Thermal mass for under greenhouse

The plan for my 20’ by 9’ lean-to greenhouse is to put insulation on the perimeter and below, then lay down PEX tubing for future radiant heat. What I’m trying to figure out is A: how thick the thermal mass should be, and B: what it should be composed of.

I want the greenhouse to buy me an extra month at the beginning of our short season. It doesn’t sound like much but here in Alaska it can make a huge difference. This pretty means the month of April with plenty of sunlight but dipping below freezing most nights.

Is there a calculator for this sort of thing?

Your frost line may be to deep for this. I’m planning on digging down to stable temperature and laying pipe. Have the pipe run to a pump and a truck radiator/fan combo. Nice cheap geo thermal heating. Not much electric either. Might could run on solar.

As a stand alone it would be too much but with one long side leaning to the house the heat dissipation from the house can remove the frost line right under said wall. 4" of rigid insulation on the perimeter of the greenhouse and underneath should slow down heat transfer enough where the thermal mass still can work.

Plus keep in mind that I don’t plan on heating the greenhouse in the middle of winter when it can be -20f outside, the plan is to bring it back alive when air temperatures are in the 40’s during the day, probably in the 20’s at night. Im thinking that once that thermal mass starts warming up that it won’t take a whole lot to keep it from freezing at night.

Got a spare wood stove? Any kind of barrier you put over the bare ground will help. Not going to be easy to raise temps where you are at.

I don’t think the parameters I’m shooting for are that unattainable. Heck by March I’m sure my worry will be to keep the greenhouse cold enough for the things overwintering there. It is a solid southern exposure that gets lots of sun. Heck when the sun is shinning the house heater doesn’t even kick in.

Another neat thing about running PEX for radiant heating is that I can run it as two zones; one for the green house thermal mass, one for the cold frames in front of it. Just thawing the soil there will let me setup cuttings and seeds for propagation.

We looked into this and there are is some info out there on this. Have you looked into what they are doing up north growing late into the season. You will likely be able to stretch your growing season in the fall easier than in the spring.

In specific I working on semi bulk scale propagation techniques of perennials, edible fruits in particular. On a good year our ultra short season can be just 5 months. That means pretty much nobody propagates up here, close to everything gets shipped from the lower 48 and Canada. Towards that goal adding a month at the beginning is a lot more useful and offers better control, as I don’t need an extended season to ripen anything. Well when the greenhouse is sorta empty in the summer I do plan on doing me some tomatoes and peppers but that’s just a crime of opportunity.

Heck haskaps are very promising. I can start bundles of cuttings in March on a heat pad select most viable ones and put them on nursery voxes in April thanks to the greenhouse, then bareroot them in the fall for winter storage. Next April they can be potted and kept in the greenhouse to fatten for June timeframe sale.

Sounds like too much to me unless you need warm soil. Just to gain a month in April with nights in the 20s can easily be done with any small heater. I’d be thinking two months with that good exposure and protected on one side by the house. It does depend on how much sunshine you get. Even that location won’t warm much with heavy clouds. But any kind of sun and it should warm nicely even in March.

A month may not sound like much but where I am, that is a 20% increase to my growth season. It is a significant increase. Plus I’m going to be laying insulation anyways, so mostly I want to put more thought on building a thermal mass that can be directly heated.

Let me put it this way: people put 55-gallon drums full of water because of the thermal mass effect they have. That’s a bit over 7 cubic feet of mass. A 1’ thick base that is insulated below and on the sides would hold 180 cubic feet of thermal mass. There has to be a way I can put this to a good use.

Sounds like a nice swimming pool…!! Hey just kidding. Next to water, gravel or stones would be an option for thermal mass. But I’m not sure how that helps you in early spring. Coming out of winter a big thermal mass is just something that takes a lot of heat to warm up. In fall a warm mass of stones or water would help lengthen the season. In summer it could keep nights a little warmer.

That’s where the PEX comes into play. The greenhouse will by it’s very nature heat up the soil under it, insulation will help to stop that heat from being dissipated away, and the PEX should let me top off the mass temperature as needed. Heck with a bit of luck that “as needed” will be minimal. Once I initially bring it up to working temperature sunlight warning the greenhouse during the day may be able to do most of the work.

I’m propagating Alaska hardy perennials, most of them consider nights Above freezing warm enough.

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