I appreciate the input, gives me more to consider!
im 100% sure im not as hardcore as a lot of permies. i like to value engineer my projects for total lifetime cost/maintenance/impact, theres not enough time or money for perfection.
Al, id be interested to know if you could offer some input as to metal structural member sizing. you say you use 2x2'' tubing on 10' spacing, what wall thickness? Do you use trusses between the 10' spaced columns or just more 2x2 tube as a top plate? do you use purlins for roofing to attach? any chance you have a picture or two of what you are describing.
Al, whats your average rainfall and humidity level? in Virginia where I live most of the summer is 80% plus humidity and would likely be higher within a greenhouse. I had considered building a red steel structure but had concerns of rust, especially underground in our clay which holds a LOT of moisture. that being said i have had several pieces of 1/8'' wall 2x2 sitting outside for about 4 years (use them as lightweight forks for my little tractor) and they are rusty as can be but still appear solid
polycarbonate/glass vs poly sheet - its possible that the UV blocking of glass and polycarbonate causes some loss in photosynthesis but it cant be that much, people have been growing under glass for over a hundred years and polycarbonate is one of the most used greenhouse materials. For me, the strength of the polycarb and glass to support a snow load, not have to be replaced for a decade plus, and much higher Rvalue outweigh any possible weakness for UV light transmission.
i went by a local dealer and my only real concern at this point is the stiffness of the structure. given, this example unit had no end walls, but you could shake the entire structure side to side. i would think end walls would rigid it up a good bit though.
fwiw carolina carports will sell the frame and purlins for 75% of the entire building cost (so $1125 on a normally $1500 carport) but then I need to come up with the roofing on the other half.
The Forest Garden Greenhouse
Book by Jerome Osentowski
The Year Round Solar Greenhouse
Book by Lindsay Schiller
both good reads about passive solar greenhouses, lots of overlap and each has a different name for the same earth to air heat exchange system. both have good concepts and examples of passive solar greenhouses but don't expect any construction plans out of either.
After speaking to some local metal building guys... It seems like one of the higher end carports might be a good base to start from. they can be braced to take up to 130MPH winds and 60lb snow loads. My understanding is the roofing provides very little structure so changing 1/2 out to polycarbonate shouldnt cause any issues.
just tossing the idea around, the galvanized frame and roof for a 12x24 is only $1500. seems like you could add extra hat purlins and wall bracing to really strengthen the structure for cheap.
The 2x2' roof was my only real concern. I believe it would likely hold up even under the max snow we get in northern Virginia but I think the amount of deflection would be undesirable. I'll stick to 2x4 tubing for the roof
Comparing a couple engineering charts it looks like 2x2 tube with 1/8" wall is comparable in compression to a 4x4 Douglas fir.
Ty, what kind of reaction were you referring to between aluminum and fasteners that you referred to as deadly.
Alternatively I could source out cedar or cypress but it's been really hard to find it in dimensional lumber sizing and I'm not paying the crazy prices for it by the board foot from local suppliers of cherry, oak, walnut boards.
After some consideration and a good bit of reading I think i have decided to go with a more conventional roof pitch and triwall polycarb roof. this way I can still use all my 1/2'' glass vertically.
I know you could stick build this fairly easily but I want to use galvanized steel for any exposed structural members. I have found 2''x2'' 1/8'' wall galv tube locally but am concerned about the functional combining of the two construction materials. I would think I could just bolt the 2x2 tube to the 2x8 ridgebeam using a bracket system but am unsure. I was thinking a 2'' tube every 24'' on the roof and maybe every 4' as support in the wall. I would think that a 2x2'' steel column would be as strong as a 4x4 wood especially if beefed up by horizontal bracing members.
if you look at any long term reviews of pool covers on amazon i think you would find they dont last long. it could be the constant daily install/removal causing strain or it could be material related. im sure it would vary by manufacturer as well. would be interesting to try out.
I am interested in both the "earth tubes" (long lines run underground not under structure) and the "climate battery" systems (tubes under structure in insulated envelope) I've spoken to a number of professionals who recommend one or the other. I would love to see direct data comparisons between the two systems in different climates. The earth tubes is a better system for a retrofit, climate battery essentially has to be done in the construction phase.
Personally, from the research i have done, to keep a greenhouse warm enough at night the key is keeping your daytime gain inside the structure. If you have sufficient sun for good daytime solar heat gain then developing an insulation system to keep that heat contained through the night will do more than either of the earth coupled systems. if you dont get sufficient solar heat gain during the day then theres not much you can do except supplemental heat. but I would think in that case you might not have enough light to grow anything during the winter anyhow...
the glass panels i got have no frames. you can see the edges, 3 layers - 1/4'' glass, laminate, 1/4'' glass. I asked a local glazier about how to install them. he gave me a couple good tips, call them "hurricane glass" apparently rated to take the impact of 2x4 at some obscene MPH
i may be leaning more towards the solawrap/ETFE now and using the glass for a sun room/solar heat gain room for a garage i will be building. solarwrap seems easier to arrange for installation but that could be because I cant seem to find any info on how to attach ETFE without weakening it
Lindsey, are you the author of that book you recommended? If so, have you evaluated Solawrap and ETFE materials?
ive been interested in the idea of gabions for creating microclimates around some of my plantings. i have found that used chain link fence off craigslist is the cheapest source of good strong galvanized wire base.
from a sustainability/recycling perspective the 8' sheets of tempered and safety glass i have now would be the ticket. im a bit unsure as to whether to use them or not since they weigh about 80lbs each. this requires a MUCH stronger structure than most greenhouses i have seen.
from an R value they are about R1.3...so essentially nothing. i believe solarwrap advertises 1.7...also not much and their minimum order is shocking. ($2200 for a 4' x 328' roll delivered) I can build a pretty strong support structure for that much. but i found a local steel supplier than can have any stock item galvanized dipped so steel rectangular tube is a good possibility.
i'm aiming for the long haul, i dont want to rebuild this thing again - hence the request for design inputs.
thanks for the replies and input. i have spoken to a couple builders who successfully use a similar greenhouse design in colder temps. i believe they rely heavily on the underground piping system for thermal mass and even that is depleted if there is a cold spell - especially one that is cloudy since the solar gain during that period is minimal.
i am ok using it for cold hardy greens in the winter. i would think I could continue to grow kale, salad greens, turnips, chard, beets, and the like. without supplemental lighting I dont know what else i could really grow anyhow. Only light we get is from 9am to 3pm in the dead of winter.
I would love to be able to keep it warm enough to grow bananas but im not interested in the time required for active heating systems like a RMH or the cost associated with electric or gas heat. Alas if that makes it a season extender vs a full four season greenhouse - thats fine. im not trying to do the impossible or subsidize my veggies growth with propane.
I have seen insulated shutter systems for greenhouses that should at least greatly minimize heat loss. i think i'll explore that further. while more difficult it would help "plug the giant S facing hole"
A roll out mylar system inside was in my mind to help with retaining some heat.
Mike - I went the opposite way and started with an A frame design like Virginia Techs passive solar house but changed to something with a vertical back wall as it would allow me to garden vertically in the winter and grow considerably more greens on a shelving type system than in the ground. plus i envision mounting a solar panel or four on top of the N side roof to power various things
Ty - i'm not sure what you meant about sealing all the walls and trapping vapor. it seems no matter how well i seal the inside if vapor gets in it will get out through the outside tyvek/siding side...
Hoped to use for year round growing of greens and getting a jump start on the season. Plus housing some of my more temp sensitive plans like figs. summer growing not as important since i have another section of ground high fenced for the regular season. I'm afraid it's going to be scorching inside even with the underground cooling system and vents.
I had thought about insulation on the exterior of the concrete block/foundation but was concerned about termite/carpenter ant infiltration. Ill need to research more
Yes, those lines inside are height of floor inside. I should label better
I have given this a lot of thought and think that modeling after a garage door or roll up door is the easiest most reliable idea. Gets harder the bigger the area is though. Small ones pretty simple to put up two rails and a roll of plastic. Could be regular poly or something type of aluminized poly like mylar to reflect heat back.
Other low tech option is just to cover the exterior with an insulating blanket. I think I saw a Chinese version that used straw rolls
Edited: you should probably account for condensation and put it on a slight incline with a reservoir to collect/route the run off
Looking for constructive input on preliminary design. Use is for home production for family/friends. I want to go passive solar with the S side being glazing and the three remaining sides superinsulated 2x8 structure. Interior walls/ceiling will be covered with thin gauge gloss white aluminum flashing material to allow max reflectivity.
Rough dimensions are 12' tall, 17' deep, and 24' long. Would have an "airlock" entryway of about 4-6' long that will also be glazed.
The three choices for glazing:
1.) commercial 1/2'' tempered/safety glass (I got about 16 panels that were 8' long and various widths off craigslist)
2.) Solarwrap "bubblewrap" poly keder film
3.) ETFE film.
I have the glass but to use it the support structure needs to be far sturdier than the two film options so any cost saving there might be moot by the end. And I can always use the glass for something else. Plus, if the support structure flexes too much the glass could always crack and film will not. But...should never have to replace it.
Support structure would be 4x4 1/4'' steel columns or 6x6's. Beam running the length is laminated 2x12's. Columns every 6'.
Support for the glass option would be 2x4 - 1/8' wall galvanized steel rectangular tubing.
Plan on using some sort of underground heat exchange, either the Earth Tube type (running approx 100' of multiple 4'' pipes down a hill and using a fan to bring in air through the tubes to 'heat" and "cool". Pipes would be approx 5-8' deep.
Or using some variant of the earth storage battery system where air is pumped under the greenhouse through tubes and pumped out at night.
I think as long as you slope your pipes or bore holes in there bottom then there should be no mold in a plastic line. I've been a plumber for over a decade and stick cameras down a lot of sewer lines. Never seen mold in a plastic sewer line. Pvc if sloped right looks almost new after 20 years of sewer sludge
Nice post. How do you deal with parasites? The only issue with pig tillers is the potential to drop parasites into your garden area. The worms/eggs can survive for years is my understanding. And several are transferable to humans. I guess it's not a problem if you cook all vegetables but stuff like greens?