Properties of Glass (Greenhouse application)

Why Glass?
Glass is a great greenhouse glazing because of its transmissive properties, elegant looks, and longevity.

A Little Background

There are three basic modes of heat transfer; conduction, convection, and radiation. Imagine a fire in the woods. The fire is the heat source. Convection is the warm air rising. Conduction is the warming of the ground and wood that is in direct contact with the fire. Radiation is the heat emitted from the fire in the form of electromagnetic waves.

Photo by By Clive M. Countryman (Heat Conduction) [Public domain], via Wikimedia Commons

Transmissivity is the measure of how well a material allows radiation to pass through it. In general, a clear material has a higher transmissivity than that of an opaque.

Because radiation emits via electromagnetic waves, the length of those waves may vary. The wave length is the horizontal distance between to adjoining peaks.

Photo from wikimedia commons

The wave length that's emitted is directly related to the temperature of the object. For example, the warmer the object, the shorter the emitted wave length is. The radiation from the sun (solar) is a short wave length, at about 500 nano meters. The radiation from the earth is a relatively much longer wave length, at about 10 micro meters.


Transmissive Properties of Glass

Looking at the graph, glass is highly transmissive for shorter wave lengths, but not at all for longer wave lengths.

Photo by Götz (Own work) [CC0], via Wikimedia Commons

The yellow and green lines are glass.
Photo by HellmaUSA (Own work) / [CC-BY-SA-3.0], via Wikimedia Commons

The percentage of solar radiation transmitted through glass is about 85-95%. The percentage of earth's radiation transmitted through the glass is close to zero. These properties of glass can now be reproduced in many plastics, making them on par with glass initially. However, the plastics may yellow with time depending on the UV coating. This yellowing will reduce the transmissivity of the material as it becomes more opaque, lasting around 10-15 years.

What does it all mean?

Glass allows the solar radiation to pass though and heats the inside of the greenhouse. The heat that is then radiated from the ground out, is in the form of long wave lengths, which are kept inside. That is the greenhouse effect in a nutshell.

Photo by Comsol

Note: heat will still be lost through convection and conduction.

Researching Aquaponics

Aquaponics is the blending of hydroponics and aquaculture. The fish nutrients (poop and other waste) feed your garden, and the plants clean the water for the fishes.  It's the circle of life!

Photo by Kate Field / cc by

I actually found a few books at the local library. This one talks about aquaponics amongst a slew of other topics. I think that it is an all around good read.
40 projects for building your backyard homestead : a hands-on, step-by-step sustainable-living guide
by Toht, David.

I have three main go to internet sources for aquaponics. The first is Bright Agrotech. It is run by Dr. Nate Storey. His blog is awesome. He has tons of informational YouTube videos on most aspects of aquaponics.

Photo by Kanu Hawaii / cc by

The second is Bigelow Brook Farm. He has some great YouTube videos on strawberry towers and bell siphons.

The last is BYAP (Backyard Aquaponics). They have a lot of great free info on this site as well. What I absolutely love about this site is the forum. There are a lot of experienced and well versed contributors on that forum.

Photo by Dallashomestager / cc by

 Some important factors to consider when starting aquaponics are location, fish, system design, grow beds and grow media, pump, and cost.

Location
An aquaponic system can be located inside, it would just need to be accompanied with grow lights. If the system is outside, then what is the climate like?

Fish
They can be kept as pets or a source of food. Check with local wildlife management to know what kind of fish are allowed in the area. It is easier to keep a fish that is suited to the local climate. That way the water in the tank might not need to be heated. Fish can be cycled in and out of a system too. It may be easier to stock one type of fish for the summer and another for the winter.

Photo by Cara Harpole / cc by

System Design
The plants can be grown in towers, soil less media, rafts, hanging pots, NFT, or many others. There are so many sources out there with all of these systems explained. Start with the three sources mentioned earlier and branch out from there.

Pumps
Make sure that the pump can handle the requirements of the system. Figure out how many gallons of water need to be cycled every hour for the desired set up, lets assume its 20 gallons per hour (gph).  Then you need to figure out what is the max vertical distance the water will travel, say it is 6 ft. That is called the head height. The minimum requirement for the pump is the mesh of these two parameters. The smallest pump to buy for the system would be a 20 gph at 6 ft head. To select a correct pump look at the pump charts provided by the manufacturers and pick the one that meets the requirements.
Here is an example of a pump head height graph.

photo taken from solar pond pumps

 

Researching Greenhouses

The first project on the list is the green house. It all starts with hitting the books.

I did about 6 months worth of researching before I settled on a system/design that I felt would work best for me. I want a system that I could use year round. I live in the Great Northwest (Oregon) and knew that I would have to use a greenhouse in order to grow during the winters. I was able to find lots of free info online about building greenhouses.

Photo by United Soybean Board / CC by

There is a lot to consider when building a greenhouse. You need to choose a location, size, building material, purpose, foundation, longevity, cost, and type. Here are a couple of resources that I used;

Building a Greenouse
Sundance Design Guide
Greenhouses for Homeowners and Gardners
Youtube search how to build a greenhouse

There were other sources that I used, but these were the most helpful to me.

Here is a very condensed and simplified version of every thing I learned about greenhouses.

Longevity, Purpose and Cost
These three parameters affect the rest. How long do you want to use the greenhouse for? Will it be used all year round, just to overwinter a few plants, or to get an early start on the growing season? It can be free bu using recycled materials or several thousand dollars for a fancy kit and a contrator to install.

Photo by V31S70 / CC by

Types
The type of greenhouse you use depends upon where it is going. If you want to use an existing structure as a support then consider a lean-to or solar lean-to style. Or if it will be placed out by it self, consider a gable, quonset, a frame, hoop house, or a pit style.

Each one has its own advantage and disadvantages. I suggest you read the links provided above as they give in depth details.

 Size
This mostly depends on what you want to get out of your greenhouse. Are you planting starts, feeding a family or perhaps a community? Some styles of greenhouses are easier to make additions to later if you wish to expand.

Photo by francois / CC by

Building Material
There are tons of options here. The frame material could be plastic, wood, or metal. But it is not quite that simple. You have types of metal, wood and plastic. PVC would be a good option for a hoop house, it cannot hold too much weight though. If you are worried about wood rot, consider useing cedar, cypress, or redwood. Metals are the strongest, but to prevent rust use galvanized steel, or paint the metal with some sort of protection (rust-oleum)

The glazing material can be from glass, polycarbonate sheets (hard plastic), or the thin 4mm plastic stuff. Glass is hands down the best material.  It is also the most expensive and lasts the longest. Most polycarbonate sheets have a ten year warranty from UV protection. The poly sheets will yellow from the sun and will not let as much light it. The cheapest option is the thin plastic that will lay over the frame (usually it is used with hoop houses). This will need to be replaced more frequently than the other options.

Photo by Axel Kristinsson / CC By

Foundation
when you walk into your greenhouse what do you want to be standing on? Would you be ok with dirt, grass clippings for the walkways, crushed rock, pebbles, brick, stone, or concrete. Each has an associated cost, look, and structural foundation support. If you are doing raised garden beds and it is a permanent sturcture, perhaps laying concrete would be a good idea.

Other Considerations
Will you need water or electricity? The air in the greenhouse will need to cycle. There are passive designs for ventilation that can minimize the use of needed cooling and heating. You may need a fan to help the air move. If you are using in the winter you can add heat to the green house with rocket stoves, space heaters, thermal mass, and a bunch of other ways.

Photo by brewbooks / CC by

There are a lot of options to consider. I do not think that there is a one right answer for greenhouse design. Do your research and find out what will work best for your climate and needs.