How do you heat a greenhouse during the winter?

[Greenhorn]

Low temp outside last night was 27, low in the greenhouse was 44. I use one of these set to low for really cold nights. Greenhouse is basic Harbor freight 6 x 8 over cement, Wife said modest use of the heater isn't too bad on the electric bill. There are only a few months of the year where using it might necessary.

The heat is gradual and gentle. Next helpful thing I could do would be sealing it better to retain heat better.

http://www.homedepot.com/p/DeLonghi...ed-Radiant-Portable-Heater-EW7707CM/204631985

 

[CanadianLori]

Low temp outside last night was 27, low in the greenhouse was 44. I use one of these set to low for really cold nights. Greenhouse is basic Harbor freight 6 x 8 over cement, Wife said modest use of the heater isn't too bad on the electric bill. There are only a few months of the year where using it might necessary.

The heat is gradual and gentle. Next helpful thing I could do would be sealing it better to retain heat better.

http://www.homedepot.com/p/DeLonghi...ed-Radiant-Portable-Heater-EW7707CM/204631985

I wish I could afford to run one of those. Our electricity is extremely expensive because we no longer have coal powered plants and the replacement green powered producers cost mightily.

 

[MoonShadows]

A main consideration is heat loss through the ground in cold weather. I imagine most folks here have a greenhouse that sits on a slab, pavers, gravel bed or directly on the ground.

The soil underground, if properly insulated, can act like a huge battery for the greenhouse, storing thermal energy and stabilizing indoor temperatures both in the summer, but more importantly, in the winter.

We all know heat loss can occur through a greenhouses' walls and roof, but it can also occur through the floor, accounting for about 15% of an average greenhouses total heat loss. This is because the ground, if left un-insulated, freezes in the winter just like the air does. Preventing this heat loss is imperative for growing in a winter greenhouse.

By preventing heat loss to the surrounding topsoil, the soil underneath the greenhouse can now trap and store heat, creating a pocket of warmer soil underground. Soil is a source of thermal mass. Like water, stone and concrete, it stores thermal energy (heat) and slowly releases it later. Insulating underground connects the greenhouse to this huge store of thermal mass.

A neat and rather inexpensive way to keep the soil directly under the greenhouse floor stable year round is a method called a "Swedish Skirt". You can incorporate this method into the building of your greenhouse or retrofit it even to an existing greenhouse.

The idea with the Swedish Skirt is that you lay rigid foam board horizontal starting at the base of the greenhouse, and extending 4’ away from the greenhouse. This insulation needs to slope gently away from the greenhouse for drainage, which will require some minimal digging or raking. It’s also best to cover this foam with gravel or soil, as direct UV rays will damage the insulation. This system prevents very cold or frozen soil from creeping under the greenhouse, while keeping the warm soil where you want it, directly under your greenhouse. Ideally, you want the ridged foam board to be 4" thick.

This image is of a greenhouse where the surrounding soil has been prepared for a Swedish Skirt.

Unfortunately, I do not have a Swedish Skirt around my greenhouse but am seriously thinking of retrofitting one. It makes a lot sense!