greenhouse design input/critque

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.

anyhow. input?

Are you hoping for year round growing or season extension?  Where are you located (roughly)?  Are the greenish lines grow beds?  What's the red line inside the greenhouse on the first drawing?

Here's my initial input:
1.  Depending on how cold it is where you are at and your winter growing goals, I'd likely aim for the best R value.  I'm guessing the solarwrap would be the best bet, or maybe a double poly layer with a blower to give an air gap.
2.  Your roof angle/glazing allows for full sun penetration during the winter solstice but there will be shade on a third of the floor in the summer.  Maybe there's enough diffuse light for that to be ok?  I don't actually know so if anyone else can chime in on that it would be great.
3.  If you get snow where you live, the second layout may be too flat to shed snow off of the uppermost part of the glazing.  On the other hand, snow will probably sit on the insulated roof and add to your insulation there.
4.  I'm working up my own greenhouse design and on the latest iteration (#16) I changed from a design like yours with 4 walls and two roof surfaces to an A frame design.  My thinking is that it will not require beefy walls and a center post/beam system.  I'll be able to get away with a footer around the perimeter and then the A frames will support themselves.  Downside is that snow will pile up and block some light on the bottom of the south face.  And I'll have a really tall building (14' ceiling).  In my case it works out because I'm hoping to grow bananas...
5.  Don't forget about insulating the foundation.

Zone 7, bordering 6.

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'd be interested in seeing your designs

I think if you design for summer venting you should be ok.  I think there's a number out there for how many square feet of vent space you need for a given floor plan.  

You're a bit farther south than I was initially thinking so foundation insulation may be less critical.  A foundation with frost or frozen ground on the outside of it will sap heat from the greenhouse.  I'm not sure what they do for ant prevention but I'm sure there's a way to resolve that.  Somewhere in the US there are termites/ants and insulated foundations.  Just need to figure out what they use there.

I just got a passive solar greenhouse book for Christmas.  Once I read that and update my design a few more times, I'll sketch it up and put it on Permies for input.

Francis Gendron has a very good e-book, "The Greenhouse of the Future" which was once a promotion item at Permies.  

I am sure there are many who will agree/disagree with my following comments, but they are based on years of practice and actual installations.  Take what you like and leave the rest.

I just completed a greenhouse for a client in our balmy zone 7a (Idaho Treasure Valley) that is 15 feet by 40 feet, solar shed style to existing enclosed space with a 3/4" triple wall polycarbonate on three exposed sides, long side facing due south.  Originally is was planned to extend the growing season.  Recently the client has decided to use year-round, so we have had to deal with heating.

The original design was calculated to provide about 20 Deg.F difference between inside and outside (a canvas or nylon tent is worth about 10 Deg. F).  That meant when the temp outside got down below about 15 Deg. F, the inside temperature would start to reach 32 Deg. F.  By about 2:00 AM on the first winter storm night the thermal mass of deep compacted gravel floor, earth berm, and reflective north wall had ceased to radiate any heat and the plants started to show frost-damage.  The lemon trees took it hard.  Fortunately, we had wired the greenhouse to the local residence and a couple of cheap electric space heaters saved the day.  Since then, they have changed over to propane catalytic space heaters.  They are not happy because they have to check the temperature, nightly in the cold part of the winter.  We did alarm the house with an interior set temperature of 35 Deg. F, but it has been brutally cold for the past three weeks, causing them to go tend the greenhouse at least twice a day.

The heat loss through the poly cannot be accounted for in insulation or thermal mass alone. Some sort of active system will be required. After, all the south facing 'glass' is a really big 'hole' with little possible insulation value, even if coated, low-e, gas-filled triple pane glass was used: that's why it works so well for solar gain.

The rear wall should be moisture-tight and have at least an R-19 (2x6 studs with batt) exterior wall system with an interior surface that is white or foil faced.  Due to the high amount of moisture in a greenhouse environment, I don't think super-tight sealing of the other walls is a good idea: the moisture could actually freeze in the insulation causing all sorts of problems.

Francis encourages an interior reflective tarp be utilized to reflect the trapped heat back into the greenhouse at night.  This is why curtains were invented for windows too.  It is brilliant, and helps with too much heat gain in the summer.  Reflective mylar is super, as it can be made in a directional reflectivity, but you really need two rollers: one for summer, one for winter.  The other issue is these require active participation by the greenhouse steward to operate them in the morning and at night.  There are automated systems that are used in commercial applications but they are not withing the realm of the average greenhouse client.

The next most successful way to provide year-round growing is with a heater.  My personal choice is the rocket mass heater (RMH) that's what brought me to permies.  The other options that Francis discusses with active thermal mass storage from different sources such as warm earth or heated water in addition to the obvious solar heat-gain are also very viable.  I still advocate the RMH as it can save you off-grid when the temperature plummets during a freaky event and you simply do not have enough stored heat.  Seems like there is always wood to heat with.  Electric or propane space heaters can also work, but they are very expensive to operate.

I am less sure about the need to insulate below grade, as that seems to be counter-productive to berming or partial sub-grade designs which utilize radiant mass heat.  Certainly, insulating to frost depth is important to avoid slab and foundation heave, but begins to reduce the ability for thermal transmission through mass (It's insulating like a coat).  The whole notion with thermal mass is to get the mass warm and keep it warm in the winter and then reverse the process in the summer with cool night-flush.  Usually for earth mass to get to a stable 55 degrees, it has to be deeper than frost depth.  Here that is 30 inches.  In Maine it is 96 inches!  Plus, it takes time to heat a big mass up, or cool it down, usually longer than a plant can handle the cold.

I like the RMH or solar hot water mass in tanks like a fish farm.  Both are active systems that require a fair amount of attention, but then so do the plants and greenhouse occupants!

Last I will say:  My most recent polycarbonate order delivered to the site was US $0.23/SF (screws, trim, gaskets, shipping, taxes).

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...

What kind of polycarb did you get for $.23/sqft??

Good sound understanding.

I think managing expectations is part of the experiment.

All I was trying to say with the whole water vapor transmission is: that some people expect a plastic vapor barrier which stops both air and water vapor.  A vapor retarder will allow the healthy migration of moisture usually from inside to outside, but not the other way, especially a greenhouse.  By using a retarder (not sealed vis-queen, but tyveck or the sort) the wood structure and batt insulation are less prone to rot and mold.  This is counter-intuitive to the European Passive House Standards, but should be acceptable since you don't 'live' in the greenhouse.

The polycarb was from:

https://www.sundancesupply.com/index2.html

I used the 3/4" x 4' by length  three layer sheets.  Imagine two rectangular super-heavy straws fused next to two more, etc. Super-light weight, super strong (Class 4 Hurricane - Dade County Rated).  These span a 4 foot width at about 20 psf snow load.  They are directional.  They are about 80% translucent (but they are not like clear glass.  They do make some noise.  They do flutter in strong wind, unless you go with purlins at 4 foot centers too.  They telegraph rain noise very well.  The material is guaranteed for 10 years, pretty good for plastic.

I do not know the whole carbon footprint for these panels.  If you were really concerned, then you must recycle old windows, which usually means more structure, becoming it's own nemesis.  That is true of all parts concrete too.  That's why I liked Francis model that uses dirt and recycled tires, but not everyone is into that, especially strict building departments.  Although the non-occupied (you don't live in it) agricultural exemption is a pretty effective argument outside City jurisdictions.

Just like Paul & Jocelyn observed recently: part of permaculture bricks includes buying locally.  Sundance was a close a source as I could find without a healthy middleman mark-up.  The owner is quite helpful, suggesting, from experience, all the bits you need to get the job done right.  I highly recommend a conversation with him, before you finalize your order.  Some of the fasteners are pretty specialized and not at every local lumber yard.

May you have great success and abundant harvest.

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 guys, keep them coming

Sorry, forgot about the glass you had.

You are going to need a stout structure for that, but it will be worth it in the quality of light and space.  I have seen a number of greenhouses make it to the 50 year boint with galvanized steel frames, but even rust gets the better of them.  For commercial work, I have used andodized aluminum frames, sometimes with steel stiffeners.  Not inexpensive, but I think aluminum will go the distance better than anything else, although the wood frames seems to hold their own if not overly coated with paints and sealers and on the beefy side, preferably ceder or cypress.  Be watchful with aluminum on corrosion with fasteners (deadly) and contact with concrete (slow erosion through alkali reaction)

I recently tried exterior LVLs, so far, so good, but I did use a transparent water based sealed to further slow down moisture penetration.

In general for glass, error towards vertical with slopes.  Unless it is 5/16" or thicker, figure loads will require a maximum 32" spacing for support, both ways.  Gravity loading means the primary frame will likely need to be 4 x ^" or better if you get strong winds, heavy rain or snow.  Really, 1/4" glass won't cut it for a snow load or a point load of more than 100 pounds.

Any local commercial glass guy will have the aluminum extrusions.  If you are familiar with good quality wood work, you can do aluminum yourself.  Usually to get a trade on a rural site is out of feasibility unless they really need the work, and none of them do right now.

Again, I am all about managing expectations.  The most recent greenhouse cost my client about $35/SF with water and power, no heat, no concrete.  A 8 ft x 12 foot Harbor Freight kit with a floor, is nearly $50/SF and you still have to build it.  For a Solarium/Conservatory, year round, turnkey, I think you are looking at at least $100/SF  if it is Victorian Style, about $200/SF.

Are the glass panels still in the aluminum or vinyl frames?  If so that makes them easier to fasten to the steel framing.  They can be bolted with a rubber gasket on top that allows for the expansion/contraction. If they are sliders then one is wider where the latch goes. I considered trimming them to be uniform but used 8 of them vertical on the south side with plastic for the shed roof with a gutter to collect the rainwater for irrigation.  My north side is 4' higher than the ground in front so the side and corner posts are stepped up the slope to make the pitch of the plastic roof. My posts beams and rafters are round poles from my land and my north an east walls are a sandwich of old wall to wall carpet with the padding in between which allows me to roll them up for summer ventilation.
I picked the last of the tomatoes off the dead vines today. The tomatoes and peppers stay in the greenhouse all their life. one pepper may make it through the winter with a ruby red heat lamp on it. Harvested some bok choi and lettuce and the winter potatoes are growing along the north wall. The rest of the plants are in wicking barrels that I can move around for the season.

Another book to add to the shelf - The Year Round Solar Greenhouse It has a full chapter on climate battery / thermal banking systems.
I would side with the ETFE over the solar wrap for durability reasons, if both are similar R-values. Though I agree with previous comments, R-value and light transmission should be the most important factors when making a final decision.

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?

One glazing material I haven't heard much talk about but it's very interesting is solar pool covers.  Like this one.  On a greenhouse forum one lady from Texas says hers has lasted 10 years.  I'm not sure which brand of cover she used and it's hard to track down the original manufacturer of these.  I was able to get through to a guy at one company and he said they were about an R2.  So it's similar to Solawrap but much cheaper.

Unless I hear something bad about it, I'm planning on using a pool cover on my greenhouse.  They're designed to sit in the sun all day and hold in heat.  They're 16 mil so they are very tough.  Apparently people put them bubble side in so the snow can slide off.  I'm not sure if condensation would drip off the bubbles or not.  I'd also be tempted to put another layer of IR greenhouse plastic inside the pool cover and inflate them.  I'm guessing the R value would get closer to 3 but I'm not sure if there'd be enough light transmission in winter.

Mike,

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.

Good to know.  I hadn't looked at the customer reviews since the application was different.  I'm pretty sure I'll be giving it a try this summer so we'll see.

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.

input?

thanks!

I make this suggestion, based on typical Zone 6 loading.

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.

I believe the way it works is Stainless Steel to Aluminum is OK but Auminum to Stainless Steel is a problem.  In other words: use no aluminum fasteners.

Oh. So you mean deadly as in Fasteners break in the roof collapses?