It leaves you with UV resistant GH roll plastic which can be bought many places.

Since you've made the decision not to go with twinwall polycarbonate, I recommend the product linked below for your outer shell. It's UV plastic with a weave of fibreglas "tapes" inside, so it's translucent and diffuses light well. It's like iron, and will add structural support to your frame once it's stapled in place. I wish I'd discovered this product years ago, instead of replacing 6-mil over and over.

If you use wire fencing material nailed to your frame (the kind with 2"x4" mesh) to support the plastic, you can get away with four-foot spacing between studs and purlins.

You may then add an inside layer of 6-mil UV plastic on the inside of your studs and purlins. This for an air space to add to your R-value. Depending upon what you decide to grow, you may find that the diffuse (but bright) light won't require shade cloth.

If you fully-insulate the back wall and your west wall, with this double layer of plastic, two 1500-watt electric heaters will give you the potential of +65F. BTW, I realize that passive solar is an attractive concept, but the space is just too small and inefficient inside a hobby gh for it to work. You'll read about folks using barrels of water, etc., and swearing by the results, but if you do the math, and allow for charging/discharging cycles, the numbers just don't add up. Active solar is a whole different ball of wax, but requires advance planning before the structure goes up, additional material costs, etc. If it's not in your budget, then just go with the supplemental heating. Each electric heater will require between $2.50 and $3.00 per night to operate.

I would strongly consider an alternative to your thoughts about detaching the sides in the summer. You'll have a much more versatile gh if you rig up an inexpensive ventilation system, and won't have to worry so much about pests, wind, etc., getting inside. Plus, the vacuum created by the exhaust fan will "suck in" the plastic snugly against the supports, and prevent snapping...at least during the day.

You'll have more questions as you move along. This is a great Forum with a number of knowledgable folks.

-Bruce

Here is a link that might be useful: 5.2 ounce fabric at FarmTek

Bruce,

The price for the film is nice (I actually found it cheaper, by 60 per linear foot) but the $102 shipping charge is insane, especially since the charge for 9' widths is just over $9.

My intention is to grow tomato plants for the most part, with maybe a tub or two of lettuce and broccoli. I know the lettuce does not need a lot of light but I wonder if the film lets enough light through. Do you have a lux meter you can check the readings with?

I'm not committed to removing walls - I like the idea of being able to just open a window and stick a fan in it!

Have to disagree with you about the need for 3000 watts of heaters. I know the 3.41 or (wattever!) BTUs per watt output. I also know I have a room that is 16 x 12', with drafty windows and walls that are insulated but not real well. My 400 watt EconoHeat unit will keep the temps at least 30 degrees above the outside temp. The Siletz is a cold-weather tom and will set fruit in the 50-55F range, though it probably does better in warmer environments.

I realize the passive solar heating will not provide a large amount of heat - based on what a rocket scientist (seriously - my former neighbor was a scientist who once worked for NASA on their rockets) worked out for me, about 3 degrees per day.

No doubt I will have tons more questions. A lot will depend on how my experiment growing a tomato plant in a hydro chamber in the office window turns out. If it produces ~24 pounds of fruit, then I will look at growing enough toms to supply a couple of restaurants and carry-outs with fresh fruit in the winter. If it doesn't, then I will just use it to extend my growing season as well as a harbor for seedlings next year.

Mike

Hi again --

I'm not sure I follow you on the costs of the fabric. I just ordered 50 feet of the 10-foot stock (I'm adding on to my air chiller this season), and the shipping was nominal. I don't know where FarmTek gets the stuff, but their prices are about as good as you can get.

I grow orchids, and I could burn sensitive orchids through a single layer of the 5.2-ounce fabric. Tomatoes, of course, take full sun, but I believe they would still perform well under diffuse light. Unfortunately, I don't have a reliable lux meter.

I've worked out the dynamics of passive solar for folks before on other threads, but I can't find one right now to link to. I'll try to summarize:

One BTU is the amount of energy required to raise one pound of water, one degree F. Reversely, one pound of water one degree F warmer than ambient temps has the potential of one BTU.

A gallon weighs 8.34 pounds, so a 55-gallon drum has the potential for 459 BTUs for each degree F it is above ambient.

Sunlight simply striking and heating the skin on one side of the barrel would be terribly inefficient in heating the water contained inside. Realistically, it only has the possibility of occurring between, say, 9am and 3pm or so, yet needs to discharge that heat throughout the night and into the next morning.

Let's say in six hours' time you are fortunate enough to raise the temperature of the barrel 10F, such that the interior temp of the gh is 70F, and the barrel is 80F. Now, since you want to maintain a temperature of 50F at night, that means you have a potential differential of 30F, or 30 x 459BTUs per degree = 13,770 total BTUs per barrel.

Unfortunately, you can't tell the barrel to release those BTUs during the coldest hours of the night. It will begin discharging those BTUs the minute the sun stops heating it. So, from 3pm until 8am the next morning, it will discharge at whatever rate the barrel will cool. If it was linear, it would discharge at 13,770/17hours = 810 BTUs per hour*. This is equivalent to 237 watts, and with the design proposed, surprisingly enough, is worth about 3-4F like your friend suggested. But read on...

The real problem occurs the next day, and each day thereafter. There's simply no way that same barrel can recharge the 30F loss in six hours' time each day. It would most likely find an equilibrium temperature between the 50-55F floor cooling the bottom of the barrel, and the ambient temperature the gh reaches during the day. The real possibility of consecutive cold, cloudy days, would make your barrel worthless at the very times you need it most.

When space is at such a premium inside a hobby gh, it just doesn't make sense to try to utilize barrels of water (or the storage medium of your choice). Larger, commercial gh's can gain some benefit of passive solar by the sun heating the expanse of floor, but this doesn't work on a small scale.

Play around with the BTU calculator linked below. You'll first need to calculate the exterior square footage of your structure, but don't forget to subtract out the two walls you will fully insulate. For the design you proposed (plus my supposition of the additional second layer of film) I used the same heat loss value as the one for 6mm twinwall, as the combination of 12-mil on the outer shell and 6-mil on the inner would give you an equivalent insulating factor.

I have a structure similar in size to the one you propose, covered in the fabric I suggested, although not double-walled all the way around. It has approximately 470 cubic feet of exterior, and about 100 square feet of it is double-walled. I'm in zone 8a, and when I run both electric heaters (one on "high," the other on "low"), those 2500 watts (or 8500 BTUs) give me about +18F. A gh is a royal pain to keep warm in the winter...nothing like a poorly-insulated room!

All things considered, +3-4F is not going to make much difference, when your needs are more on the order of +40F (or more) on the coldest nights.

Hope this helps.

-Bruce

* What is more likely is that the barrel would cool quite quickly over the first eight hours or so, with very little potential left after that.

Bruce,

I got the estimated shipping price for 20 feet of the 120" wide material and it was $102 plus change shipping it to 45216. I wonder if perhaps their calculator is broke? Did you estimate the costs or have them applied when you checked out?

Mike

There is something wrong with their online shipping calculator. I got out my invoice for the order I mentioned above, and the shipping was $12.09. Call the 800-number to get an accurate quote, and you might mention to them the problem on the website -- that's unfortunate, and I can imagine is costing them some business.

I always call my orders in, because I usually have questions too.

In my above post, I meant to say that my structure is 470 SQUARE feet, not cubic. My cubic feet is closer to 1000.

-Bruce

Bruce,

Yep, their calculator thinks it has to go on a pallet via a truck rather than UPS. My shippings costs would be about $10; 20 feet of 12' wide would be less than $70 which is fantastic.

Your GH must be rather short, I figure in a 16x12 I will have nearly 1250 CF of room. I guess a good point is that the tall wall will be the one that is heavily insulated. I will have just 158 sq. ft. of uninsulated walls and 176 sq. ft. of ceiling area.

Mike

The lean-to design is actually the most efficient with respect to insulating possibilities and light-to-interior-volume ratios...especially for more northern zones. BTW, depending upon how you plan to use it, it will be more challenging to keep it cool in the summer than warm in the winter.

My gh is 8x14, although it's irregular, and most accurately described as two lean-tos back to back the way the roof pitches. (If you use the roof ridge dimension, it's a "shortie" standard greenhouse with a length of 8 feet...the 14 feet is side-to-side.) I did it as cheaply as possible, and also had to get it up quick -- so no glamour competitions won here! Main support is an eight-foot beam between two trees, so I didn't have to worry about a foundation or burying corner posts.

-Bruce