This is my first winter with radiant floor heating in the greenhouse, and our first extreme cold front of 2024 is here.

During my first winter with the Hartley, I had only electric heaters in the greenhouse. Unfortunately, when outside temperatures reached negative double-digits Celcius, the greenhouse temperature fell to 2°C. It was also expensive to heat solely with electricity. I could over-winter plants but not actively grow plants during January and February. So, I decided to install radiant floor heating in the greenhouse.

This post tracks how my greenhouse heat kept up during a cold snap, with tracking between January 13 and January 15, 2024.

TLDR: In short, the temperatures held very well.  Six inches off the ground, the temperature remained around 18°C at night even when it was -17.1°C outside. At bench level, the temperatures never fell below 11°C. I gained roughly 10°C in ambient air temperature at bench height during double-digit lows by installing radiant heat. I gain another 6°C if I place plants on the floor.

Temperature Results During The Cold Snap

January 13 was setup day. I moved all the plants to ground level from the benches. The floor temperature, using an infrared heat gun, runs around 30°C.

I have four temperature sensors in the greenhouse. I moved one on top of an empty pot in the middle of the foliage, about 6 inches off the floor. The idea was to measure ambient air temperature where the foliage was.

Temperatures around the plants (the blue line) never fell below 17.7C.

The outdoor temperature during this period hit a low of -17.1C between 8:10 am and 8:14 am on January 15. During this time, the air 6 inches above the floor was a balmy 18.1°C.

Even the air around bench height (the grey line) performed better than before I had radiant floors. The lowest temperature in the southeast corner of the greenhouse was 12.8°C.

In contrast, when I had only electric heaters, the ambient air at bench height was only 2.3°C at -14°C outside. So, I’m gaining more than 10 degrees Celsius in ambient temperatures at bench height in the greenhouse by adding radiant floors. And when I move the plants to the floor, I gain another 6°C.

I also have a soil sensor that I placed in one pot. During the cold period, the soil temperature ranged between 19-21°C. This compares to a range of 16-17°C when that same pot was on the bench during the previous week when outside lows were in the single digits.

I did notice that I need to water the plants more when they are on the heated floor, which makes sense.

To keep the bench-level ambient temperature up, my two electric heaters ran non-stop all night when it was -17 °C, although, at 14°C, I noticed they do go off every 20-30 minutes for a short period. I know that’s not as cost-effective as it could be. I could put insulation boards around some of the windows and may do that if I can find a way to do so only during cold snaps. I may also run some tests (when the temperatures are not quite so cold) to see how low I can set the electric heaters without affecting the temperatures a foot or so above the floor.

All of this has taught me that I need to consider using my greenhouse differently. Warm and cold zones are very different when you have radiant floors with electric heat as a backup. There are so many possibilities:

• I can use the floors as heat mats for seed sowing and propagation
• I can put plants on the bottom shelves and expect that it will be warmer there, rather than cooler
• Benches are cooler at night but will be warmer in the day, assuming it’s sunny outside.

I probably won’t grow exclusively on the floor. Watering is more complicated, and I’m too old to keep bending down. Shade from the benches or side walls will also limit sun exposure on the floor except in the middle of the greenhouse. So, I’ll continue to use my electric heat as backup during cold weather. And I know I can put the plants on the ground during extreme cold periods and not worry.

Some Detailed Temperature Readings

12:00 pm (before move)
– outside temperature: -2.0°C
– heaters set at 19°C
– bench level NE 16.7°C, SW corner 15.9°C, SE 17.7°C = average 16.8°C
– soil temperature (1 pot) 16°C

2:30 pm
– outside temperature: -0.9°C, winds 41km/h
– heaters set at 19°C
– temperature 6″ from floor: 22.8C
– bench level NE 17.5°C, SW corner 17.2°, SE 18.9°C = average 17.9°C
– differential 4.9°C
– soil temperature 18°C – warmed up 2 degrees

8:00 pm
– outside temperature: -4.0C, winds 30km/h
– heaters raised to 20°C at 5 pm
– temperature 6″ from floor: 20.8°C
– bench level NE 14.1°C, SW corner 14.5°, SE 15.7°C = average 14.8°C
– differential 6.0°C
– soil temperature 18°C (reached 19°C by 3pm until just before 8pm))

JANUARY 14

8:00 am
– outside temperature: -9C, winds 24km/h
– temperature 6″ from floor: 20.5°C (low was 19.8 briefly at 5:21 am)
– bench level NE 14.1°C, SW corner 12.6°, SE 14.7°C = average 13.8°C
– differential 6.7°C
– soil temperature 18°C
– lowest bench reading was 12.5°C at 7:17am

2:30 pm
– outside temperature: -5.8°C, winds 22km/h, partly sunny
– temperature 6″ from floor: 24.3C
– bench level NE 21.6°C, SW corner 18.6°, SE 21.2°C = average 20.5°C
– differential 3.8C
– soil temperature 21°C

8:00 pm
– outside temperature: -12.9°C, winds 33km/h
– temperature 6″ from floor: 18.8C
– bench level NE 12.0°C, SW corner 13.8°, SE 11.7°C = average 12.5°C
– differential 6.3C
– soil temperature 18°C

JANUARY 15

8:00 am
– outside temperature: -17.1C, winds 13km/h
– temperature 6″ from floor: 18.0°C (low for the night)
– bench level NE 10.7°C, SW corner 12.9°, SE 11.0°C = average 11.5°C
– differential 6.5°C
– soil temperature 17°C
– lowest bench reading was 10.7°C at 7:52 am

2:30 pm
– outside temperature: -5.8°C, winds 21km/h, partly sunny
– temperature 6″ from floor: 23.6°C
– bench level NE 20.6°C, SW corner 18.0°, SE 21.6°C = average 20.1°C
– differential 3.5C
– soil temperature 21°C