Small 6x8 greenhouse heating?

 I recently purchased a small 6x8 polycarbonate greenhouse from Harbor Freight which I hope to extend my growing seasons with cold crop's in containers.
I’m not keen with using an electric heater and been researching thermal mass which is done mostly with barrels of water which take up space and freeze when depleted.
The most promising thing I found was Finland’s sand battery which I believe I could incorporate in the ground below the greenhouse.
So here is my idea:
Dig out the whole 6x8 area approximately 2 feet down, lay foam board on the bottom and sides, pour 6” of sand, lay 36 volt heat cable, lay another 6” sand, lay 3” or 4” pipe across 3 times, lay another 6” sand, lay more heat cable and another 6” sand then top with pavers. I will also insulate the north wall.
The 2 heat cables I wish to connect 2x 250 or 420 watt 36 volt solar panels and the 4” pipes will come up above the pavers to use a fan to pull warm air up if the floor radiant heat isn’t enough.
With the greenhouse being so small this is the best thing I could possibly hope will work without taking up valuable space and won’t get damaged when the deep freeze happens. I understand this probably won’t heat enough during January and February.
Any advice or suggestions?

I'm not sure of your pv solar will be enough to make much of a difference.
I'm not saying it won't work, I'm just think it might not be adequate.
Are the tubes there to move daytime heat into the thermal mass?
Is there an advantage to using sand over using soil?

Jim Griff wrote:  I recently purchased a small 6x8 polycarbonate greenhouse from Harbor Freight which I hope to extend my growing seasons with cold crop's in containers.
I’m not keen with using an electric heater and been researching thermal mass which is done mostly with barrels of water which take up space and freeze when depleted.
The most promising thing I found was Finland’s sand battery which I believe I could incorporate in the ground below the greenhouse.
So here is my idea:
Dig out the whole 6x8 area approximately 2 feet down, lay foam board on the bottom and sides, pour 6” of sand, lay 36 volt heat cable, lay another 6” sand, lay 3” or 4” pipe across 3 times, lay another 6” sand, lay more heat cable and another 6” sand then top with pavers. I will also insulate the north wall.
The 2 heat cables I wish to connect 2x 250 or 420 watt 36 volt solar panels and the 4” pipes will come up above the pavers to use a fan to pull warm air up if the floor radiant heat isn’t enough.
With the greenhouse being so small this is the best thing I could possibly hope will work without taking up valuable space and won’t get damaged when the deep freeze happens. I understand this probably won’t heat enough during January and February.
Any advice or suggestions?

the best solar electric panel can transform less than 30 percent of the light that hits it into electricity coupled with losses in transport it's not a lot of heat. Solar thermal on the other hand can capture most of that energy as heat. You could deliver it via water. You could also look into walapini greenhouses which use the already there heat of the ground...

William Bronson wrote:I'm not sure of your pv solar will be enough to make much of a difference.
I'm not saying it won't work, I'm just think it might not be adequate.
Are the tubes there to move daytime heat into the thermal mass?
Is there an advantage to using sand over using soil?

Yes I was thinking of using the tubes to help push down hot air into the sand. I’m only contemplating on if I should use metal or copper pipe versus PVC or plastic.
I’ve seen a lot of videos where sand held got hotter and held heat longer than water which leads me to believe that probably the dryness helps. Regularly soil retains a lot of water.

Any advice or suggestions?
the best solar electric panel can transform less than 30 percent of the light that hits it into electricity coupled with losses in transport it's not a lot of heat. Solar thermal on the other hand can capture most of that energy as heat. You could deliver it via water. You could also look into walapini greenhouses which use the already there heat of the ground...
I have seen using the ground itself to transfer heat but most of them are about 4 feet deep. I don’t believe I can get that deep because of an old tree stump roots near by. I also can’t get any machinery in this area to dig that deep. I can only hand dig the 6x8 area.
I don’t want to use a water heat system because of the possible chance of freezing and breaking the pipes.

Hi Jim;
I am aware of Finland's sand heat battery, it is very cool BUT on a much larger scale than you can create in your space.
On a small scale when using an RMH, it has been proven that using sand as your mass is counterproductive.
Sand is a great insulator, with tiny little air pockets it holds its heat, unlike a solid mass that shares its heat.
You want your heat in the greenhouse not down in the sandbox.
Not all sand is created equal, superfine sand from a dune would share heat better than purchased a course-bagged sand.
Large flat rocks with soil or clay filling all air pockets will share the heat in the greenhouse much better than any sand.
Small round rock (pebbles) only works well with no air pockets (Clay)

Perhaps, if the solar will power your heating strips, your buried pipes may contain enough heat that you can move it up and into the greenhouse.
If it works well, please keep us posted.

Given your parameters, a solar solar air heating collector attached to your greenhouse might be the way to go.
This would collect solar thermal thermal energy from a larger area and pump it into your greenhouse.

Jim Griff wrote:

William Bronson wrote:I'm not sure of your pv solar will be enough to make much of a difference.
I'm not saying it won't work, I'm just think it might not be adequate.
Are the tubes there to move daytime heat into the thermal mass?
Is there an advantage to using sand over using soil?

Yes I was thinking of using the tubes to help push down hot air into the sand. I’m only contemplating on if I should use metal or copper pipe versus PVC or plastic.
I’ve seen a lot of videos where sand held got hotter and held heat longer than water which leads me to believe that probably the dryness helps. Regularly soil retains a lot of water.

As usual Thomas has put his finger on it. Creating a heat absorbing mass in the greenhouse which captures sun directly is a great place to start. I would bury hollow pipes in it myself to maybe add heat from a hot air collector built separately from the greenhouse. Cheaper more efficient and less prone to break down. And an RMH down the line maybe... One system adding on to the others until you hit a balance

It would help us respond if you gave us some indication of your weather cycles and latitude so we know generally what your sun and high/lows are likely to be.

Books I have read, suggest that it is very hard to control both cold and heat in that small a greenhouse, so it is really good that you're planning up front.

1. I would dig as deep as you can and put in a really good foundation with insulation on the outside. If your thermal mass isn't insulated from the rest of the world, it won't be as effective.
2. Tree stumps... been there done that. I *really* recommend you beg/borrow/buy a powerwasher. You can use it like a cutting tool to get the dirt away from the roots and to clean the roots enough that your saw blade won't be instantly dulled by cutting them off. Seriously - it can be done, and work up front will improve your odds of not ending up with a tool shed instead of a greenhouse. Even in my climate, I see way too many abandoned greenhouses, and it's usually because they're too much work to keep the extremes of heat/cold from freezing and cooking everything.
3. I'd listen to Thomas Rubino - he knows his stuff! Some sort of clay/rock mix with pipes through it, and your solar only has to power a fan to blow too hot air down and too cold air back up. Judging from deserts I've read about, sand gets *really* hot during the day, and then cools off *really* fast at night. I think you're looking for something more gradual.
4. This is from some of Rob Avis' work:

I think in your small space it would still help, but the greenhouse in Northern BC that uses a system like this, is much larger, and I don't know how well it will scale down.
5. Yes - not only can you insulate your north wall, but *many* greenhouse books encourage insulating some or all of the east and west walls also. The west wall is a particular risk in causing the "over heating in the afternoon" syndrome. This is less true if your climate has a lot of cloud cover as the clouds bounce light around more. However, having panels that you can add/subtract easily at different times of the year could be considered.

thomas rubino wrote:Hi Jim;
I am aware of Finland's sand heat battery, it is very cool BUT on a much larger scale than you can create in your space.
On a small scale when using an RMH, it has been proven that using sand as your mass is counterproductive.
Sand is a great insulator, with tiny little air pockets it holds its heat, unlike a solid mass that shares its heat.
You want your heat in the greenhouse not down in the sandbox.
Not all sand is created equal, superfine sand from a dune would share heat better than purchased a course-bagged sand.
Large flat rocks with soil or clay filling all air pockets will share the heat in the greenhouse much better than any sand.
Small round rock (pebbles) only works well with no air pockets (Clay)

Perhaps, if the solar will power your heating strips, your buried pipes may contain enough heat that you can move it up and into the greenhouse.
If it works well, please keep us posted.

Yeah I should try a few small experiments with course sand, fine sand and sand mixed with gravel.
👍

David Baillie wrote:

Jim Griff wrote:

William Bronson wrote:I'm not sure of your pv solar will be enough to make much of a difference.
I'm not saying it won't work, I'm just think it might not be adequate.
Are the tubes there to move daytime heat into the thermal mass?
Is there an advantage to using sand over using soil?

Yes I was thinking of using the tubes to help push down hot air into the sand. I’m only contemplating on if I should use metal or copper pipe versus PVC or plastic.
I’ve seen a lot of videos where sand held got hotter and held heat longer than water which leads me to believe that probably the dryness helps. Regularly soil retains a lot of water.

As usual Thomas has put his finger on it. Creating a heat absorbing mass in the greenhouse which captures sun directly is a great place to start. I would bury hollow pipes in it myself to maybe add heat from a hot air collector built separately from the greenhouse. Cheaper more efficient and less prone to break down. And an RMH down the line maybe... One system adding on to the others until you hit a balance

I look into solar hot air collectors a few years back and was very tempted to get one for my house until I seen how the duct values were thin and not very well sealed. It’s a great concept that I may consider down the road if pv solar on heating cords doesn’t work. I will have the hot air pipe in the sand as well.

Jay Angler wrote:It would help us respond if you gave us some indication of your weather cycles and latitude so we know generally what your sun and high/lows are likely to be.

Books I have read, suggest that it is very hard to control both cold and heat in that small a greenhouse, so it is really good that you're planning up front.

1. I would dig as deep as you can and put in a really good foundation with insulation on the outside. If your thermal mass isn't insulated from the rest of the world, it won't be as effective.
2. Tree stumps... been there done that. I *really* recommend you beg/borrow/buy a powerwasher. You can use it like a cutting tool to get the dirt away from the roots and to clean the roots enough that your saw blade won't be instantly dulled by cutting them off. Seriously - it can be done, and work up front will improve your odds of not ending up with a tool shed instead of a greenhouse. Even in my climate, I see way too many abandoned greenhouses, and it's usually because they're too much work to keep the extremes of heat/cold from freezing and cooking everything.
3. I'd listen to Thomas Rubino - he knows his stuff! Some sort of clay/rock mix with pipes through it, and your solar only has to power a fan to blow too hot air down and too cold air back up. Judging from deserts I've read about, sand gets *really* hot during the day, and then cools off *really* fast at night. I think you're looking for something more gradual.
4. This is from some of Rob Avis' work:

I think in your small space it would still help, but the greenhouse in Northern BC that uses a system like this, is much larger, and I don't know how well it will scale down.
5. Yes - not only can you insulate your north wall, but *many* greenhouse books encourage insulating some or all of the east and west walls also. The west wall is a particular risk in causing the "over heating in the afternoon" syndrome. This is less true if your climate has a lot of cloud cover as the clouds bounce light around more. However, having panels that you can add/subtract easily at different times of the year could be considered.

I live just slightly north from Boston Ma so we do get some roller coaster weather although our winters have be surprisingly mild lately.
I’m not expecting that this thermal mass will work all winter but I would be happy with just extending the growing of colder crops in the early spring and fall.
I like the manifold and was thinking of doing the same but I’m hesitant with using drainage pipes because of not knowing how hot the sand could possibly get off of the heating cords.
I probably will do a few small experiments with course, fine sand as well as sand mixed with gravel. 🤞 I don’t have access to any clay soil unfortunately.
I might try to go deeper but in my location our ground is 40% soil and 60% boulders or ledge. I’ve attempted to put a drainage system years ago and I really struggled with completing it. Unfortunately I’m not as young and strong anymore and not sure how deep I can go. 🤷‍♂️
I appreciate everyone’s input.

🤔 So what I just found is “The denser the material (i.e. the less trapped air), the higher is thermal mass.”
After seeing this chart of density it looks like sand mixed with gravel has more density than sand alone.
I think I will definitely experiment with a small 1  gallon mixture of fine sand mixed with small gravel and another with just fine sand. I’ll probably put them both in a metal bucket, put them in a oven for a hour and see what the temperatures are when they first come out and several readings after to see which one remains hotter longer.
I’ll have to check my local landscape store to see if they sell marble chips and at what cost. I’ll possibly try a sample with that mixed with fine sand.

Jim Griff wrote:I might try to go deeper but in my location our ground is 40% soil and 60% boulders or ledge. I’ve attempted to put a drainage system years ago and I really struggled with completing it. Unfortunately I’m not as young and strong anymore and not sure how deep I can go. 🤷‍♂️

I hear you! I needed to put about 10 feet of fence up to discourage deer. The first 3 posts went in with no problem. The forth went "clunk". I moved it little bits in all directions - clunk, clunk, clunk! I started excavating. By the time I was done, I had to get Hubby to haul out the rock drill and feathers and irons and the rock became 5 rocks, but 2 pieces were still too large for Hubby to get out of the hole without help from our son. I used the rock drill when I was younger, but now that I'm retirement age, I don't heal as quickly, so I'm more reticent to use equipment that powerful.
On the positive side, some of those boulders could be useful. There's a few projects where I'm thinking of making home-style gabions and filling them with all those rocks that are in places I'd prefer they aren't.

Jay Angler wrote:It would help us respond if you gave us some indication of your weather cycles and latitude so we know generally what your sun and high/lows are likely to be.

Books I have read, suggest that it is very hard to control both cold and heat in that small a greenhouse, so it is really good that you're planning up front.

1. I would dig as deep as you can and put in a really good foundation with insulation on the outside. If your thermal mass isn't insulated from the rest of the world, it won't be as effective.
2. Tree stumps... been there done that. I *really* recommend you beg/borrow/buy a powerwasher. You can use it like a cutting tool to get the dirt away from the roots and to clean the roots enough that your saw blade won't be instantly dulled by cutting them off. Seriously - it can be done, and work up front will improve your odds of not ending up with a tool shed instead of a greenhouse. Even in my climate, I see way too many abandoned greenhouses, and it's usually because they're too much work to keep the extremes of heat/cold from freezing and cooking everything.
3. I'd listen to Thomas Rubino - he knows his stuff! Some sort of clay/rock mix with pipes through it, and your solar only has to power a fan to blow too hot air down and too cold air back up. Judging from deserts I've read about, sand gets *really* hot during the day, and then cools off *really* fast at night. I think you're looking for something more gradual.
4. This is from some of Rob Avis' work:

I think in your small space it would still help, but the greenhouse in Northern BC that uses a system like this, is much larger, and I don't know how well it will scale down.
5. Yes - not only can you insulate your north wall, but *many* greenhouse books encourage insulating some or all of the east and west walls also. The west wall is a particular risk in causing the "over heating in the afternoon" syndrome. This is less true if your climate has a lot of cloud cover as the clouds bounce light around more. However, having panels that you can add/subtract easily at different times of the year could be considered.

A little update:
 I was only able to dig down to about 20 inches when I ran into some serious hard like concrete sand/gravel mix that took 10 minutes of chopping with a pick axe for only 1 shovel full. 🥵 I gave up and chose to heighten my greenhouse base using 2x12 pressure treated lumber so I can get a full 2 feet.
 I utilized some old plastic pallets to help hold back dirt and possibly adding extra insulation by having an air gap between the soil and 2 inch foam board that will go over the plastic and 2x12.🤞
 I found someone on FB market place that’s selling 4 in aluminum pipe for $2 per foot which I’m going to pick up in a couple hours. I’m thinking this aluminum pipe should work good for my manifold since aluminum has great thermal conductivity.
I pulled out a lot of granite boulders and I have many more in our flower gardens which I will put back in the hole with the sand thinking that they could hold more heat for longer time. 🤞
 My next step is to put up the greenhouse but leave a few panels out so I can keep rain from getting in the hole while I start insulating and filling it up.
Edit: I added more photos in my next comment below 👇

My few photos didn’t load correctly on my last post so I’m attempting again.

thomas rubino wrote:Hi Jim;
I am aware of Finland's sand heat battery, it is very cool BUT on a much larger scale than you can create in your space.
On a small scale when using an RMH, it has been proven that using sand as your mass is counterproductive.
Sand is a great insulator, with tiny little air pockets it holds its heat, unlike a solid mass that shares its heat.
You want your heat in the greenhouse not down in the sandbox.
Not all sand is created equal, superfine sand from a dune would share heat better than purchased a course-bagged sand.
Large flat rocks with soil or clay filling all air pockets will share the heat in the greenhouse much better than any sand.
Small round rock (pebbles) only works well with no air pockets (Clay)

Perhaps, if the solar will power your heating strips, your buried pipes may contain enough heat that you can move it up and into the greenhouse.
If it works well, please keep us posted.

Unfortunately I tried my best to see if I could get clay but couldn’t find any local suppliers.
 So I did a little bench experiment with coarse sand, fine sand, coarse sand with gravel and fine sand with gravel.
 Of course the fine sand with gravel heat lasts the longest but not by much.
I did more research online and found interesting information that silica sand (pool filter sand) has a much higher thermal conductivity than regular sand. 🤔
This is what I found as comparing the 2…

Regular sand:
0.27-0.34 W/mk  1.8-2.4 BTU*in/ft2.h.F
Silica sand
6.5-12.5 W/mk.   9.6 BTU*in/ft2.h.F

I think I will go to my local pool supplier to buy a bag and do further bench testing with it. It would be quite costly filling the whole 6’x8’x2’ area with silica sand so I’ll try a few tests with different mixtures with regular sand and gravel.

Jim Griff wrote: I think I will go to my local pool supplier to buy a bag and do further bench testing with it. It would be quite costly filling the whole 6’x8’x2’ area with silica sand so I’ll try a few tests with different mixtures with regular sand and gravel.

If it seems to help, I would consider contacting a few pool maintenance companies and see if you can get "used silica sand" from when systems need replacing or upgrading. I don't think people replace it often, but it might not hurt to ask?