Anna,

I'm not sure what your intentions are for the "ground heat sink". In other words, I don't know whether you want it to keep the greenhouse from getting too hot in the summer or to keep the greenhouse from getting too cold in the winter.

Normally a heat sink is used to keep something from getting too hot. For example, heat sinks are mounted on computer CPUs to keep them from getting too hot. They are intended to carry heat away as quickly as possible. Their analogy in an electrical circuit would be a ground wire, which "shorts" the circuit to the ground voltage through as little resistance as possible.

I am in the process of excavating a foundation for a kit lean-to greenhouse that we ordered. Here in Maine the problem is to keep the greenhouse warm enough during the winter without spending a lot of money for fuel or electricity. During the summer we may not even use the greenhouse at all. So, during the winter I definitely don't want my heat to "sink" into the cold ground underneath the greenhouse. Instead of a heat sink, I am going to use a thermal mass of a rather limited amount of sand, gravel, and concrete pavers. That thermal mass will be insulated from the soil underneath by insulating foam, as much as I can afford.

The thermal mass could be likened to an electrical capacitor, that stores current when the voltage is higher and releases current when the voltage starts to drop. That's what I want my thermal mass to do, namely warm up significantly during the day and release heat back into the greenhouse during the night as it cools off. Too much thermal mass won't warm up enough during the day. (A really large electrical capacitor can act rather like a short to ground.) Too little thermal mass will cool off too quickly during the night. There is a "happy medium" thermal mass that I am striving for.

I intend to use enough insulation to prevent the floor from acting as a heat sink. I'm excavating deep enough to lay down several inches of insulation, and several inches of thermal mass over that. I certainly don't want to try to heat up the planet through the floor of our greenhouse.

MM

MM,
Thanks. I should have written thermal mass instead of heat sink. I was thinking of how hot it can get here in the summer. However I am glad I didn't because your explanation of thermal mass and comparison to an electrical capacitor was very helpful for me. I realize that in order to determine where that "happy medium" is for my greenhouse I will need to balance my needs of preventing the greenhouse from getting too hot in the summer (105+ days with nights ranging from 60 to high 80s) and from getting too cold in the winter (40 to 75 days with 9 to 35 nights).

This makes me think that what I will need to do is determine if the soil under the greenhouse is even a good of a conductor of heat. It appears that below the top few inches the soil has not been disturbed for a very long time. It is very dry fragile adobe-like clay with almost no organic matter. When digging in the garden I found that it was possible to dig a rectangular hole two feet with vertical sides where the sides will stay without crumbling for at least a year. On the other hand, disturbed soil is very fine clay with only a little sand, dusty when dry and sticky when wet.

I will look for my soil thermometer tomorrow and do some tests at various times over the next few weeks. The ground is currently damp and may be dry next week. That should give me a range of values. I have an undisturbed area where I was planning on putting another garden bed next year where I could dig various holes for the tests now. That may determine if, or how much, I need to insulate under the block foundation walls and under the pavers that will be the floor of the greenhouse. In addition I was thinking of using water for thermal mass. That will have the advantage of being able to change the amount of thermal mass between winter and summer.

Thanks again,
Anna

Anna,

You are welcome. Our different circumstances do give us different viewpoints for greenhouse energy management. In my case here in Maine, I am solely concerned with energy conservation during cold and cool weather. How warm the greenhouse gets during the summer doesn't concern me. Our summers are quite mild, and a lot of tourists come here for that reason, so I don't expect our greenhouse to overheat. But if it does, I will simply open the vents and doors. I don't think we will be growing anything in there in the summer, anyway.

In your case, dangerous overheating during the summer is a possibility. You seem to be taking a very reasoned approach to your greenhouse planning, and that is a good thing.

MM

Do I understand right that the best way for a floor to hold the warmth in winter is to first put down as much insulation as you can then gravel the top of that?

Which kind of insulation is best to put on the ground for the floor?

Thank you

pcdur,

"Do I understand right that the best way for a floor to hold the warmth in winter is to first put down as much insulation as you can, then gravel the top of that?"

Yes. The more insulation, the better, within practical limits. I haven't bought the foam yet, but right now I'm thinking in terms of maybe a foot of foam insulation if I can afford it. Maybe more.

First I'm going to level the bottom of the excavation with an inch or so of sand. Then I will lay in some hardware cloth as a deterrent to rodents. Then I will put in several inches of foam insulation, probably in more than one layer.

"Which kind of insulation is best to put on the ground for the floor?"

I'm not sure which kind is "best" because budget may be a factor and 12 inches of cheap polystyrene foam insulation may be better than 6 inches of higher quality insulation like polyurethane foam (PUR) or polyisocyanurate foam (PIR). Depending on what is available locally and how much it costs, I may use one, two, or all three kinds of foam. I will probably put the cheapest foam on the bottom layer, but I haven't worked out the specific details of the foam on my project. I'm sure there will be some trade-offs between quality and price.

I may add some more hardware cloth over the top of the foam as a second barrier to rodents. I may lay some landscaping cloth over that. The landscaping cloth is optional, but I have some left over from another project. Next comes a layer of gravel or crushed rock to act as thermal mass. I'll be using some crushed granite from a pile that we have on hand. I will lay concrete pavers on top of the crushed granite because some of it has somewhat sharp edges. Crushed granite definitely wouldn't feel good to bare feet. The concrete pavers will also act as thermal mass.

I suspect that there are a lot of houses or basements with cold concrete floors because the builders skimped on foam insulation under the concrete.

MM

MM,

Thanks very much for the detail of the project, sounds like a great plan.

Charles

I put Polystyrene - Extruded (XEPS)***under my floor then added several inches of sand then concrete paver's on the floor. The paver's make a nice floor and also a good thermal mast to help stabilize the temp
To learn about insulation values check out the link that I have provided below. This will make you a more informed shopper when you go shopping for insulation.
When comparing remember the higher the R- number the better the insulating value.

Here is a link that might be useful: Insulation Values

gardenerwantabe,

Thanks for mentioning Polystyrene-Extruded (XEPS) and the link to the insulation data. I saved the link for later reference.

MM