up the breezes that come mostly out of the southwest, especially on our warmer spring days.   The other sits high in the east wall so the heat can escape.

According to the Bayliss website, the autovent is powered by the expansion of a mineral wax in its cylinder.  This wax expands rapidly when temperatures rise to its solid-liquid phase change point.

...Autovent expanding and opening window as the day warms.

As the wax expands, it pushes a piston that opens the window up to as much as 12” wide.  The operational vent opening temperature can be lowered or raised by turning an adjuster on the end of the piston.  Once the desired opening temperature is set, the autovent travels from its closed position to its fully opened position over a 5°F – 10°F temperature range.  Three springs close the window again as the evening cool sets in.

We don’t expect to grow year-round in our greenhouse.  Based on last winter’s temperature measurements, our best guess is that we can expect extreme cold to freeze us out by Christmas.  Shortened day lengths will stunt growth before that.

Each fall, we’ll see just how long we can keep fresh spinach, chard and kale on our plates from a late-summer second planting.  Come late February, we’ll gear up to sow and transplant.  We’re hoping for a ten week jump on the outdoor Wisconsin growing season.

...Heat escaping even when we can't be there.

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3 replies

  1. Super greenhouse, especially now that you’re adding the ventilation. We measured the temp in our mini-greenhouse the other day when outdoor temps were around 60 degrees F. The top shelf of the greenhouse was 114 degrees F. Thank goodness we could move it to reduce sun exposure for part of the day.

  2. We are also still figuring out the best way to handle the heat. But aren’t greenhouses great?

    How soon were you able to start frost-free growing this year?

    • Our last frost date is April 15. Ordinarily I would have planted tomatoes much earlier in Wall-o-waters, but between family health problems that kept taking me out of town (and thus got me a late start on my tomato seedlings) plus my surgery a few weeks ago, I won’t be putting in any warm-weather plants until around May 1. I hate to wait that late, but I’m not supposed to dig, bend, lift, etc.

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