I grow a rare variety of Portulacaria Afra (Dwarf Jade) for bonsai. Typically I bring my trees indoors into a grow tent in winter, but I'd like to build a greenhouse to expand my hobby further. These plants can stand temperatures down into the 30's degrees (F) , but grow well when temps are 65+ degrees F, up to about 100 degrees F.

I plan on ordering a greenhouse from ACF (GrowMore GM16) and heat it with a natural gas heater (Hotdawg HD45). I'll have gas/electricity and water available.

My main question is about the greenhouse base. I can do a gravel pad or concrete with a drain. I'm leaning toward concrete with a drain, but I'm wondering if having a concrete slab will make it harder to heat in winter. Will the concrete pull all of the heat from inside the greenhouse since it will be exposed to the cold weather outside? I'm located near Chicago, so we get pretty cold winters.

For summer I'll have huge roof vents and also electric fans pulling air through the greenhouse.

Welcome to NGA, @rrubel

@subarctic (aka Chip) and @MoonShadows (aka Jim) are 2 members who come to my mind as possibly being able to help you with that - best of luck!

(I should probably add that they will be notified by my post and will likely show up here soon.)

Thanks for that! I'd love to hear these guys thoughts.
I've actually been reading all relevant posts on this forum and have learned quite a bit from both of their previous posts!

Now that I've learned quite a bit I can ask better questions. It sounds like insulating my slab is a must, especially if I want to keep warm temperatures inside my greenhouse in our cold winters. I wont be running radiant heating, but I'll put insulated foam board under the slab and around the sides. I'm curious how this will look on the sides once finished though. I don't like the idea of having insulation showing on the visible slab (likely a couple inches shown on the front, and several inches along the back since the greenhouse will sit on a slightly angled part of my yard). Does the insulation only come up to the ground level? If so, aren't we still losing heat from the slab where the slab is exposed to the outside ambient air?

Here's my plan right now:
Through ACF I'll purchase a GrowMore GM16 greenhouse
- It has 10mm triple wall poly
- Insulate the interior north side of the greenhouse with bubble wrap in late fall through spring
- Insulate the slab
- Heat using a natural gas vented heater (HotDawg HD45 - 45,000 BTU Heater)

Questions:
1) Details on insulating the edges (per above description) would be appreciated!
2) I'm trying to get a rough gauge of what energy costs will be to keep my greenhouse warm, so I used an online calculator (results below). I'm pretty encouraged by it, since it seems quite affordable relative to the joy I'll get out of having a greenhouse to step into during the cold drab chicago winters.

Screenshot of the calculator results below, but here's a summary:
- I'll use 6,728 cubic feet of natural gas (~70 therms) each of the 3 coldest months of winter in my area. For me this equates to about $140/month for those cold months.
- This factors in my location, greenhouse wall materials, etc.
- The other 9 months are significatly warmer and will require less or zero gas.

Sorry it took me so long to see this! and welcome!

It would be real nice if you could do a ground to air heat exchange, but you would need to dig down 4-6 feet to install it.

Concrete. Take a look at these two methods: https://ceresgs.com/solar-gree...

You could do the Swedish Skirt around the perimeter, or you could dig down, insulate the dirt walls, refill, top with gravel and pour a slab. These would be the two most efficient IMO.

If you can't do anything like that (I couldn't do either), go with crushed rock tamped well, a good layer of paver dust tamped well, and a nice dark color brick or stone that will absorb a lot of heat. You could put rigid insulation somewhere in that sandwich, but I don't know how well it would stand up over time.

BTW...expect water on the ground...a lot! One thing I like about the dark bricks I have is they absorb some water and help evaporate it fast, as opposed to the water having to "find" the drain.

Please let us know what you do. Now, I should go back and read your 2nd post.

Wish I had read both posts before responding to the first one.

As far as your 2nd post. The link I gave you in my first reply should show you how to put foam board around the perimeter. Also, look up Swedish Skirt on YouTube. There are a couple of videos there. One is for a house, and one is for a huge greenhouse if I remember correctly.

I was going to put a Swedish Skirt around my new greenhouse, but never got to it before the winter, but was pleased with the temps without it, but I was growing all cold crop plants. I may still do it at some point. If so, I will slope it down, put a layer of protective plastic underneath and above it, and then put a layer of gravel on it. No soil, although, I'm sure you could do that, too.

The little bit of slab sticking above the ground will have some negative effect, but it should be minimal. You could always cover the foam board with something if you don't want to see it but want it all the way up the side.

Dinner's ready! I'll come back later from "Here's my plan right now" and continue to put my 2 cents in for what it is worth.

MoonShadows said: Wish I had read both posts before responding to the first one.

As far as your 2nd post. The link I gave you in my first reply should show you how to put foam board around the perimeter. Also, look up Swedish Skirt on YouTube. There are a couple of videos there. One is for a house, and one is for a huge greenhouse if I remember correctly.

I was going to put a Swedish Skirt around my new greenhouse, but never got to it before the winter, but was pleased with the temps without it, but I was growing all cold crop plants. I may still do it at some point. If so, I will slope it down, put a layer of protective plastic underneath and above it, and then put a layer of gravel on it. No soil, although, I'm sure you could do that, too.

The little bit of slab sticking above the ground will have some negative effect, but it should be minimal. You could always cover the foam board with something if you don't want to see it but want it all the way up the side.

Dinner's ready! I'll come back later from "Here's my plan right now" and continue to put my 2 cents in for what it is worth.

Thanks for the info so far! I'll need to look into the Swedish Skirt, although I'm not sure it will work for me as I'm planning on pushing the greenhouse as close to a corner of the yard where there's a fence and a power pole. It will be about 2-3' from the fence on 2 of its sides. I like the idea of insulating the walls and filling with dirt/sand/gravel then concrete. I'll need to explore that a bit more.

I'm not sure if this next question can be answered or not, but I'm curious what type of energy savings there would be (in Therms ideally) from insulating a slab vs not insulating it. It's a fair amount more work to insulate the slab well, and if the difference means $25/month in savings for those 3 coldest months I'd probably skip it.

Not sure about the energy saving, but you lose either 15 or 20% of your heat through the floor. Can't remember which it is, but you can look it up.

I looked up your greenhouse. Nice size.
Triple poly. Great. I have triple poly too, 16mm triple-wall poly

I would cover North and East walls. (I haven't covered my east wall yet in this new greenhouse, but I did in my old greenhouse.) If I remember correctly, AFC sells a bubble wrap that is made specifically for greenhouses. Looks durable. Don't buy cheap bubble wrap. It won't hold up. Even better, think about using Reflectix. https://www.lowes.com/pl/Refle... It is a thin layer of bubble wrap with aluminum coating on each side. It is better than plain bubble wrap if you install it properly (air space between the Reflectix and wall) and reflects the heat back into the greenhouse. I've used it in both my greenhouses.

You'll love it in the cold Chicago winters. Sometimes I spend a good part of winter afternoons just puttering in the greenhouse. I also bought 2 high deck chairs so we can sit and relax in the winter.

MoonShadows said: Not sure about the energy saving, but you lose either 15 or 20% of your heat through the floor. Can't remember which it is, but you can look it up.

I looked up your greenhouse. Nice size.
Triple poly. Great. I have triple poly too, 16mm triple-wall poly

I would cover North and East walls. (I haven't covered my east wall yet in this new greenhouse, but I did in my old greenhouse.) If I remember correctly, AFC sells a bubble wrap that is made specifically for greenhouses. Looks durable. Don't buy cheap bubble wrap. It won't hold up. Even better, think about using Reflectix. It is a thin layer of bubble wrap with aluminum coating on each side. It is better than plain bubble wrap if you install it properly (air space between the Reflectix and wall) and reflects the heat back into the greenhouse. I've used it in both my greenhouses.

You'll love it in the cold Chicago winters. Sometimes I spend a good part of winter afternoons just puttering in the greenhouse. I also bought 2 high deck chairs so we can sit and relax in the winter.

I love the look of your greenhouse, and the Reflectix actually looks good too. I've seen people insulate their GH's where it looks like an eyesore but yours definitely doesn't!
It looks like the ones from Lowes and from Reflectix website has a foil side and a white side - where did you get the all white version? It looks like yours has textured squares - is that how it came?

rrubel said: I love the look of your greenhouse, and the Reflectix actually looks good too. I've seen people insulate their GH's where it looks like an eyesore but yours definitely doesn't!
It looks like the ones from Lowes and from Reflectix website has a foil side and a white side - where did you get the all white version? It looks like yours has textured squares - is that how it came?

Actually, what you are looking at is Phase Change Material attached over the Reflectix. The PCM are 2' x 2' panels (4 sections) that have a substance (think like wax) that begins to melt at 65 degrees. As the substance melts, it absorbs heat. At night, when the greenhouse cools down and falls below 65 degrees, the panels begin to solidify and give off the heat back into the greenhouse. Each 2x2 panel can hold 400 BTU's of heat energy. I have 63 panels, so I have the potential to store over 25,000 BTU's of heat energy. In the summer it works the opposite, helping to keep the greenhouse cooler during the hottest part of the day and give the heat back at night when the greenhouse cools down a bit.

Here's a pic when I first started hanging them over the Reflextix (double sided aluminum layer). I used the 24" x 25' rolls; 4 of them; run horizontally on 1x2 pine boards I attached to the greenhouse (with a 1.5" air space between the reflectix and the outer wall of the greenhouse) so I could staple the Reflectix and then screw in the PCM panels. https://www.lowes.com/pd/Refle...

MoonShadows said: Actually, what you are looking at is Phase Change Material attached over the Reflectix. The PCM are 2' x 2' panels (4 sections) that have a substance (think like wax) that begins to melt at 65 degrees. As the substance melts, it absorbs heat. At night, when the greenhouse cools down and falls below 65 degrees, the panels begin to solidify and give off the heat back into the greenhouse. Each 2x2 panel can hold 400 BTU's of heat energy. I have 63 panels, so I have the potential to store over 25,000 BTU's of heat energy. In the summer it works the opposite, helping to keep the greenhouse cooler during the hottest part of the day and give the heat back at night when the greenhouse cools down a bit.

Here's a pic when I first started hanging them over the Reflextix (double sided aluminum layer). I used the 24" x 25' rolls; 4 of them; run horizontally on 1x2 pine boards I attached to the greenhouse (with a 1.5" air space between the reflectix and the outer wall of the greenhouse) so I could staple the Reflectix and then screw in the PCM panels.

The PCM Panels sounds great. I'll have to look into that. Can you tell me where you got these? I'm not able to find them easily by searching, but I love the idea.
I believe they're Templok panels, but not 100% sure. Thanks for sharing the knowledge!

They are Templock panels.

I purchased 7 cases last year from Insolcorp, however my understanding is Insolcorp has since sold the rights to sell these PCM panels to another company. I do not know what company, and I do not know if Insolcorp is still making them and this other company is selling them, or this other company is making/selling them now.

I do know the CEO of Insolcorp, Peter Harworth, has left the company and is now working for another company. You could try contacting Jon Lippin at [email protected].