mack1

The idea of the green house is to let in as much light as possible and don't have any reflective material to sent it back out. It needs to be absorbed by ground, plants, dirt, anything inside the green house. When it is re-radiated by the inside objects, it is at a wave length such that it will not escape the inside of the green house. Can't go back through the glass roof. Same princepal applys to a clear plastic top, but the plastic lets a lot less of the rays in to start with.

The next order of business it to keep the heat inside the house from conducting through the walls to the cold outside. Use insolation in the walls and an insolated door. Double glass letting the rays in also serves as an insolator at the roof. Again, when re-radiated, the rays are at a longer wave length and won't penetrate the glass or clear plastic.

There are three ways that heat is transmitted. Conduction(through a material), convection(carried by air movement across a material), and radiation (transmitted from the material through air or vaccuum). If you have ever heated a piece of metal and felt the heat some distance from the place that it was heated, that is heat conduction. Copper is one of the best heat conducting material. If you have felt a warm breeze, that is heat brought to you by convection. If you have ever felt the heat from a heater with a reflector, but can't feel the heat if something gets between you and the source, that is radiation heat transimition.

All three of these are significant. Consider this; have you ever wondered why it don't frost when there is an overcast sky? (even in Ireland) What happens is that the heat absorbed by the earth during the day is radiated at night towards the sky(in infrared wavelenghts). If up in the sky there is something to absorb it, it is re-radiated back toward the earth, which again absorbs it. Heat is traded back and forth between the earth and the clouds. On clear nights, the radiation goes out and don't return. Thus the transmitting object on the ground just keeps getting colder from the loss of heat. When earth objects reduce its temperature to below freezing, moisture from the air freezes on the surface of that object. Dark objects absorb and retransmit heat much more readily that light, reflective objects. Dark ojbects get much hotter during the day (also re-radiate more). Have you noticed that dark roofs have more frost on them that lighter roofs?

Those three principals come to play in a hot house. Sunlight comes in through the roof, is absorbed and re-radiated in a longer wavelength that can't exit through the glass or plastic. Heat is traded back and forth between all objects inside the hothouse. Walls absorb some of it and conduct it to the outside where the cold air is. The outside wall is kept cool by radiating and conduction to the outside air. Insolation lowers the amount of conduction from inside to outside.

I bet I've told you more that you wanted to know.

Have fun with the hothouse and keep the principles above in mind.

Best regards,
Mack1