A hobby greenhouse gives an opportunity to start plants earlier and extend the growing season. Moreover, it shields the plants from rain, snow and wind. However, it is very important to monitor day and night temperatures inside the greenhouse on a daily basis.
Sometimes a greenhouse can get too hot during summer months. However, on other days you can notice that temperature inside is lower than outside after checking the thermometer readings in the morning. Generally, greenhouse is colder than outside due to conduction of heat, air blowing away from the structure or prolonged cold spells. The later means that structure doesn’t get a chance to heat up during the day.
In this post we will discuss the possible reasons why greenhouse is colder than outside and will offer solutions to each problem.
How much higher can be temperature inside a greenhouse as compared to outside?
When the outside temperature falls to 20°F ( -7°C), a minimum temperature of unheated greenhouse should be around 28°F ( -2°C). So, a temperature inside a greenhouse can be 8 to 10 degrees higher than outside. In this conditions plant growth is approximately 3 to 4 weeks ahead than of outdoor plants.
#1 – Conduction of Heat.
Most of the time greenhouse is colder than outside during the night. As the sun goes down, the temperature outside gets cooler than in the greenhouse, therefore heat leaves the structure in a process known as conduction. Conduction is a process of heat transfer through the material when there is a difference of temperature. It results in a colder greenhouse temperature than outside.
As you can see from the figure above, most of the heat from a greenhouse is lost due to conduction across the plastic cover and through the soil. The objects inside the greenhouse that store solar energy such as soil, greenhouse frame, plants and benches release the energy into the cold sky during the night.
So, to make sure that the temperature inside is warm enough, aim to maximize sunlight, keep moisture in the soil and insulate your greenhouse. Moreover, try using plant covers which are breathable fabrics you can place over the plants at night.
#2 – Air blowing away from greenhouse structure.
The air that holds solar radiation can leave though vents, cracks and holes in the plastic because of the air movement. It is one of the most common reasons why greenhouse is colder than outside. Also, greenhouse looses heat by convection as wind blows across the structure.
So, when preparing a greenhouse for planting it is very important to find the spots of chills and seal the cracks or gaps in glazing panels or greenhouse frame. The easiest way is to use an adhesive foam strip to seal any gaps. Don’t forget to replace broken panels, doors or vents.
If you are an owner of pop-up plant house or hoophouse that are poly-covered, winds can blow plastic covers away. To secure the plastic cover and the greenhouse, screw the structure into the ground with anchor stakes and place paving slabs on the base. To find more tips on how to keep a greenhouse from blowing away read this post.
If you leave in the area with strong winds, a storm can break the glass or plastic covers. That is what happened to our greenhouse roof a couple of weeks ago. In this case you need to replace a broken panel as soon as possible.
My dad ordered a polyester roll and cut it with hacksaw to fit on a greenhouse roof. I have put together a step-by-step guide on how to install plastic on wood frame greenhouse. As much as we don’t want to let the air out of the greenhouse, ventilation is very important. So just keep it in mind when replacing greenhouse glazing.
#3 – Prolonged cold spells.
Greenhouse warms the inside of the structure by trapping energy inside. Therefore, prolonged cold spells especially when its cloudy can lead to greenhouse being colder than outside. Let’s remind ourselves how greenhouses work.
Sunlight enters the structure and plants, soil, frame, staging and shelving absorb solar radiation and reflect the rest. The plastic glazing traps this radiation inside and structure warms up. However, this energy is also leaving the greenhouse as described above. So, what can you do to trap the heat inside the greenhouse?
Soil and moisture are materials that absorb a lot of energy from sun. That is why many greenhouse growers put a few 1 gallon jugs of water inside. These will absorb the energy during the day and release it at night as temperatures drop. Moreover, there are 10 ways to increase a temperature inside a greenhouse without electricity.
However, in very cold climates you might need to use other ways to heat a greenhouse, especially in winter. Although heating cables and heating mats can increase soil temperature for germination and propagation, it might not be enough to warm the whole structure. In this case you can run a heater (hot water pipes, electric fan heater, gas or paraffin heaters).
