Very early planning for a greenhouse

Slippertalk Orchid Forum

Help Support Slippertalk Orchid Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
Feb 16, 2019
Reaction score
Nottingham, UK
I'm starting to put some plans together to build a new greenhouse next year. I have a space 18ft by 10ft but could go a bit bigger if there was a very good reason (other than to fit more stuff in!!!). I'm not planning a conventional glass box so bear with me!

Over here in the UK double glazed greenhouses I've seen on here just don't seem to exist plus my wife would like it to disappear into the garden a bit. Our garden also slopes away from the house.

This greenhouse will also be a bit multifunctional - I don't grow enough orchids currently to fill it - so it will have to grow the usual seedlings in spring and over-winter some half-hardy plants from out patio. If my other hobbies take precedent again, I'm also planning that it could be converted into a fish-room!

So current idea is to have an unequal roof with the long span facing south-west to catch plenty of sun. The greenhouse will be built into the ground up to bench level at the top. The taller side wall and both ends would be built out of bricks/blocks with an insulated cavity in the wall. The roof will be twin wall polycarbonate (possible two layers with a large air gap for further insulation). There will be vents all along the apex on the north side. The lower side will have the door in the centre and glass up to the roof (probably double-glazed like the door).

The brick end facing the house will be covered with a climbing plant.

I am thinking of a large water reservoir at the back made with a brick wall up to bench height (lined black to act as a heat store and filled from the roof to store rainwater for watering the orchids) and was thinking of stepped staging on top mainly for orchids and other permanent residents and then more conventional staging at the glass side. I'd put a tap at one end and a potting station at the other.

I am also thinking I'll need a fan in the roof ridge and some way of getting cool air in low down at the bottom end. Heating wise will be electric of some sort but I'm hoping to not need much.

Does any of his sound feasible? Will brick ends block too much light? Will polycarbonate, even two layers, provide enough insulation? Will they cut out too much light? Any other thoughts would be welcome at this early stage too!
I love the "into the ground" concept, but think having the wide dimension facing directly south, with the east- and west walls glazed will serve you better.

The closer the design is to being spherical, the better it will be from an energy efficiency standpoint. I'm not necessarily recommending you build a geodesic dome, but what I'm saying is that a square will be better than an elongated rectangular structure with the same interior volume - it's a surface-to-volume ratio thing. Similarly, the greater the volume, the better, so if you can go bigger, both laterally and in height, go for it.

The degree of light degradation by horticultural polycarbonate panels is of little concern. The more layers, the better, but they have to be sealed.

Paint any non-glazed, interior walls eggshell- or flat white to maximize the scattering of incident light.

Yes, gentle air movement is good.
Thanks Ray. I'm rather restricted with the orientation of the greenhouse as the garden is a typical British long and thin back garden (I'm lucky that it's longer than most at about 150ft though). I might be able to persuade the wife to rotate it a bit to get the slope facing in a more southerly direction though. Will take some photos and try and sketch up an outline drawing in the next couple of days to make it clearer.

I could go wider to make it closer to square.

I was thinking white above bench height from other threads I've read on here but black below to absorb heat into the concrete walls. Will take on board about the glazing one the two ends - these cutting down the light is one of my biggest concerns.
I am describing my greenhouse for the benefit of all the readers not just the person starting this discussion. Your mileage may vary.
My greenhouse in Columbia Missouri is roofed with triple wall 16mm poly-carbonate. I use 60% Aluminet shade cloth laying on the roof for nine months of the year. Nov, Dec. Jan. I remove the shade cloth because of snow and the sun angle is low enough that much of the light is reflected off the roof. I have eight foot side walls and twelve foot to the peak. I was able to minimize my heating needs by choosing better materials initially and saving money on heating for the life of the greenhouse. Build it tight and cut down on air leaks. My winter minimum is 60F. summer maximum is 94F. I have a two speed fan that will change the air every minute if needed in the summer. I use cool cells for evaporative cooling in the summer. My orchids are on the same level that we live on so I can take care of them as I age and get more infirm. What I am trying to say is, Plan for the future. The most important thing you can do for orchids is give them pure water. I use an RO system and many problems that other people have do not exist for me. Plants grow better, potting media lasts longer, I earn more ribbons at the AOS show. I have a system (for watering) of fogging nozzles above the plants controlled by a timer so I can go on vacation. With your earth contact greenhouse, please use a thermal break between the ground and your greenhouse. Soil is a good conductor of heat. This includes the floor. Here we use 2 inches of extruded polystyrene. Put a continuous layer behind your brick wall, cement is a poor insulator. The floor needs a water drain. No need for sewer, just drain to air. I enjoy a ceramic tile floor. A gravel floor can increase the humidity, but lots of bugs, weeds and problems live in the gravel. Storing water in the greenhouse seems like a good idea, but plants need warmth to grow, so the plants will not grow well until the water is same temperature as the greenhouse. Solar greenhouses works in theory, not in reality. I was able to use treated lumber to build my greenhouse foundation. A greenhouse is cool and wet, a sun room is warm and dry. It took me 18 months to negotiate with my wife to build my greenhouse, cost was $12,000 for a 20 x 20 foot space 16 years ago. I hope you get to enjoy your new space.
Thanks for your reply Hardwood.

Already planned cavity insulation in the walls and under the slab. I will use PIR sheets (Kingspan / Celotex type) for the wall cavity. The slab might have Jablite sheets or PUT spending on price!

Will fit a floor drain to a simple soakaway as it will be below ground level.

Does your greenhouse have solid walls or polycarbonate sides? Looking again at the sun this time of year yesterday, I will definitely need one end wall in polycarbonate.

Looking at prices of polycarbonate it looks like two 4mm sheets with a cavity will be cheaper than 16mm triple wall. The question is whether it will insulate as well or better!
4mm polycarbonate wil need a very 'close together' fixing system. Its way too flimsy to fix over large areas.. Remember that twin/triple wall poly has an enclosed air space for insulation.
You can always add horticultural film/plastic during winter.
As hardwood said, 'spend now and save over the life of the GH'.
i'll ditto on the spend now part to be smart for multiple reasons....

i had an insulated glass (vacuum sealed type glass panel) greenhouse as a first incarnation was dumb and got the fancy schmancy curved eave... in less than 10 years when the panels started failing and cracking, i started looking for a plan. Realized that replacing the glass was going to cost almost as much as new house with single pane glass... Decided that the heating cost didn't warrant any thought about insulation. Although that was mostly true, the bigger issue was that growing up close to the glass was a way bigger concern. Now i am looking to retrofit acrylic in the roof ('drip proof' i've been told) and acrylic on the side walls. THe weekend before our outside temps dipped to -4 F (outside phila - usa) i put up a plastic film on my west facing sidewall.. Made a huge difference in both internal temperature and ability to grow next to the glass. I'm looking for some good solution to keeping air circulation completely off the aluminum and side glazing in an effort to give more usable space.

Another thing to consider is how you can get fresh air exchange in a cost effective way during cold weather. Heat recovery ventilators/units are the only options i can see (would love to hear others thoughts on that)

...and one more what are you thinking about for water source/infrastructure... if you need to 'make' your water, that may need to play into your structural planning.

Hoping some of that rambling may tweak some new ideas or thoughtways for you...

Good luck...
Thanks guys.

With the 4mm, I'm thinking a frame made of 2 by 4 with one layer screwed outside and another inside (so 4" space between panels which I'm worried is too much) and siliconing them to between frame and panel to seal even more. The frame I was thinking with uprights at about 2ft intervals. I'm very much in two minds and might end up with triplewall ploy carb and plastic sheet pinned inside in the winter for extra insulation.

Nice idea with the heat exchanger. I'm wondering if an air source heat pump might be the most efficient way of heating the space depending on cost to buy it.

Planning to store collected rain water from the roof in the water store inside but I will also be running mains water down there too. I've got an RO unit in the house so can use that at a push too.
Last edited:
My greenhouse walls are double glass sliding windows, 4 x 5 feet. three feet knee walls below the windows. My wife insisted on being able to see the view, not just vague shapes that poly-carbonate allows. Sliders do not take up any interior space when open. I then use a heat shrink inside storm window covering on the windows to keep the condensation off the windows sills. The window literature says that a space thicker than .75 inches between the window pains allows air to thermal-siphon up and down. So I would recommend putting your 4mm layers closer together. I need to walk on my roof to deploy the shade cloth and do maintenance. You would have to put down a layer of 3/4 plywood across the rafters to support your weight. My exterior air comes in the floor drain. So when it snows I go out and shovel the snow off the outflow. That way the cold air is at the floor level and close to the Southern Burner Propane heater. It is controlled by a milli-volt thermostat so the greenhouse can be warm when we lose electricity. Again plan for the future and extreme events before they happen. St. Louis, Missouri lost electricity last year and several orchid collections were frozen. My 16mm triple wall roof will almost not allow snow to melt. I get worried about the weight and use a snow roof rake to get the snow off the roof and let the light in. I used 2x6 treated lumber for the wall studs and rafters. Since the wall is mostly windows, I thought 2x4 would not be strong enough in our windy climate. Since the greenhouse is constantly humid I did not want it rotting from the inside out. I used Styrofoam to insulate the walls. The insulation should not absorb any water. You are not building a house. Spray foam would be better. The doors are 36 inch wide and made from fiberglass. They are wide enough to get a wheelbarrow in and are rot resistant. For orchids, the windows are important in the fall and spring to get the proper diurnal temperature change. I slide the windows open in the evening, spring and fall, to let the greenhouse get to the outside ambient night temperature. They need 15 to 20 degrees of temperature drop at night. The cooler night temperature allows the plants to use less energy (build up reserves) and initiate flowers. I hope you get to enjoy your new space.
I'm finally coming back to this greenhouse design. Orchids have taken a bit of a backseat to cacti and succulents but I plan the basic principle of the greenhouse to be the same regardless so I could change it over to orchids (or other plants, or tropical fish etc. Etc. Etc.!)

The design has evolved from some further reading about passive greenhouses and trying to incorporate some of those ideas as well as previous advice on here. I'm going for an almost square design, 18' long by 16' wide to incorporate two 3' wide benches, a 6' wide bench in the middle and two 2' wide aisles.


I'm planning a 16mm polycarbonate roof and ends, double glazed windows on the front facing south on the side and then making the back (north facing) a solid wall with reflective insulation.

All areas below bench height will be concrete blocks and all of this will be insulated on the outside to allow the concrete to retain heat. I'm thinking about painting this black but I'm worried it might get too hot in the summer! Also, the middle and rear benches will sit on large raised ponds to store captured rainwater for irrigation and also as a major heat store in winter.

Has anyone tried or seen something like this? I will be making this myself using a wooden frame that will be completely covered on the outside and thoroughly painted with white paint on the inside to reflect the light.

I hope this design stays frost free over the winter with minimal to no heating (enough for the cacti!)

For ventilation, the double glazing will all be on automatic openers and there will be exhaust fans upstairs (possibly heat exchangers to allow air exchange in winter without losing too much heat.)

Can anyone see any problems or offer any advice? Has anyone heard of anyone who might have done something like this in cold climates?
Sounds like a great project. I don't see any major "gotcha's", but I would like to throw out a few bits:

Paint your "reflective insulation" white. A matte white surface scatters light better than does glossy and silver, as they tend to act more like mirrors and reflect the light down to the floor.

If possible, dig out your floor, lowering it below ground level. The earth is a remarkably good insulator, plus that will increase your interior volume, which lends more thermal stability. If you can work it so the structure approaches a cube in dimensions, that'll be even better.

My only use of water as a heat sink was when I was in South Carolina, which hs sunny winters. There, I use steel barrels topped with bread flts as my benches. the barrels were filled with water and antifreeze (to prevent rust) and painted black. They held onto a lot of heat. after a couple of sunny weeks, my heater hardly ever came on. In fact, it was so warm that I didn't get a sufficient temperature dip for my phals to bloom.

Your open pond idea might not be so effective, unless you have a good way to circulate the water into a sunny area to absorb sunshine. Otherwise, you'll be paying to heat that large mass - don't forget that evaporation absorbs heat.
Thanks Ray, appreciate your input.

Point noted on white v reflective material. Might save me some money too!

It is going to be partly in ground (should have said that, sorry!) as the garden slopes away from our house. Unfortunately it slopes away to the North so I will have more exposed on the North side rather than being able to dig it into the bank more.

My plan is to try and seal the ponds as well as possible. I am hoping the full metal bench tops I am planning on top will seal enough but if not there will be a layer of pond liner going on top beforehand. I'm planning to use the collected water in there to water the plants so can't tightly seal them like barrels. With growing cacti and succulents, I need to keep the humidity down so I have considered a bladder type of arrangement if I really have problems! Really hoping it doesn't come to that though!

I am hoping the black concrete wall will heat the water up enough but, if not, I have considered some form of solar panel / pipes to circulate water into a body with a higher surface area. Again, hoping I don't need that too!
Believe me, I understand dealing with slopes. My last greenhouse was on a hillside. Right front corner was at grade, LF was 2 feet above it, LR was 6 ft, RR was 4 feet.

My foundation was pressure treated 4 x 4 posts at the corners and every 4 feet in-between, topped with horizontal 4x4s to make a level surface. Outside was pressure treated 5/4 x 6 (a true 1" thick") boards, inside was waterproofed plywood, and I filled the interior space with expanding polystyrene foam.
Your greenhouse project sounds very exciting! I sometimes wish I could start over on mine. I am on #3, #4 and #5 of my life. An 8' x 12' attached to the front of our library.. it faces north west, is shaded by enormous trees and has no source of heat or water. You also have to go through our carpeted library to get into it. It looks great full of decorations at Christmas time, though! My other two are a 10' x 20' lean to curved eve ancient Janco, which is VERY awkward due to a concrete stairwell smack in the center of it, and a newer 12' x 25' poly that replaced and even older glass Janco that was falling apart. I have the back two heated with natural gas heaters, controlled by thermostat. When we first moved in, however, they were both plumbed to the house, and had baseboard hot water running through them (as did the house)... just copper pipes fitted with exposed fins. These worked very well until they started to leak, of course. I am in Virginia, and tried black barrels full of water as a heat sync, with little success. It gets too cold, and my exposure was not sufficient to expose them to enough direct sun. I don't know how cold it gets at your home, but the hot water heater is a good idea, and will also provide you with warm water for watering your plants. My only comment to anyone planning a greenhouse, is to go as large as you possibly can.. even if you don't think you'll use the space, you will!... and go for all the improvements you can afford up front. I find it impossible to retrofit anything in my greenhouses, as they are so full of huge and well established specimens that are impossible to move now. Also, make sure that you are planning enough space for yourself to move around inside the greenhouse. My benches are too wide.. I wanted to maximize plant space, of course, and my rows are narrow and feel very cramped. I also recommend an overhead misting system, if your plants will tolerate it, with an auto timer. Our summers get blazing hot, and having the mister come on several times a day really helps control heat. It's all very exciting, though, and I wish you the best in your building.


I would have thought the barrels would have worked well to conserve heat. See here: Collecting Solar Heat at Marble Branch Farms by Mark Reinke. They were more active in collecting solar heat. What once could do is to sink the barrels into the ground to make use of heat from the earth to moderate temperature.

Like ray I am skeptical of ponds to keep the temperature because of evaporation. I struggle to heat aquariums without a lid on them. The combination of low ambient temperatures and humidity mean that you care constantly losing water and heat to evaporation. One way around this is to use very large volumes with low surface areas such as these 1000 L tanks: These are used to transport foods, solvents etc... and when the product is delivered the tanks can be picked up cheaply. In my green house dreams I also imagine having fish ponds with orchid benches over them but it gets hard to do this the colder and drier the climate gets.

Rolling benches are an awesome idea.

Latest posts