Advice sought re. Cold greenhouse for alpine plants

I'm building a greenhouse for overwintering alpine plants. I want to keep the winter temperature just a few degrees above or below freezing, similar to what the plants would experience under deep snow cover in the wild. The greenhouse measures about 12ft by 8 ft. Outside temperatures will vary from minus 30 Centigrade (-20F?) to plus 10 Centigrade (50F). Can anyone advise me about insulating, heating, and ventilating please?

Hi June,

Congratulations on your GH. I have a few thoughts, but first could I ask what material is the GH made from? How far along are you with the construction?

Insulating the foundation would be helpful. This involves a layer of rigid styrofoam or urethane board in the ground around the perimeter. If using a concrete foundation, it goes around the outside of the foundation wall. If the GH is just sitting on the ground, then the insulating material is sunk into the ground.

The priniciple factor in heating is knowing what you will grow (therefore, what temp you will need in the GH), what minimum temps you have, and what the rate of heat loss is from your house. Then you can determine from a formula or a calculator how much heating capacity you need. So we need to know GH details to go any further with that.

As far as the type of heater, pretty much anything goes. I don't think you want electric in this situation, but kerosene, nat gas, propane, even wood have been used. Greenhouse owners debate ad nauseum about vented or nonvented heaters. If you use non-vented heater, you will need little cracks or vents, even in winter, for O2 to the heater.

Passive ventilation may be adequate using a ridge vent +/- intake shutters. To be safe you may wish to add an exhaust fan. Standard way of sizing an exhaust fan would be to make the cfm of the fan = volume of air in the house, but you might be able to shade that back if not using the house in the warmer months.

Consider insulatiing the entire north wall with a polyisocyanurate, foil faced foam board, with the foil side in. Maybe even do the north end of the roof this way, too, if you have a rigid type of GH.

A solar pool cover has been used and is good for adding perhaps 0.6-1 R value to a hoop style house. I have not been impressed with the performance on a rigid, square-end GH.

Just a start. Hope this is helpful. Get back to us with details of your house!

SB

lol SB...are you sure you aren't an undercover greenhouse construction/engineering professor???? I think this is your true calling!

June, you are lucky that SB has responded to your questions, just give him the info he wants...he craves it!! lol

Dear Stressbaby,
Thanks for your detailed response. I'll do my best to describe the greenhouse project in technical terms:
It's a stand-alone aluminum and glass "kit" imported from Britain. The aluminum frame is secured to a base of 4"x4" timbers, which rest on buried builders tubes filled with concrete. This, we hope, will eliminate frost-heave. For the flooring, we have installed 1" of gravel, then a layer of rigid styrofoam, then another layer of gravel which is probably going to end up about 2" deep and will have concrete pavers in places to support bench legs and foot traffic. We will caulk the glass panes and gaps in the frame, so I guess we'll have to install a heater-vent.

An electric line has been installed, so I'm thinking of using an electric heater, with propane as a backup in case of power-outage. I only need to heat the green house to 5C, and the plants will withstand periods at minus 5C, but outside temps can go as low as minus 30C. However, I don't yet know what type of heater to get, or where to get it from. (I'm finding that many US suppliers don't ship to Canada.)

The greenhouse has four ridge vents. I purchased two automatic vent-openers from the kit supplier, but I find they are designed to operate in summer, not winter, and the lowest setting on them is 15C (at which temperature my alpines will "cook"). I need an opener that will open the vents at 5C and will not be damaged by freezing. Does one exist?

I've been told that sticking bubble-wrap to the inside of the glass will help with heat-loss, but I'm not sure what to use as adhesive. The greenhouse is oriented north-south, with a wind-break of pine trees to the north. I like the idea of insulating the north wall. Luckily, the door is at the south end!

Is there anything else you'd like to know?

Single pane glass?

Glass to floor or knee wall?

How many amps of electrical service?

Any vents besides the ridge vent?

Maker and model of your kit?

Hi June, my son's girlfriend is from Toronto, are you close to there? She really misses it!

Stressbaby:
Single pane tempered glass, to the floor.
30 amps
No vents apart from roof
I don't see any maker's name on the kit, but the model is called "Viscount". It was imported into Canada by Europa Greenhouses Ltd and marketed under its name.

Tigerlily123:
We are out in the countryside, about 85km north of Pearson International Airport. We have 69 acres of deerfly and mosquito infested woodland, a stream, and a couple of fishponds. Lovely if you like nature: lots of deer, rabbits, raccoons, squirrels, porcupines, mink, muskrat, beaver, foxes, coyotes...and too many kinds of birds to mention. Wildflowers too. Pity about the insects, though.

June, here is your adhesive: http://www.charleysgreenhouse.com/index.cfm?page=_productdetails&productid=2567&cid1=-99&cid2=-99&cid3=-99

More re: heat later.

SB

June, the main problem I see so far is electric heat and the electric service of 30 amps.

BTU/3.413=Watts and W=amps*volts

Without any modifications, like bubble wrap, you'll need about 20,200 BTU for heat (assuming inside temp of 30F and min outside temp of -22F (-30C) and GH surface area of 344sq ft). This translates into about 5900W. That's four of those little 1500W heaters that we can get at Walmart or HD here. Each of those draws about 12.5 amps. Using 110/120V, you would need 50amp service to the greenhouse, just for the heaters.

There are electric heaters of about this size out there, but they all require 220/240V and they draw 23-24 amps. (240V*24amps=5760W; still with me?)

It is hard to know the additional R-value of adding bubble wrap, but lets assume that it is close to 4mm twinwall polycarb or inflated double layer poly; then it probably gets your heat requirement down to around 12K BTU, around 3500W. Adding urethane board to the north wall might get you down to the point where you can run on two 1500W electric heaters, but this will draw 25amps and lead you with little to no additional electrical capacity.

Keep in mind that as a general rule you don't want to draw more than about 80% of the amp capacity; that's 24 amps on your 30 amp service.

Any thoughts on other sources of heat? Maybe go with the propane, with electric backup?

I like what you have done with the floor, that will really help. Make sure you have some holes or gaps in the styrofoam, you'll need the drainage.

SB

go propane, June, its much cheaper in the long run, and I don't think a small, vented heater will cost that much, and if you can buy the heater from the company that will sell you the propane they will always come out to repair it right away, and they set up the copper pipe for the gas etc for free ( at least here they do), and they can hang the heater for you as well.

Your place sounds great, there's nothing better than being outside in the country!! Where I lived in Hawaii, there were an excessive amt of mosquitos too. We wore lightweight long sleeve shirts and pants-even during the day and always had a mosq coil going by the door so they didn't get inside. I lived way up a valley though and I think that was partly why they were so bad, it wasn't that bad in town.

Thanks, SB. It occurs to me that maybe I wouldn't need to heat the entire greenhouse, if I confine the plants to only one half of it in winter. Could I hang some kind of thermal barrier - heavy plastic sheeting, or bubble wrap, perhaps - to cordon off half the house, and leave the other half unheated? In any event, I'll give serious thought to using propane rather than electricity for heating.

I checked out the garden supplies vendor website you recommended and I have ordered their catalog (out here in the "boonies", we have a dial-up Internet connection that is infuriatingly slow when purchasing on-line. It's much better for my blood pressure to order by mail).

June

Tigerlily123, your last message was posted while I was replying to SB. I see you agree on the heating issue. To talk about heating on such a hot day is making me sweat just sitting here. Hope you are keeping cool.

June

June, yes you can cut off half your grhouse with heavy plastic to cut down on the heat, I do it on a 100' grhouse for a few weeks until I need the space. I guess since you are not having any fans in there you can leave the plastic up during the day as well as at night.

June, if you cut the GH in half and used bubble wrap, you'd be down to a heat requirement of about 8150 BTU, easily within the reach of a couple of the electric milkhouse heaters.

Now we need to find you a vent opener...

I see what you mean about the roof vents. Everything is set to open at 59-65*F.

Here is a thought: set up a standard exhaust fan but at a reduced size. Here is a link with the idea: http://www.umass.edu/umext/floriculture/fact_sheets/greenhouse_management/jb_ventilation.htm

Generally, for summer ventilation, you want your exhaust fan sized to equal in cfm the volume of the GH. This way the air is turned over every minute. This link suggests that for winter ventilation you could get a fan sized at 1/4 of volume of the GH. The reduced size is necessary in the winter to avoid shocking the plants with a blast of cold air.

I'm guessing your GH volume at about 700 cu ft. If you cut the GH in half in the winter and take 25% of that, you are talking about a fan the size of a bathroom fan...better yet, a 4" in-line duct fan: http://www.smarthome.com/3016.html either one controlled using a thermostat: http://www.littlegreenhouse.com/accessory/controls.shtml

You would have to check on how moisture or water resistant these fans are, since they are not designed specifically for GH use. But, if moisture resistant, it would be simple to install by leaving out a pane of the glass, using a small piece of 4mm twinwall PC instead, cuttiing a hole in the PC, and installing the fan in the hole, covered with a small louvered shutter like you would have on the end of you dryer vent.

I know of a guy who did this with a HFGH, I'll see if I can find a pic.

SB

SB, you are brilliant! I was so focussed on vent openers, I never gave a thought to exhaust fans. Now that I think of it, I once attended a talk given by a professional alpines grower, and I recall he said that stagnant air in the greenhouse is death by mold to alpines. A fan would be just terrific.

June

Conifers, or Dax, did something like this for his seedlings, trying to keep the temps under 50*F as I recall. I Dmailed him and asked if he would come over and comment about how his system worked. Maybe he could post some pics, I couldn't find any good ones. I think he ended up with a 6" in-line duct fan...maybe two.

This kind of setup is undersized for a greenhouse used in warmer months, and that is why you cannot find a standard GH exhaust fan in this cfm range.

Have an idea about bubble wrap.
Make a tent like structure of the bubble wrap, use ductape to attache the parts to one another. Put hooks (glue)along the sealing and where the seiling meets the sidewall. Glue/tape/whatever rings or hooks at the same places on the bubblewrap tent.
If you need openings for vent and such, use ductape around the opening as big as the opening have to be and where it have to be, cut out the inner part and tape it in place. The first tape around the opening is so that when you take it down the bubble wrap dont get destroyed.
This way you can just unhook the bubble wrap in the spring and just put it back up in the winter and you dont have to clean after the gluespray.
Janett

Hi June,

I have a 8 x 6 "Harbor Freight Greenhouse" and I've brought in (3) 120volt receptors to power my heaters and fans and in my case, my heating pads.

Each heater has it's own power source and everything else is ran on the third outlet. Charleys sells the exact heater I have but Charley's is 100 bucks and this link, they're 60 bucks: http://www.heatershop.com/indoor_space_heater_dfh_480m.html

For fans, I have two clip-on fans - this is what they look like:
http://smg.photobucket.com/albums/v280/Cultivar/Harbor%20Freight%20Greenhouse%20Assembly/?action=view¤t=SuperSafeCompactClampFanbyCaframo.jpg&refPage=&imgAnch=imgAnch12
I found them on the internet for cheap - once again I think 'Charley's Greenhouse sells them but I found them online for much cheaper. I don't have a link for you for these, but fans are fans and you can use what works best for you:)

"My" thermostat/ductstat (made by 'Suncourt')- which can be adjusted to turn the exhaust fans and any other fans at a given temperature - purchased at Menards - or you can find it online or Home Depot - http://smg.photobucket.com/albums/v280/Cultivar/Harbor%20Freight%20Greenhouse%20Assembly/?action=view¤t=CloseUpofPlug-InThermostatDuctstatb.jpg&refPage=&imgAnch=imgAnch5

The exhaust and intake fans are the same and are either placed to pull air in or exhaust air out. I used wood on both sides of my greenhouse all gobbed up with silicone caulk - waited a few days for it to dry and used a tool/bit called a 6" hole saw to cut the holes to hold the 'inline fans' which also are made by 'Suncourt' and can only be purchased at Home Depot or online I'm sure as well. The fan is called a 6" inline duct fan I believe -
Her's a photo of the outside "hood" (dryer hood) showing how the wood was painted, glued and finally caulked around the edges of both the wood pieces and caulked around the edges of the hood on the outside as well as caulked around the edges of the wood inside and fan (again) - "lots of caulk in other words".
(See link below for wood pieces and caulking)

These inline exhaust/intake (reversible) 'Suncourt' fans:
http://smg.photobucket.com/albums/v280/Cultivar/Harbor%20Freight%20Add%20Ons/?action=view¤t=6inchintake.jpg&refPage=&imgAnch=imgAnch1

Also these three things are a necessity as you wouldn't believe how HOT a greenhouse can get during winter on sunny days. For example, even if it's 20F outside or 0 F, "My greenhouse" gets too hot in it (even with the fans) - SO - I built a screen door that can be clamped into place and heres a photo of this:


A 'Cool Puff Mister' which can be purchased at Walgreen Drug Stores or other Drug Stores:
http://smg.photobucket.com/albums/v280/Cultivar/Harbor%20Freight%20Add%20Ons/?action=view¤t=UltrasonicCoolMister.jpg&refPage=&imgAnch=imgAnch2

And you definitely need shade cloth. Upon recommendation I was told to get a 50% shade cloth and for my dinky greenhouse, I bought a 12' x 20 foot which covers all but the north side of the greenhouse - so a larger one would have been preferred. (I can't tell you how many times the shade cloth saved me)
I bought from Gemplers - just one of my favorite stores - here's a link:
http://www.gemplers.com/keywords/shade-cloth.asp

Now all this aside, I can tell you it takes a lot of work to keep a cool enough temperature - maybe since your greenhouse is larger, you'll have better luck. You could add (4) of those fans instead of three like I have - I just don't have the right set-up for four, here.

I'm sure I'm forgetting some stuff, and of course there are many ways to get a job done - but this is how I did it (against the grain of the pro's) - using everyday household-type items instead of pricey greenhouse intended items.

Plus you can view my images, any other images I have at this link:
http://smg.photobucket.com/albums/v280/Cultivar/

Dax/conifers

P.s. with all this junk - I can keep my greenhouse at 50-55 pretty successfully, but even with "everything", it shoots up to 65 some days but that won't hurt your alpines as long as they aren't kept at that temp for too long. Previously as Stressbaby said, I was shooting for 45 F, but 55 is my new temperature for grafting/etc....

Thanks, Dax.

June, Dax raises some interesting points.

The larger the GH, the easier it is to manage the temps. With a GH twice as large, you will have an easier time with the temps.

Charley's is on the expensive side, he's right.

I doubt if you would want the mister, as I suspect the alpines won't appreciate the humidity the way his cuttings would.

The products he uses are not listed for GH use. When he says "against the grain of the pros," he is referring to a lengthy discussion on another forum last year regarding whether or not it was acceptable to use a product like this in-line duct fact for an application for which it was not designed. You'll have to make up your own mind on this, but in the link I gave above, they specifically mention it's use in ventilating bathrooms, which are humid environments like a greenhouse. Personally, I wouldn't have a problem giving it a try.

Use conduit and waterproof wire nuts, please. While you're at it, be sure you have GFCI protection and in-use outlet covers as well.

Keep in mind that the greatest need for ventilation will be on the hottest days, the same days on which there is least risk of damaging plants with blasts of cold incoming air. This may mean you could up-size your exhaust somewhat.

Here is something we haven't touched on at all: thermal mass. You know that a tremendous amount of heat is given off by water as it freezes. If you can afford to divide your GH space in half, maybe you can afford 55gal drums of water, painted white on the north half, black on the south half. This will be very effective in moderating your temps. People who use thermal mass in this way cite examples of extremely cold temps outside, when the greenhouse temps hover around freezing all night long as a result of the heat given off by the stored water in the GH as it freezes. Yours would seem to be an ideal application for water as thermal mass.

SB

I agree with Stressbaby - you may need nothing else except the barrels of water to keep it from freezing thus eliminating the need for more than one heater, etc. You might still keep one like I have because it has a built in thermostat which usually is only running when it isn't light outside.

When I upsize, ...someday, I plan on using thermal mass.

Still you need shade cloth - the heater also, again, I referred you to I believe can be set only as low as 40 degrees. Which is perfect for your application...

Stressbaby is right about the cool puff mister because ultimately it causes the humidity to raise however I don't have the knowledge to know how much "cooling" it can do for a greenhouse. I can tell you the reason I bought it was because a conifer grafter in Canada told me it would help cool the greenhouse on sunny days. So in retrospet, it does cool actually as humidity is already heavily present with or without it. It makes sense to me anyway.

Also this, since you're overwintering them, you could keep that shade cloth on 100% of the time as plants don't need sunlight to survive when they're completely dormant. Maybe I'll present the question to you and others that a heavier density cloth of 80% would be better than the 50-percenter I mentioned of earlier.

You'll get it dialed in. It takes group thought to get it all together and Stressbaby I can tell you is one that can help you. If it wasn't for him last year, I would have only had two outlets thinking I could heat my greenhouse with one heater. One simple mistake like that and I lose an entire greenhouse of grafts and seedlings -

There's my ideas guys,

Dax


Oh yah, p.s. - here's that dumb screen door I simply framed from 2 x 4's and a strip of screen 2 feet wide I purchased at a hardware store (and the clamp which I stick on the door track - and recently I resized it so it wouldn't bash into my new intake fan). Good LUCK! Everybody is pulling for ya.
http://smg.photobucket.com/albums/v280/Cultivar/Harbor%20Freight%20Add%20Ons/?action=view¤t=ScreenDoorStationaryShowingClamp.jpg&refPage=&imgAnch=imgAnch4

Thanks a million, everybody. I think it's going to take me a few days to follow up on the hyperlinks and and digest all the information. I'll let you know how things work out.

June

I hope this isn't piling on...here's one more link, this one on thermal mass and approximating number of gallons of water needed: http://www.greenhousegarden.com/Material%20properties.htm

Shane Smith's book is excellent BTW.

Let us know if any other questions, and keep us posted!

SB

Now that winter has finally arrived, I can report back on how the greenhouse is working out. The first thing I have to say is that I wish we had built from scratch, instead of buying a kit. The aluminum frame is flimsy, it warps, and has gaps everywhere. The sliding door is not designed for a cold climate and gets iced-up. The roof vents are prone to sliding out of position, making them difficult to close. However, my worthy spouse did an excellent job of caulking the air gaps and constructing a wood-frame inner door that is held in place with magnets. When we had a night-time low temperature of minus 20 Celsius, the greenhouse interior stayed above minus 5C, which is what I was hoping for. Keeping the greenhouse cool during the day is the problem! As soon as the sun comes out, the interior temp quickly rises to above 10C. The extractor fans can't cope, and the "automatic" vent openers don't work until the temperature is way too high, so I have to depend on someone being home during the day to manually open the vents or the door.