supplemental heat in the greenhouse

It has been cloudy and cold and the greenhouse is not always warming up during the day and is getting below freezing at night.  The seedlings are not getting bigger.  Our plan was to try to keep the greenhouse at or above freezing without supplemental heat.

Our efforts so far:

We have containers of water to use as heat sink.   Right now we have 36 cat litter boxes and 30 mushroom boxes filled with water.  We also have four 55 gallon barrels filled with water.  We are in the process of covering the fronts and sides of the boxes  with black tape to hopefully help the water absorb more heat.  We are also hoping that the porcelain tile floor and the dirt in the planters will absorb warmth.  We have passed the solstice and the days will be getting longer and the nights shorter.

We have scattered five thermometers around the growing area looking for cold spots and warm spots. The thermometers are sitting in different locations and at different heights.  So far they don't have permanent locations.  There is one near the glazing at table height, one in a planter at about six feet in the air, and three that have been moved around trying to measure the east and west corners and the north wall at different heights.


What we know:

On a cloudy day the temperatures throughout the greenhouse are pretty consistent. On a sunny day the temperatures can vary significantly, as much as 20 degrees or more of difference when it is moderately sunny. Since we added the additional thermometers we haven't had any really sunny days yet, so it may vary even more.

When there is a difference it is consistently cooler near the glazing.  The warmest area that was measured is in the center of the room at about six feet in the air. It is probably warmer still near the rafters and roof.

When we had heat sink buckets on the south wall below the glazing, those buckets had more ice than the buckets on the north wall.  The air temperature in that area is lower and that area is not hit directly by the sunlight coming in through the glazing.  Plan A had been to line the area below the glazing with the pallet tables and put the heat sink buckets under the tables and plants on top of the tables.  As the greenhouse filled with plants the gutter planters would be hung from the rafters and the table would hold larger containers with bigger plants. Now that we know that is the coolest part of the greenhouse at least in the cloudy short days of late fall and early winter, we may have to rethink that plan.

All of the heat sink buckets are now on the north wall. They are stacked three deep.  In moving them around to apply the tape it is apparent that in the buckets that I was working on, the top row of buckets have no ice or less ice than the bottom row of buckets.  I did not check all of the buckets only the few in the center of the room that I was taping.  A new question then is why the lower buckets are colder.  Is it because they are closer to the floor?  Is it because there were tables with planters in front of the stack that prevented direct sunlight from hitting the lower buckets?  Now the tables have been moved into the middle of the room to allow greater exposure to the sun.  I think there will be a lot of trial and error in finding our way this first growing season.

On New Year's Day we purchased this:

Commonly known as a milk house heater, this is a small electric heater used to provide supplemental heat to farm buildings.  It has two settings, high and low, and a dial type thermostat.  Even on high it doesn't put out a large amount of heat but I am hopeful that between this heater and the heat sink we can keep the growing area above freezing on most nights.