Hello all I am new to this forum and am hoping the communities collective knowledge can lead me in the correct direction.
I have designed and hope to be building soon a passive solargreenhouse to be located in MA. I am hoping to gain some insight as to what type/R value of insulation I need for my walls/ceiling. My thought is to use closed cell spray foam which is supposed to provide R7 per inch. I have also been readying recently about Radiant Barrier insulation. Would a combination of the two types of insulation allow for less foam insulation? The greenhouse itself will be 12' x 24' and the lower 3 feet will be below grade. From 3 feet to 8 feet below grade will be my fish tank and earth tubes. My back wall will have a recessed area for (27) 55 gallon drums of water for thermal mass. For the glazing I am hoping to get triple wall polycarbonate. I do not know what kind of temperature I can maintain with out any added heat source but I am hoping to at least keep tomatoes alive. Any recommendations/advise would be greatly appreciated.
I don't know anything about radiant barrier insulation so my comments may not be totally useful. As I'm doing math on the heat loss for my future greenhouse, it's amazing how horrible the heat loss is through the glazing vs the insulated walls. Taking the walls up from R20 to R40 is a drop in the bucket when you see how much heat is pouring out through the glazing. In my case with R20 walls and R1.4 glazing I'm going to lose 3,000 BTU/hr through the walls and 24,000 BTU/hr through the glazing. Doubling the wall insulation to R40 would save me 1,500 BTU/hr. Doubling the glazing R value would save 12,000 BTU/hr in comparison.
I'd recommend reading Bioshelter Market Garden by Darrell Frey. He puts a lot of awesome calculations in the book that you can use to determine the heat properties of your design.
My struggle with insulation is the moisture in the air. I'm not sure if there are any good options other than styrofoam or spray foam. Fiberglass would likely get wet and stop working. I'm very open to suggestions in this area.
I have a couple other thoughts on your design (as I understand it). First is that the "below grade" section of soil will be pretty cold so I'd insulate that from the interior. Frost goes a few feet deep and having frozen soil against your foundation or wall will cool the interior. Secondly, if you're intending for the thermal mass to pick up solar gain, it has to see the sun. So tall plants may shade it. It would still warm up from the warm air of the greenhouse but that gain may be less than you are counting on. I'm tempted to put my thermal mass in the shade in my greenhouse but plumb the tanks/barrels all together and circulate the water to a big radiator in the ceiling where it's getting blasted by the sun and is at the hottest part of the greenhouse.
Sorry it's taken so long for you to get some input on your design. I read your post a month ago but was hoping others would chime in that I could learn from
Thanks Ed and Mike, I had despaired of getting any advise. I was hoping to build 2x6 walls to gain a R value of 35+ but the cost is stupidly expensive for a backyard greenhouse, so I am going to stick with 2x4 walls with an added gap of 1" for a radiant barrier placed on the inner wall. Plus since the radiant barrier is reflective it should bounce the sunlight around. I like your idea of circulating water through my thermal mass barrels to maximize heat gain, though I am not looking forward to figuring out the plumbing to circulate water through 27 barrels. I am hoping to create a thermal blanket that i can use to cover my glazing at night in the colder months and just roll up during the day. This should hopefully reduce the BTU loss through my glazing. I had planned on placing 2" foam insulation on the outsides of my below grade walls, would putting them on the inside be better?
I just called Owens Corning to ask if they have insulation options that will work for a damp greenhouse. Basically the only options they had were Styrofoam or spray foam. Might you be able to save money by piecing in Styrofoam for your wall insulation instead of spray foam? Make the pieces about 1/2" smaller than the stud cavities and use great stuff to fill up the perimeter. Just a thought. My greenhouse will have curvy walls (gothic arch) so Styrofoam would be a challenge to fit.
Do you already have the barrels? If not, IBC square cage totes may be easier to work with. They are about 300 gallons each so you could get by with 5 of them. And maybe keep fish in them And the plumbing would be easier. I see them on Craigslist around here for around $100.
What kind of blanket were you thinking of using? That sounds like the Chinese Greenhouse system. I was thinking of using those construction blankets that they use to keep cement warm when it's curing in winter. Then it would be insulating, waterproof and tough as nails. But probably expensive... And a pain in the butt if you get 8" of snow overnight on the blanket. Another thing people do is have a curtain on the inside that covers the glazing. The challenge with that is getting a good seal all the way around the curtain so it completely traps your warm air under the curtain. Another issue with them can be condensation or frost on the inside of the glazing.
I also have heard that pool covers are used by some folks for an insulating glazing. The one I was directed to supposedly gives you a R3 but I can't find any manufacturer contact info to verify (Diamond pool cover).
I think you'd want the Styrofoam on the outside of the foundation so the foundation can act as thermal mass for you.
It feels as though i have spent an eternity researching, it never even crossed my mind to call an actual insulation company. From what I understand Styrofoam will not give me a very good Rvalue for the thicknesses they offer so think I may be better off with 3.5 inches of spray foam which should hopefully gain me an Rvalue of 24 or so not including what I will gain from the radiant barrier which appears to have a double layer of bubble wrap in its makeup, though I am sure its Rvalue is minimal every little bit helps. As for the IBCs my fish tank is also going to be below grade, from 3 feet below grade, which is where my floor is going to be, to about 6-7 feet below grade. This is to save space and use ground surrounding the tank as a thermal mass to help regulate the water temp. I think the barrels stacked up in 3 rows of 9 along the north wall would save me grow space. I agree tall plants will likely block sun and thus reduce thermal gain to the barrels but i think it unlikely i will be able to maintain big plants through out the winter, likely just cold hardy greens which are typically rather short in my experience. For a blanket I am thinking of taping together more of the radiant blanket material and attaching it to the inside structure of my south wall. I am thinking of using a pulley system similar to that used in window blinds to raise it up and then find a way to lock it against the wall to minimize air leaks. Still working on that, but seeing as I am just now filling my hole in after laying in the first layer of earth tubes that is a looong way off.
Thanks, I was calling them to figure out if mineral wool insulation could be an option and it isn't. With Styrofoam you get R10 for every 2 inches. With polyiso insulation you can get R12 from 2 inches. So if you could swing 4 inches of poly iso you'd be up to R24 ($61 for a 4x8 area per Home Depot). It also has a reflective surface for what that's worth. Can you share a link for your radiant barrier material? I'm still not sure what that is...
What price are you getting for spray foam? Maybe it isn't as bad as I'm imagining.
How warm does your fish tank need to stay?
You're right that if you're doing greens and such, shade on the barrels won't be an issue. And then you wouldn't have to worry about plumbing them together and all that. I'm hoping to put perennial shrubs/trees in mine so I have to keep different things in mind.
Spray foam is pretty expensive per r-value. Rigid foam is much better--if you have the room for it. Neither should really be exposed to sunlight, and the rigid foam is easier to cover with something like drywall. Either can be painted to prevent UV damage.
For example, Tiger Foam (available to the do-it-yourself person) costs 13 dollars per cubic foot, not counting labor and wasted product.
2" pink foamboard (like Dow Fomular) works out to $4.55 per cubic foot, not counting labor and wasted product. Tiger foam does not give you triple the insulating power per dollar spent.
As noted, the glazing is the giant heat loser. Triple layer polycarbonate is not terrible....
A very useful google search:
movable thermal insulation for greenhouses
You can have your cake and eat it too.
I found Jerome's book to be very informative with regards to heating a greenhouse in a cold climate:
Troy thanks for the info, I purchased that book and have started to read it.
While researching foam board insulation I noticed a link installation instructions and also found in a question forum on a review for foam board where I learned that apparently the best place to put foam board is on the outside of our studs/joists as it keeps them from becoming thermal bridges. I am thinking if I put a 2" foam board on the outside of my walls with a 2"foam board with radiant barrier in between my studs/joists I should get close to a R25 on the walls and greater on the roof depending on how thick of foam I use between my joists.
I had researched movable thermal insulation, mostly by mistake a few years ago. Which is why I am planning on have a thermal blanket to attach to the underside of my glazing.
I plan on having Koi for my fish as my wife is a vegetarian and would never allow me to eat the fish I raise for my aquaponics. They can withstand water temps in the 50s and still be fairly active. Plus they live for a long time.
Why perennial shrubs, is this for hobby or income?
Mark Simmons wrote:Why perennial shrubs, is this for hobby or income?
I guess to be more specific, I'm thinking perennial shrubs or small trees that have food on them. If we produce enough it would become a source of income. I'm hoping for tropical stuff but I may be deluding myself. If that fails I'd go for Mediterranean crops. If that fails I'd go for stuff that grows in Georgia. If that fails I'd still have a season extension greenhouse.
Also when placing foam insulation on the outside of your sheathing you have to make sure you use a minimum amount based on your climate zone so the sheathing will remain warm enough from interior heat to prevent condensation issues. I had thought of placing foam board on the outside of my sheathing and another layer cut and block on the inside of my sheathing but that may cause an issue with drying condensation which likely would cause mold/rot. From what I understand, and I may not actually understand it properly, I think I am going to use an open cell spray foam between the studs/joists with foam board on the exterior of my sheathing. Open cell foam is much cheaper and apparently is not a vapor barrier which should allow for "drying to the interior". All in all its much more involved than I had originally thought just for insulation never mind every thing else.
Mike I like the idea of possibly growing tropical plants year round, I recently went to an event in Holyoke MA help by Jonathan Bates and Eric Toesmeier. Eric is the co author of Edible Forest Gardens and Perennial Vegetables. Jonathan and Eric also wrote Paradise Lot which is about their experiments in edible forest gardening on their 1/10th acre lot in Holyoke. Anyway they have an insulated hoop house in which they are growing, among many plants, a citrus fruit which I can only describe as cherry sized lemon. Jonathan states that his green house has never gone below 25 degrees with out any active heating.
1. Keep the vapor barrier warm enough and airtight enough that it never gets cold enough to hit the dew point (the dreaded condensation) AND the water vapor can't penetrate to the colder outside parts of the wall, or
2. Make all the materials highly resistant to water damage and allow enough air movement to keep the interior of the wall dry despite some water vapor intrusion and the subsequent condensation under some temperature/humidity conditions.
summary: make the interior vapor barrier very very good.
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