Montana greenhouse project

Next season I am planning to build a greenhouse over a poured cement foundation here in Montana. The structure will be about 9'x20'. I would like to keep it warmer in winter months with a rmh solution. I'm not sure how I should best install a mass for it. Should I bury the pipe in the cement without anymore insulation? Should I have the cement guys leave a ditch cleared that I can insulate the pipe inside? Should I build a mass inside the greenhouse that also could act as a bench of sorts? This will be my first real rmh, but I have made a small test one that has worked as I would expect. Thanks for any suggestions!

Unless you don't want any raised beds, the common practice seems to be to run the heat duct through a cob/masonry bench which has a planting bed on top of it, or else is used to hold starting trays. Putting the duct under the slab without massively insulating below it will send a lot of your heat into the earth, and it would take a very long time if ever to stabilize that heat flow situation.

Thanks Glenn, I have been given some great advice much in line with yours from a fellow member that did this very type of installment. He suggested the same thing you did which I will be implementing. He suggested to have the j tube and mass in the greenhouse to keep the heat inside as opposed to having the j tube outside the greenhouse and piping the heat in, which was my first thought. He also suggested to have the mass as an above ground bench type thingy on which to have plants happily warmed. Buy was I wrong! I will just have to "skin" the mass with brick to keep it waterproof from the humidity and also watering plants, duh!

Thanks for your response again!

I have seen photos of one such design that used backer board as the skin. Matt Walker has skinned outdoor benches with tiles to good result.

Hi William, thanks for the idea of the backer board. I'm leaning towards brick on the sides and tile on the top of the mass. I'll have to post some more details as I pry them from my head. This is going to happen next spring, so I've got lots of time to build it incorrectly over the winter (in my mind...) Keep the ideas and input flowing gang. Thanks!

I remember that Erica and Ernie Wisner built one with a rock facing on it. Not sure what the top of the bed is made of, but here's the video.