# Greenhouse Question



## Mack (Nov 8, 2004)

Hi all:wave:

I have just about finished building my greenhouse 6x5x6Feet aprox, built mainly for spring next year.

I have used a corrugated perspex instead of glass.

My question is do I need to hang a plastic curtain internally or does it matter. I have seen other greenhouses with something hanging inside either nets or plastic sheeting but not sure why they do it. Is it to block harmful rays from the Sun or for extra insulation maybe?

Another question. I would like to if possible to grow something over the winter. Any ideas? Needs to be greenhouse friendly as I don't have an area for growing other than that. Usually a little below zero here in winter but not for long. I'm growing chillies at the moment but might need to take them inside the house soon.

Thanks

Mack


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## mack1 (Nov 27, 2007)

Hi Mack,

Here are a few tidbits that I know about green houses that you may, or may not know. Light in the visual spectrum can pass through glass better than plastic. Car manufacturers did us a dis-service when they switched from sealed beam headlights to plastic. Once inside the green house, light needs to be absorbed by something black and not shiny. If the light is caused to reflect back, it will leave the green house. Dark objects absorb the light rays and re-radiate light energy back, but in the infrared spectrum which won't go through glass. That is the essence of the green house effect. If you use well insolated walls and double pane glass in the roof, it will stay much warmer inside. The plants, of course, need the sunlight, but under the plants, dull black plastic would help trap the extra rays. 

Oddly, I learned most of this at an infrared techschool. 

Best regards,
Mack1


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## birdjaguar (Aug 1, 2008)

How cold does it get in your part of Ireland in the winter?


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## Mack (Nov 8, 2004)

Thanks Mack1. I have a weed out material on the ground which is black so that might help. I take from that your saying that the plastic im using may cause the greenhouse to heat slower but retain for longer?

birdjaguar, Usually not too cold and not for sustained periods -1 or -2 but not much more. It usually sits in and around 3 to 4 degrees in winter.


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## mack1 (Nov 27, 2007)

> I have a weed out material on the ground which is black so that might help. I take from that your saying that the plastic im using may cause the greenhouse to heat slower but retain for longer?


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. :grin:

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

Best regards,
Mack1


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## Mack (Nov 8, 2004)

I have been looking at greenhouse builds on various sites and many use plastic sheeting whch is too fine for me since I have 2 young boys who would love to poke holes in it. Glass for the same reason is out. The plastic panels are durable and if one gets damaged it can be replaced so the plastic corrigation was a compremise in my case. Some people seem to be conserned about uv and ir rayes getting into the greenhouse. It does effect the yield of Tomatoes for example but not significantly but that's not a big concern for me since my greenhouse is small.

However, your information has answered my question. A second layer would improve the isulative properties and possibly block some of the harmfull rays. Since the GH is woden framed and with the perspex attached to the outside, I would be able to tack some light plastic sheeting inside. I think based on what you have said that would improve things considerably.

Thank's for the great information Mack1.


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## BLUES4U (Aug 4, 2006)

Hi I live in the UK and weather is similiar to yours. I fix bubblewrap inside my glass greenhouse and have kept geraniums and fuschias all winter. Reduce the amount of water to the plants in winter


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## BLUES4U (Aug 4, 2006)

I use XP Professional LOL not Home Edition


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## BLUES4U (Aug 4, 2006)

:wave:
Hi I have a glass greenhouse and insulate it with clear bubblewrap in winter. I have kept geraniums and fuschias all winter like that but make sure to reduce the water you give plants.


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