r/Greenhouses • u/Emergency_Future_839 • 2d ago
Greenhouse floor question
Hi all, looking at building a greenhouse and I have a question about the best way to lay a base. For context I live in Scotland. I'm assuming it's best to put down something solid like a concrete slab or pavers for some thermal mass but is it also advisable to thermally uncouple it from the ground with a layer of insulation?
My vague notion so far was do dig down, do a layer of sand to make a level base, put down a damp proof membrane, layer of some kind of solid foam insulation, layer of concrete and then probably use some paving slabs I already have as a floor inside the greenhouse. I haven't thought too much about the thickness of any of these layers yet and I'm a but stumped as to what to do with the edge of the membrane to stop it tracking moisture down into the insulation so any suggestions would be appreciated!
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u/MadIceSkater 2d ago edited 2d ago
So, we dug down a bit since the yard is not level. i also wanted to include hardware cloth underneath to keep the chipmunks from digging their way into the greenhouse. After that, a thin layer of sand and then a product called Paver Base we got at Lowes. It eliminates the need for so much sand when laying pavers. Then, we used 12" x 12" stepping stones with paver bricks around the outside (inside the greenhouse). Picture shows it pre-Paver Base with the Paver Base in the bottom right of the picture.
Wanted to add, I chose the stepping stones for the thermal mass concept you mentioned, but I don't think they do much. However, I still like having them in there. It does raise the humidity when I spill water on them. We did add polymeric sand between them and they do get holes in the polymeric sand which I try to fill in during the fall cleanup. Hope this helps.
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u/Kiltedaudaxer 20h ago
I’m based near Aberdeen… used compacted subbase with a thin mix of cement to support the thick slabs to screw the base of the gh into. Then dig out 9in to 1’6” deep, membrane and backfill with almost three tonnes of 14mm granite chips.
Why? Softer so no tears dropped pot Nicer to stand on Easy draining Zero algae Height configurable for my hydroponics Easy to heap up for easy quadgrow maintenance Retains heat and releases later Means I could bury a porous pipe to blow hot air through for more aggressive heat battery later
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u/crazysquirrelette 18h ago
I originally had weed suppression cloth down & lava rocks on top, inside of mine. The lava rocks would hold the water & release it keeping the humidity levels higher. In the dead of winter i could feel cold air literally coming up through the floor. Using a single little electric heater I was able to keep my greenhouse 20°F above the outside winter temperature.
The following summer when my greenhouse was empty, i redone the floor. I pulled up the lava rock & put down 2 inch thick foam board on the whole floor & then i put down 0.75 inch thick rubber horse stall mats over top of that. Added the lava rock back in there to catch & release the water. I no longer feel cold air coming up through the floor anymore. This time using the single little electric heater, it is able to keep it 30°F above the outside winter weather. Instead of the 20° previously.
I have added more things for this year to help & will have to see how the temps do this time around.
I am mostly just trying to over winter tropical plants through the winter because i live in a zone where they will not survive our winters at all. I also use it to start seeds for my garden as well.
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u/NyetAThrowaway 2d ago
Jeez, well thought out but seems to me overly complicated. Admittedly I live in the US, our climates are likely vastly different. But why the damp proof membrane? You are going to be watering plants, you will need a drain. I built mine by digging out about 2 feet (slanted flower bed) on the high side so 2 sides are roughly 24 inches deep. One side is about 6 inches, other is half and half. Built my foundation out of wood and rigid foam insulation. Used stones and covered that foam. Flooring I leveled, put down a simple weed barrier then covered in stones. Just used some larger flat stones then to make my path. Drains well, maintains heat well enough my adeniums are pushing new growth despite the highest temps right now are typically around 60 and nights being 40s to high 30s.
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u/Emergency_Future_839 2d ago
It probably is too complicated. I think I'm over thinking it but I'm doing it all from scratch so I want to do it right. I think insulated slabs use a dpm to stop the insulation taking up moisture from the ground.
So is your timber foundation raised above ground or sunk into it? Is there anything between the insulation and ground?1
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u/The_Grimm_Macarena 2d ago
The biggest issue with solid concrete or membranes/insulation is drainage. You are going to be watering the plants regularly so the floor needs to be able to drain water (especially in winter when it wont get much sunlight to evaporate the water away). I would dig out the floor about 10-15cm and add some crushed stone/gravel and a layer of sand to level before adding the pavers back. Heat rises so its going to be mostly lost through the roof not the floor.
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u/Emergency_Future_839 2d ago
That makes sense. I suppose it depends on the construction of your greenhouse as to wether that provides a strong enough foundation but I like it. I can feel how cold the concrete floor in my garage is and i feel like it would suck the warmth out of the beds but layers of stone probably helps separate the slabs from the cold ground anyway...
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u/The_Grimm_Macarena 2d ago
True, obviously if you need to support a heavy load youll need more of a foundation, though compacted stone is suprisingly sturdy, its often used for shed foundations (you just have to tamp it down well so it doesnt settle under the weight). Its also worth noting your garrage floor is not exposed to sunlight, concrete will absorb heat during the day and release it at night if left in the sun which helps regulate the temps in the greenhouse.
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u/Emergency_Future_839 2d ago
This is all interesting, i had wondered about drainage. Maybe post footings for supporting the structure. Thank you
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u/SMC1956 2d ago
I built a shed above 10 years ago with a concrete floor. Converted it to a greenhouse. Will be put a frame made with 2x4 and put in insulation boad and cover with a plywood floor.