I'm not a gardener personally but I'm building a mini greenhouse for my girlfriend for her birthday.

I want to combine the raised flowerbed table design with a mini-greenhouse. The other catch is I want her to be able to use it in the winter.

I worry about the cold air below the table and the type of insulation needed while also allowing drainage for water.

Any advice on how I get around this problem?

I attached two pictures to show the two concepts I want to combine.

Thank you for your help! 😊

Insulating the table bottom and sides, according to your unstated climate needs, and putting and insulating cover over it might work, again, not knowing your climate. Mass would help in winter, like filling in the sides of the base, moving the insulation down to the earth, and filling in the space below the bed with water containers.

Orienting E-W by length, and replacing the north roof panel with a white insulating panel would help with winter also. Burying some self regulating heat tape under some sheet metal under your bed might also be helpful...it could be on a thermostat for very cold nights.

Heat build up, even in winter on sunny days is also a serious issues for such a cold frame - like design. Automated panel openers- even the wax cylinder type could be plant heroes. I used a frame like this as a season extender for years.

That is essentially the same as a row cover. And row covers need to be easily opened so they don't overheat. So construct something to prop the top open, perhaps to different degrees. And independently. Having both side of a roof open and air flowing through will provide more cooling, or too much.

These are indeed rally season extenders, rather than season reversers. So they will help fall plants carry on somewhat in winter and will allow earlier spring starts. The above described practice of using water mass is remarkably effective at stabilizing temperatures.

You can find black 5-gallon buckets with lids reasonably priced in 10-packs if you search. I most often find Argee brand in 3.5 gallon and 10 gallon for about $38 and $40. I get mine from Amazon and Walmart, depending on which is cheapest. I buuy a lot of these for hydroponic projects. But you can also use used 5-gallon paint buckets. You don't care if they're hard to open or have dried paint inside. Paint them black to absorb daytime heat and pack them under the table as closely as you can.

The corrugated poly is great stuff. The thicker the air space and more air spaces (three-wall), the better it will hold heat.

And if you are going to have her planting in a raised box like that, a masonry floor will also hold heat. Concrete pavers and such, bedded in sand.

If you want to get fancy and give her some ability to monitor the greenhouse, you can use devices and apps with temperature alerting like this one: (Not recommending. Just for example.)

https://www.amazon.com/dp/B0081UR76G/

I'd forgotten to mention that only via covering the glazing with some serious insulation- like a blanket of fiberglass insulation wrapped in waterproofing such as plastic tarps can you expect much winter performance. Double wall plastic is only R2, triple R3. This is too much loss without throwing energy at heating in most cold winter climates.

A roll down insulating blanket is one solution illustrated by the China greenhouse style. It becomes more practical for smaller cold frames. Polyester fiberfill, such as bargain sleeping bags come to mind. One of the China greenhouse variants is widely spaced double glazed (film) with insulating blanket rolled down between the protective film layers. This makes good sense in climates with snow and freezing rain issues in winter. Exterior (steel roof panel covered) foam insulation panels are used by one of my local greenhouse owners; they dramatically improve winter performance.

It is tempting to ignore hard design issues. Insulating shutters for greenhouses have been largely avoided due to design difficulty, complexity. The era of just throwing lots of energy at heating and cooling is coming to a close. We can and must do better. A proven, exceptionally good design for a specific climate is future treasure for all the gardeners in that climate and beyond. A forum like this can now spread a new, exceptional design, as well as share the equally valuable mistakes.

Cold frame type growing boxes maximize growing area with the lowest amount of construction materials and cost; all the access space around the bed is no longer enclosed, heated or ventilated. They need more design attention to go beyond the modest season extension similar to a row cover.

Best wishes for your design.

Bruce