The thread "Additions to FAQ" has a few in it. I will cut and paste them to this one with yours. If any one else has some then if they can cut and paste this list with the new one(s) at the bottom we can generate a list everyone can see, without duplicate additions.

OK, so start the cut here....

:)
*How do I heat my greenhouse?

*What are the different types of glazings? What are their relative strengths and weaknesses?

*What modifications should/can I make to my HFGH?

*What is thermal mass and how do I use it?

*How can I make my greenhouse more energy efficient?

*Where can I get plans for building my own greenhouse?

*Is humidity important? How can/should I try to control it? (I'll take this one!)

*What is IPM?

*What are some good links to GH information?

end of cut.

Then paste that into the next post with more at the bottom,

Then I add one.... Mine is "How much will my greenhouse cost me to heat and cool FAQ"

maineman suggested 3 more and mollyd asked one

so I added them.

*How do I heat my greenhouse?

*What are the different types of glazings? What are their relative strengths and weaknesses?

*What modifications should/can I make to my HFGH?

*What is thermal mass and how do I use it?

*How can I make my greenhouse more energy efficient?

*Where can I get plans for building my own greenhouse?

*Is humidity important? How can/should I try to control it?

*What is IPM?

*What are some good links to GH information?

*How much will my greenhouse cost me to heat and cool?

*What are the recommended procedures for preparing the foundation for a greenhouse or greenhouse kit?

*How do I keep the copper in the pressure treated timbers from attacking, corroding, and even dissolving the attached framework of aluminum greenhouses?

*How do I protect a lean-to or attached greenhouse from falling icicles and snow slides from the eaves of the house?

*How to deal with condensation?

and so the list grows.

"You want it to be general. We dont want questions about the nit-pikin stuff. People can give answers for the little stuff. We want the big questions."

So then, which of these questions qualifies as general?

IMHO, IPM, heating, glazing, thermal mass, and condensation are sufficiently general, with broad application. Aluminum corrosion, icicles, humidity, building plans, glazings may not make the cut.

I would still like to see Laserfan and Gardenerwantabe write the HFGH modifications FAQ...even though it doesn't meet Orchiddude's requirement, this is among the most frequently asked questions that we see here on the forum ;-)

Merry Christmas, everyone!

SB

I'm relatively new but have gotten a ton of information from GW and the structures/HFGH pages particularly.

My FAQ has nothing to do with the specifics you've listed. Rather, can someone explain the organization or administration of this group? Is someone such as orchiddude or milwakueedave the "list owner" such as on Yahoo groups and as such has the keys to add items to a FAQ list?

Second, is there a way to sort the main messages? If you pull up the forum, you get a page of run on topics, nothing sorted by date etc. There ought to be a way (short of ""search"") to get everything posted about a given topic in one list.

Third, some people include photos in their posts. How is this done?

Thanks
Steve
Wisconsin

Steve,

There is no list owner, as far as I know. Orchiddude, how did you come to have the Faq password, anyway? Did Spike give it to you in the beginning?

As for pictures, here is what you do. Upload your picture to a web photo host site such as Webshots or Photobucket. It may help to resize the photos, in my experience. Then, at the web host site, you will find an "HTML Tag" under each picture. Copy that tag and paste it into the message.

SB

I am the one that requested that this forum have a FAQ, therefore, I was given the ability to make one. Me and a few other people had started one many years ago and something came up where we had to stop. We left it like it was. So, when yall brought up the subject of adding stuff to it, I jumped on the bandwaggon. Maybe this time we can get more added.

There are general rules stated by Gardenweb on guidelines. List below:

Please leave out all company or personal names, links, email addresses, phone numbers, mailing addresses and other information that's likely to change. These things change frequently and adding them will make maintenance of the FAQ a nightmare.

I have no problem in adding the questions and answers and giving credit to the ones that offer the information. I think we need to be orgranized about it to make it easier.

I dont own anything here nor does anyone else. As far as pictures, the problem comes up when you have links that changes, pictures that move, hosting sites that change. Photobucket or other posting sites do not want you to use their bandwith for stuff like this. If you have personal webspace and want to link off of it, thats ok, but I believe way way back it was looked down upon because of the bandwith issue.

The whole idea of a FAQ was to give general information about this subject. It was not intended to cover everything about the subject nor make it a one-stop shopping area for information. Bookstores have books that cover details. FAQ should give a general insight and give enough information to help people, not make is sound like its the only way to do it. Also remember, your answers need to be of general content that greenhouse owners have been using through the years. You don't want to place bias information or information that might one day come back to bite you, that is the reason I am suggesting that we use general information, then if people want to talk about nit-picky stuff, they can ask questions.

No matter what, I will be glad to post into the FAQ whatever questions and answers you would like to place into it.

I have placed the questions that Chris has listed into the FAQ. If people want to begin answering the questions, we can compile the answers and credit information. If you dont want to be listed in the credits, let me know.

Rob, a question I see often is how to calculate the number of BTUs needed to heat a greenhouse. I have a Univ. of Ga booklet that factors glazing, etc., into the formula. I'll be glad to send you a copy if you will send me your address? - Terry Smith

Orchiddude, where do you suggest we post our answers? Here?

stressbaby...you can post them here are you can email them to me. If you want credit, let me know. yes or no

If you would like to post them here and anyone wants to add something that might be good.

Thanks for your help!

"Put me in, coach!"
-John Fogerty

Greenhouse foundation (I think that question should be reworded):
A greenhouse may be built upon a foundation of timbers, block, brick, or concrete. Elevating the foundation has the advantage of gaining height and headroom in some greenhouses. The greenhouse foundation should be stable, level, and square.

A popular material for the floor is " crushed rock, but the floor may be concrete, dirt, mulch, brick, or tile. The installation of perforated drains beneath a porous layer of flooring helps to assure good drainage.

When building a greenhouse, it is useful to insulate the foundation. This will prevent the loss of heat through the ground and permit the ground to act as a "heat sink." The installation typically consists of 1-2 inches of polyurethane or polystyrene insulation 1.5-2 feet deep around the foundation.

SB

Energy efficiency/conservation:

You can save a considerable amount of energy in the greenhouse with a few simple steps.

1) Insulate the greenhouse foundation (See Foundation FAQ).

2) Insulate the north wall and north roof. Little light comes through the north wall of a greenhouse in the northern hemisphere, so insulation of this wall helps with energy losses without sacrificing any sunlight. One approach is to install foil-backed foamboard insulation, with the foil side facing into the greenhouse.

3) Seal off vents, weatherstrip doors, and caulk or otherwise close off other areas of air infiltration.

4) Double up on glazing. A double or triple layer of glazing can be added with an additional layer of polyethylene or with the addition of "bubble wrap."

5) Add thermal mass to the greenhouse (See FAQ on thermal mass).

SB

OK, folks. A possible addition to the FAQ's page.

What's kept me from building or buying a greenhouse is the variable of cost. I suspect we tend to think mostly in terms of structure cost which can be sizeable...but what about overall cost...i.e. heating installation, required electrical, watering system, benches, flooring, shade cloth, etc. I suspect they can easily double the overall cost of having a greenhouse. I might be mistaken, though.

I don't know if a comment like this fits into the general FAQ's category but it seems to me it does. It's certainly what I and others seem to question the most. I have no idea how to reduce it to a manageable, concise question. If you think it belongs in the FAQ's category I'll have to leave the honing of the topic to someone else. I'm not savvy enough to do it.

The only title I can come up with is...Overall Costs.

thanks

Lilies, you have it exactly right, IMHO. The cost of my kit (I won't even tell you!) was just under 1/2 of the total cost of the GH, counting foundation, knee wall, plumbing, electrical, fans, controllers, shade cloth, blah, blah, blah.

A different question, one that was asked, is how much does it cost to RUN the GH once it is built. I wrote a detailed answer, with examples, for DG, and I could copy it here if anyone is interested.

SB

In the beginning I tried posting answers to all the questions about the modifications to the 10x12 HFGH but soon realized that it was going to be a never ending thing.
In the past week I have compiled all of the modifications that need to be done in a very detailed way making it easy for anyone to follow what I did to mine.
Rather than post it and have it be lost I typed it in my word processor so that I can copy and paste it.
If those who are doing this want to include it in the FAQ I can make it available but it is very large two pages worth so it don't fit your outline of keeping it brief. When you post on this subject their are always a lot of follow up questions but with what I have made up now should pretty much solve that because it outlines all of the modifications including a fast and easy way to strengthen the roof for a snow load and how to stop the back wall from flexing.
I plan on keeping this in my computer so I can paste it to a post at any time. This will save me a lot of typing.
Me and my HFGH will soon have our first anniversary I bought it on January,10th 2006

gardenerwantabe... I will be glad to add a question and then post your material to that question. You can email me the material if you like.

Orchiddude,

How soon do you think you would be able to post the material from Gardenerwantabe?

Thanks

Orchiddude,

The foundation question is in FAQ, but the energy efficiency question is not. Does it need more work?

"I have no problem in adding the questions and answers and giving credit to the ones that offer the information." Well, I didn't know you spelled Stressbaby O-R-C-H-I-D-D-U-D-E! LOL! Just kidding with ya!

Here is DG answer I wrote to the GH costs question. Personally, I think it's probably to long for a FAQ, but you all can decide:

Some people have asked how to calculate the cost of heating and cooling a greehouse. I hope that this thread is helpful in this regard. It is fairly technical, but I hope to explain it fully enough to allow anyone to plug in their own numbers.

First some formulas:
Watts=amps*volts
1 KWH = 3,413 BTU
1500W electric heater produces 5120BTU
Propane is 91,600BTU/gallon
Q=deltaT*A/R
Q=BTU heat requirement per hour
delta T is the difference between outside temp and desired inside temp
A= the surface are of the greenhouse is square feet, not including the floor; ie, the roof, side walls, and end walls
R= the R value of the glazing

Two important points about this formula...some heating calculators use "heat loss value" (HLV) instead of R-value. Don't get confused, the heat loss value is just 1/R. The formula using heat loss value is then Q=deltaT*A*HLV. The other important point is that you use a different temperature when calculating your average energy costs over a month than you do when you you calculate what size heater you need. Heater calculations are used to determine how much heat you will need on the coldest night of the year. For this, you use your coldest winter temps, for example the colder end of your zone. For calculating heating costs you need the average temperature for the month.

Here are some typical R-values for common glazings or coverings
4 mil polyethylene 0.83
4 mm (5/32") twinwall polycarbonate 1.43
6 mil polyethylene 0.87
6 mm (1/4") twinwall polycarbonate 1.54
6 mil poly double layer (inflated) 1.43
11 mil woven polyethylene 0.95
3 mm (1/8") glass (single layer) 0.88
16 mm (5/8") triplewall polycarbonate 2.5
Polycarbonate / fiberglass (single layer) 0.83

OK, let's say you are contemplating an ACF 10 x 12 Cottage Greenhouse. We first need to calculate the surface area of the greenhouse. Using the diminsions found on the website and some simple geometry, I calculate the surface area of each sidewall to be 55 sq ft, each roof side to be 64.6 sq ft, and each end wall to be 75 sq ft. Therefore the total area (A) is sq ft is (55*2)+(64.6*2)+(75*2)=389.2 sq ft.

The website tells me that this GH comes in 6mm twinwall polycarbonate, so the R-value is 1.54. A quick Google search shows that in this particular zone 7a, the average January temperature to be 28.4 degrees. I want to keep my greenhouse at 50 degrees. So the "delta T" is 50-28.4=21.6 degrees. Now we can calculate the BTU per hour necessary to heat the greenhouse in January. Q=389.2*21.6/1.54=5459 BTU. To figure daily use, multiply by 24 hours to get 131016 BTU/day. To figure monthly use, multiply again by 30 (days in the month) to get 3,930,480 BTU/month.

Now, lets say you heat with propane. Knowing the BTU in a gallon of propane, we can take our BTU/month and divide by the number of BTU in a gallon of propane. 3,930,480/91,600BTU/gal propane to get 42.9 gallons of propane we would expect to burn in January. We just bought propane at $1.69/gal, so this would mean my January cost would be 42.6*$1.69=$72.52/month.

Let's say you heat with electric. Take the same BTU value and instead of dividing by the BTU/gallon of propane, divide by BTU/KWH. 3,930,480/3413BTU per KWH=1151KWH. If my energy cost is 7.169cents/KWH, then heating electric should cost about $82.52 for the month of January.

OK, well, our hypothetical DW says that the winter costs are too high and she cannot be convinced to go ahead with the greenhouse. Let's look at what we can do to lower the winter costs.

Let's insulate the north wall...no sun comes through this wall, and if we insulate it with foil-backed polyisocyanurate 1" board, which has an R-value of 7.04, we raise the R-value of the north wall to 1.54+7.04=8.58.

We can recalculate Q using the same formula, but adding together the BTU lost through the north wall and the BTU lost through the rest of the greenhouse. The formula would be Q=(deltaT*A/R)+(deltaT*A/R). Our greenhouse is oriented north-south, so the north wall is an end wall with a square footage of 55 sq ft. Our BTU formula changes:

Q = (334.2*21.6/1.54) + (55*21.6/8.58) = 4826 BTU/hour

This equals a monthly use of 3,474,720 BTU, equal to about $72.99/month heating electric.

What else could we do? Well, we could add some thermal mass. Let's put 8 black 55 gallon drums in the greenhouse. One BTU is defined as the amount of heat required to raise one pound of water one degree F. The water will warm during the day and release this heat at night. One chap I know has measured the temperatures of his water at dawn and at dusk. The temp is 74F at dusk and 66F in the morning, a drop of 8F.

Eight 55 gallon drums hold 440 gallons of water which is 3520 pounds of water. A 1F drop in the water temp releases 3520 BTU. An 8F drop releases 28120 BTU overnight. Over the course of a month, this is 844,800 BTU. Therefore, the total monthly BTU could hypothetically drop further to 3,474,720-844,800=2,629,920 BTU. This is 770.55KWH heating electric. At $0.07169/KWH, this is $55.24 for the month of January.

I want to thank my fellow Missourian and TL for pointing out the problem with heater (in)efficiency. Our calculations assume 100% efficiency. As you say, heaters, particularly vented heaters, are not 100% efficient. If you know the efficiency of your heater, you can adjust the calculations accordingly!

Now on to summer costs. The greenhouse in the example will need an exhaust fan and HAF fans. First the exhaust fan. The greenhouse is 900 cubic feet in volume. Therefore, we need an exhaust fan that is rated at least 900 cfm. I found a 16" exhaust fan at Charley's rated at 1095 cfm. The website tells me it draws 1.8 amps. Remember that watts=amps*volts. So when it is on, the fan uses 1.8*120=216 watts. If this fan runs 16 hours per day in the summer, it will use 3.456KWH/day. This is 103.68KWH/month, which at the same price of energy we have been using above, amounts to $7.43/month for the exhaust fan.

Let's say you use two 8" long life HAF fans. This may be overkill for a greenhouse of this size, but you will get the idea. Each fan draws 0.6 amps, and at 120V, this is 72 watts. Running 24 hours a day, each fan uses 1.728KWH/day, or 51.84KWH/month. Total use with the two fans is again 103.68KWH/month or $7.43/month.

I can clean it up if it is worthwhile for the FAQ. If not I won't bother!

SB

Hey SB,

My name comes up on the bottom because of the system. I dont have any control over that part. I was going to give credit under the information that was posted. It will still have me as the poster.

I will be glad to post something to the affect of: "Information provided by Stressbaby" or something like that. Does that sound ok, or we can go with something else.

Your info above looks good. I will stick it in there.

Pcdur...right now if I get the info. As soon as I get it. I will post.

I came down with the flu this week so I did not take some of the pictures that I want to up load to my web hosting site so I don't have this done yet.
Also I think it may be too large for the FAQ but since instead of posting on the GW like in the past trying to answer individual questions I typed this one in my word processor so If it gets lost I can copy and paste to GW anytime.I wish this forum had a edit function that a post could be updated. What I have to date I will put in another post. I will post it under Modifying the 10x12 HFGH

SB...Sorry, I missed the energy efficiency question, but I am up todate now. See if the credit looks ok to you. I can link your name to your page if you want.

gardenerwantabe... at least send me the information so I can test it out. I have cable internet, so you can send me anything and I will get it. Send me the file at my name @charter.net We can add or take away later if we need too. Lets get something up and go from there.

stressbaby,orchiddude,

The "Calculate the cost of heating and cooling a greehouse." FAQ is great! I had to read it at least five times before I understood it and I thought I knew what I was doing!

Scary, daunting stuff!!!!!!

Can you add a nice note at the end of the FAQs saying something along the lines of,

"After you have read this FAQ if you still need more detail, or have a specific question not fully answered by this FAQ, please feel free to start a thread on the forum and we are sure other members will do their best to help you. Unless it is a question about unvented heaters. If it is then post your question anyway, then sit back and watch the fur fly!!!!!"

Or words to that effect.....

:)

chris: perhaps we could keep our greenhouses warm just from the flames about unvented heaters?

The FAQs are looking good (although a little unpolished).

Nathan,

I am hosting a conference on the unvented heater question. Registration is free, and the conference is limited to 127 attendees. The debate I hope will be a very heated one.

It starts on 1st March and I hope it will continue until mid May. The daily conference will start at sunset and last until an hour after sunrise.

Why limited to 127? Well I would realy feel stupid powering up my exhaust fans at 3am in Feb just because the seminar was overbooked!

127*80w is.......

A 10Kw heater! which is 34,130btu/h Nice.....

Now I am convinced Chris has been experimenting with the refrigerator again!

I agree with Nathan. Orchiddude, if we want to "polish" an answer, or perhaps restate a question, can we just post it here for you?

SB

Well, OD, you didn't answer. May I post a polished version anyway?

First, restate the subject as a question: How do I calculate the costs of running a greenhouse?

Answer:

The question is best answered by using an example, but first some formulas:
Watts=amps*volts
1 KWH = 3,413 BTU
1500W electric heater produces 5120BTU
Propane is 91,600BTU/gallon
Q=deltaT*A/R Q=BTU heat requirement per hour
delta T is the difference between outside temp and desired inside temp
A= the surface are of the greenhouse is square feet, not including the floor; ie, the roof, side walls, and end walls
R= the R value of the glazing

Two important points about this last formula...some heating calculators use "heat loss value" (HLV) instead of R-value. Don't get confused, the HLV is just 1/R. The formula using heat loss value is then Q=deltaT*A*HLV. The other important point is that you use a different temperature when calculating your average energy costs over a month than you do when you you calculate what size heater you need. Heater calculations are used to determine how much heat you will need on the coldest night of the year. For this, you use your coldest winter temps, for example the colder end of your zone. For calculating heating costs you need the average temperature for the month.

Here are some typical R-values for common glazings or coverings: 4 mil polyethylene 0.83; 4 mm (5/32") twinwall polycarbonate 1.43; 6 mil polyethylene 0.87; 6 mm (1/4") twinwall polycarbonate 1.54; 6 mil poly double layer (inflated) 1.43; 11 mil woven polyethylene 0.95; 3 mm (1/8") glass (single layer) 0.88; 16 mm (5/8") triplewall polycarbonate 2.5; Polycarbonate/fiberglass (single layer) 0.83

OK, let's say we are contemplating an ACF 10 x 12 Cottage Greenhouse. We first need to calculate the surface area of the greenhouse. Using the diminsions found on the website and some simple geometry, we calculate the surface area of each sidewall to be 55 sq ft, each roof side to be 64.6 sq ft, and each end wall to be 75 sq ft. Therefore the total area (A) is sq ft is (55*2)+(64.6*2)+(75*2)=389.2 sq ft.

The website tells us that this GH comes in 6mm twinwall polycarbonate, so the R-value is 1.54. A quick search shows that in this particular zone 7a, the average January temperature to be 28.4 degrees. We want to keep our greenhouse at 50 degrees. So the "delta T" is 50-28.4=21.6 degrees. Now we can calculate the BTU per hour necessary to heat the greenhouse in January. Q=deltaT*A/R. Substituting the numbers, Q=389.2*21.6/1.54=5459 BTU. To figure daily use, multiply by 24 hours to get 131016 BTU/day. To figure monthly use, multiply again by 30 (days in the month) to get 3,930,480 BTU/month.

Now, lets say we heat with propane. Knowing the BTU in a gallon of propane, we can take our BTU/month and divide by the number of BTU in a gallon of propane. 3,930,480/91,600BTU/gal propane to get 42.9 gallons of propane we would expect to burn in January. We just bought propane at $1.69/gal, so this would mean my January cost would be 42.6*$1.69=$72.52/month.

Let's say we heat with electric. Take the same BTU value and instead of dividing by the BTU/gallon of propane, divide by BTU/KWH. 3,930,480/3413BTU per KWH=1151KWH. If my energy cost is 7.169cents/KWH, then heating electric should cost about $82.52 for the month of January.

OK, well, our hypothetical DW says that the winter costs are too high and she cannot be convinced to go ahead with the greenhouse. Let's look at what we can do to lower the winter costs.

Let's insulate the north wall...no sun comes through this wall, and if we insulate it with foil-backed polyisocyanurate 1" board, which has an R-value of 7.04, we raise the R-value of the north wall to 1.54+7.04=8.58.

We can recalculate Q using the same formula, but adding together the BTU lost through the north wall and the BTU lost through the rest of the greenhouse. The formula would be Q=(deltaT*A/R)+(deltaT*A/R). Our greenhouse is oriented north-south, so the north wall is an end wall with a square footage of 55 sq ft. Our BTU formula changes:

Q = (334.2*21.6/1.54) + (55*21.6/8.58) = 4826 BTU/hour

This equals a monthly use of 3,474,720 BTU, equal to about $72.99/month heating electric.

What else could we do? Well, we could add some thermal mass. Let's put 8 black 55 gallon drums in the greenhouse. One BTU is defined as the amount of heat required to raise one pound of water one degree F. The water will warm during the day and release this heat at night. One chap we know has measured the temperatures of his water at dawn and at dusk. The temp is 74F at dusk and 66F in the morning, a drop of 8F.

Eight 55 gallon drums hold 440 gallons of water which is 3520 pounds of water. A 1F drop in the water temp releases 3520 BTU. An 8F drop releases 28120 BTU overnight. Over the course of a month, this is 844,800 BTU. Therefore, the total monthly BTU could hypothetically drop further to 3,474,720-844,800=2,629,920 BTU. This is 770.55KWH heating electric. At $0.07169/KWH, this is $55.24 for the month of January.

Our calculations assume 100% efficiency. Some heaters, particularly vented heaters, are not 100% efficient. If you know the efficiency of your heater, you can adjust the calculations accordingly!

Now on to summer costs. The greenhouse in the example will need an exhaust fan and HAF fans. First the exhaust fan. The greenhouse is 900 cubic feet in volume. Therefore, we need an exhaust fan that is rated at least 900 cfm. A 16" exhaust fan at Charley's is rated at 1095 cfm. The website tells me it draws 1.8 amps. Remember that watts=amps*volts. So when it is on, the fan uses 1.8*120=216 watts. If this fan runs 16 hours per day in the summer, it will use 216*16=3.456KWH/day. This is 103.68KWH/month, which, at the same price of energy we have been using above, amounts to $7.43/month for the exhaust fan.

Let's say we use two 8" long life HAF fans. This may be overkill for a greenhouse of this size, but you will get the idea. Each fan draws 0.6 amps, and at 120V, this is 72 watts. Running 24 hours a day, each fan uses 1.728KWH/day, or 51.84KWH/month. Total use with the two fans is again 103.68KWH/month or $7.43/month.

Know the usage pattern and amperage draw of each electrical appliance in the greenhouse allows us to calculate the cost of running each item.

OD, I cleaned it up a little and also reinsterted the line breaks in the formulas. Thanks. SB

What is thermal mass?

Thermal mass is anything in the greenhouse that absorbs solar energy and heat from the sun during the day and releases that heat at night. Using thermal mass can moderate greenhouse temperatures and may reduce energy costs.

Many things can act as thermal mass in the greenhouse, including wood, soil in raised beds, concrete, block, or brick foundations, or even the floor of the greenhouse. But the most effective and least expensive thermal mass is water. Water can store far more heat for a given mass than any other commonly available material. Water is also useful because of its latent heat; it release up a tremendous amount of heat energy as it freezes, and often this is just when greenhouse heat is needed most.

A common practice is to store water in containers along the back wall of a greenhouse or in 55 gallon drums used to support benches. A commonly recommended minimum volume of stored water for solar greenhouses is 2-3 gallons per square foot of glazing.

What is IPM?

IPM stands for Integrated Pest Management. It is a approach to plant pests and diseases that combines a variety of cultural, physical, and biological practices while minimizing the use of pesticides. IPM emphasizes control of pests and diseases, rather than eradication.

Some elements of IPM used in greenhouses include the use of disease-resistant plant varietes, pest monitoring and scouting techniques such as yellow sticky cards, biological controls such as parasitic wasps and lacewings, strict sanitation practices, exclusion practices to limit entry of pests, insect growth regulators, and minimally toxic biologic pesticides such as horticultural oils and insecticidal soaps.

What are the different kinds of glazing materials, and what are their strengths and weaknesses?

The ideal glazing will transmit light well, yet not transmit thermal infrared, the wavelengths at which the solar energy absorbed in the greenhouse is reemitted. The ideal glazing will also have a high R-value, will be inexpensive, and will last a long time. The most common types of glazing are glass, polycarbonate, and polyethylene films.

Glass, the original glazing, is the longest lasting of the glazings (in the absence of large hail), particularly if the glass is tempered. It is expensive relative to other glazings. It transmits light very well, yet it is opaque to thermal IR. Glass does not diffuse light very well and may result in shadowing and plant burn. It is more difficult to work with and in fact tempered glass cannot be cut at all. Single pane glass has an R-value of about 0.8.

Polycarbonate (PC) generally comes in twinwall or even triplewall sheets in thicknesses from 4 to 16mm. It is easy to cut. Twinwall PC has an R-value of about 1.4, and triplewall PC has an R-value of 2.5. PC has a high degree of light transmittance and low thermal IR transmittance. Light is diffused with PC, lowering the risk of foliage burn. PC lasts 10-20 years.

Polyethylene (PE) films are relatively inexpensive and are the easiest with which to work. Single layer of PE film has an R-value of about 0.8 but and inflated, double layer PE glazing has an R-value of about 1.4. Light is transmitted and diffused well, but plain PE film is not opaque to thermal infrared wavelengths. Therefore radiation energy losses at night can be greater with PE films than with other glazings. PE with infrared additives are available, and these films reduce heat loss by about 20% compared to plain PE films. The lifespan of PE films is generally felt to be about 3-4 years.

Other less commonly used glazings include acrylic, fiberglass, and PVC.

OK, folks, on the humidity FAQ, do we want to get into a discussion of VPD or just keep it to RH? SB

No response, so here's my answer to the humidity question:

Proper greenhouse humidity is important both in preventing plant diseases and promoting healthy plant growth. High humidity can promote the Botrytis and other fungal diseases. High humidity also restricts plant transpiration, which in turn limits evaporative leaf cooling and can lead to overheating of plant foliage. If high humidity persists for a long time, the restriction of transpiration can limit the "transpiration stream" of nutrients and can lead to nutrient deficiencies.

Low humidity is best avoided because it may increase foliar transpiration to the extent that the root system cannot keep up.

Humidity is most accurately measured in terms of the "vapor pressure deficit" or VPD. This can be thought of as the drying power of the greenhouse air. However, most people measure humidity as relative humidity. Charts and programs are available to convert relative humidity to VPD.

Humidity is perhaps the most difficult of the greenhouse conditions to control. Most growers simply aim to avoid the extremes of humidity. Over most temperature ranges, a GH humidity of 50-85% is generally safe.

Low humidity can be managed with misters and foggers. It is also useful to shade plants under conditions of low humidity to reduce the rate of transpiration.

Transpiring plants add water vapor to the greenhouse air, increasing the humidity inside the greenhouse. Therefore, managing high humidity starts with ventilation...replacing warmer, humid greenhouse air with cooler, drier outside air. Ventilation also involves significant energy losses, and therefore ventilation often must be accompanied by heating. Therefore, lowering greenhouse humidity with a combination of ventilation and heating can cost a lot of energy.

Where can I get plans for building my own greenhouse?

Greenhouse plans are widely available online. These links contain some plans for greenhouses:

http://bioengr.ag.utk.edu/Extension/ExtPubs/PlanList97.htm#Greenhouse%20Plans

http://www.cerc.colostate.edu/Blueprints/Greenhouse.htm

How to build a Hoop House? Cold Frame?

Sqftgarden, I believe the above links also have plans for cold frames and other structures.

SB