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I'm putting together a greenhouse, 10x13-feet base. Unfortunately it will be on a concrete-paved section of my backyard, and I know it will lose a lot of heat to the ground via conduction in wintertime. My goal is a 4-season greenhouse.

I would like to insulate the subfloor of the greenhouse to prevent heat loss via the concrete floor.

What is the best way to achieve this? "Best" meaning bang for the buck, and durable, and won't degrade over time due to direct sunlight, or being in contact with a frozen concrete floor?

Appreciate the help!

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  • Frozen concrete floor. Does this mean you will be heating your greenhouse space in winter ? Is so, How ? I am thinking sub-floor with radiant heat, this will turn your concrete slab into a giant thermal mass and have it work for you instead of against you. – Alaska Man Jul 14 '20 at 18:37
  • The goal is passive heating, but there will be some supplemental heating. Heating the concrete floor only works if I can insulate a good depth around the greenhouse, which is not possible. The concrete paving in my backyard is very large (area-wise), many times the size of the greenhouse. – Jinren Jul 14 '20 at 18:46
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Your concrete floor even if not insulated is still the best flooring for the small greenhouse you are planning for. I would use wood pallets where you place containers on the floor and use tables or racks for raised bed. If the concrete has drainage, the floor is easy to clean and could just be spray with a hose or a blower to do quick cleaning. Any sub-floor like foam would trap water and bacterial and other dirty stuff underneath. Just keep it simple. Any sun light coming in will heat the concrete. The concrete is easy to maintain and clean.

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  • You could use those steel frame car port 10x20, shorten the length to the size you want, cover with the material you want to use, and it is done in no time. – Programmer66 Jul 14 '20 at 21:10
  • Wow...makes sense to me. – Lee Sam Jul 14 '20 at 22:07
  • So you're saying the concrete won't really conduct away the heat from the air trapped in the greenhouse? I just need to put the planters on pallets? Even in the coldest time of winter? – Jinren Jul 14 '20 at 22:59
  • On sunny days, the concrete will absorb heat and assist in heating after the sun goes down. On cloudy days, you were planning on supplemental heat and the amount of heat the concrete extracts should be minimal. It's a trade off between the additional cost of adding an insulated sub-floor with all its maintenance, problems (moisture underneath, etc.). I would start with the existing concrete and see how it works before putting in an insulated sub-floor. – Programmer66 Jul 14 '20 at 23:21
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    @Jinren If this is your first greenhouse, I'd definitely recommend sticking with the concrete floor. If you've done this before and know more about the challenges greenhouse maintenance presents you could optimize, but for a start, concrete really isn't that bad and for a first build definitely one of the easier floors to start with. – Mast Jul 15 '20 at 13:17
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You will lose VERY little heat through the floor . Heat loss will be through glass/plastic and a little through the walls ; assuming you insulate the walls. I have some experience ; an 8 X 12 semi-greenhouse with tropical fish aquariums , about 70 F minimum water temperature. The only heat is a few hundred watts of aquarium heaters. And several tender plant pots sitting on the floor. My coldest weather is about 20F but many nights below freezing. Concentrate all your heat loss plans on the window area.

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For a small greenhouse?

Cheap, effective, durable: degrades slowly but easily renewed - straw, 6 inches to a foot deep (15-30 cm.)

If you want to spend a lot more money you could use eXtruded PolyStyrene panels (XPS foamboard) covered with cementboard (you can get them pre-bonded, but unless you find them surplus (roof insulation and surface for commercial flat roofs) that might cost you more than a layer of each, unbonded. The cementboard protects the foam from sunlight and point impacts. You could also use that ONLY where you intend to walk to spend a bit less, and use straw for the rest. It would be thinner than the straw for similar effect (2 inches/5 cm.)

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  • Can I walk on strawbale and how much will it compress? I imagine the compression from walking won't affect its function too much? – Jinren Jul 14 '20 at 18:50
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    I was not suggesting you leave it baled, but yes, you can walk on it baled or unbaled. Straw (as opposed to hay) is moderately decent at resisting becoming a compressed mess when underfoot, though it will likely compact some. – Ecnerwal Jul 14 '20 at 18:53
  • Would its insulative properties be better baled or unbaled? – Jinren Jul 14 '20 at 19:05
  • It's would matter much. Packed loose straw will be similar to baled. The fact is that you can't use it baled unless you lay something over the top to create a floor, so the question is moot. – isherwood Jul 14 '20 at 19:06
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    The greenhouses I’ve been involved with have lots of water sprayed around and lots of dirt and fertilizer tossed around. Seems like straw would become wet and messy several times a day...maybe even frozen to the concrete slab in your environment. – Lee Sam Jul 14 '20 at 19:15
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Extruded foam in your choice of thickness with 1/2" plywood over the top. Easy and foolproof. You could use some project adhesive to secure it, build a simple frame, connect the plywood sheets with flooring tape underneath, or just let it float. You could also use play mats or livestock mats. The idea is just to get a layer of wood off the concrete a bit. Beyond that the benefit will be minimal.

Note that this assumes good water management on your part--drainage trays and other means of keeping water off the floor. If it's routinely sprayed down it'll begin to degrade after a year or two.

Don't overestimate how much heat you'll lose through the slab, though. Concrete is not a good conductor, and the layer of air at the floor will always be cold anyway without a heat source or lots of circulation.

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