Unusual greenhouse?

This is my first full winter with my new greenhouse. I designed it with some specific problems in mind, particularly temperature control. I've understood from conversations with various people who have greenhouses that they're pretty much unusable during the summer because of heat.

My greenhouse is entirely passive and was designed with a couple of features that seem to be working well. It's small, only 9x11, with a door on either side in the direction of the prevailing winds. It has a solid block wall on the North, backed by an existing hill. I created a waterwall as well.

The other feature is that I sloped the roof to the North rather than the South. During the winter the sun hits the walls almost head on while during the summer the higher sun won't hit the roof directly.

So far these features appear to be working. When there's the slightest wind and both doors open the greenhouse is no warmer than the surrounding air. If there is no wind, the greenhouse still doesn't get much warmer than the outside with both doors open. Last winter it never got below 26 inside (manual measurements just before dawn) and that was when the temps outside were in the single digits. This summer it was actually warmer inside when the sun started to lower in the last month, so I can assume the roof part is working but I have no real information.

My problem is that I don't know of anyone else who has built this way so I have nothing to compare to. Can anyone give me any first hand information about the effectiveness of the backward sloping roof, thermal mass, or the waterwall?

Hi Lauren, that's a great greenhouse!  

Sloping the roof to the back is not unheard of in passive solar greenhouses.  Burying them into a hill is also commonly recommended so it's great you were able to.  Same goes for water walls.  From what I've read, the larger the water container, the longer it takes to heat up and cool down.  I've heard 1 gallon ones heat and cool in a day while 55 gallon drums may take several days to heat up and cool off.  

In your climate, I'd think the smaller jugs would work well.  Unless you get lengthy inversions in which case you may need bigger water jugs.

None of this is based on my direct observations, just from reading many books on greenhouses

Can I ask some questions that may lead to some possible suggestions?  
What is the roofing material?  Does snow sit on it all winter?
Can you pile snow around the base on all four sides to insulate the ground a bit?
If the block wall behind it has open cores (square holes), they may be allowing cold air to travel within the wall, cooling it down and reducing its helpfulness.  
Could you insulate the North wall?  You probably don't get any sunlight though it so it is a wall that gives no solar gain and a fair amount of heat loss.

Mike Jay wrote:
What is the roofing material?  Does snow sit on it all winter?
Can you pile snow around the base on all four sides to insulate the ground a bit?
If the block wall behind it has open cores (square holes), they may be allowing cold air to travel within the wall, cooling it down and reducing its helpfulness.  
Could you insulate the North wall?  You probably don't get any sunlight though it so it is a wall that gives no solar gain and a fair amount of heat loss.

The roof is the same polycarbonate as the walls. Snow collects, then slides right off toward the back. Unexpectedly, this creates another layer of insulation as the snow piles up hanging off the edge.
The polycarbonate material is buried in the ground by about 6 inches (the panels were 8 feet, and rather than cutting them I buried the bottom few inches). The inside is filled (this year) with compost, with a layer of soil up to the top of the foundation blocks. I'm hoping the compost will create another source of heat.
The block literally has a hill behind it. The soil goes up to the level of the top of the blocks, then the space between the roof and the ground is filled with leaves. The blocks were filled with concrete a long time ago so there should be no drafts there.

I spent a good part of last winter (the greenhouse was finished at the end of November) chasing down the drafts and plugging holes. I have also put in 2 55 gallon drums in the back corners this summer. That was planned, but I had to wait until spring to take care of it so it's not in the picture.

Any suggestions you have would be appreciated. In all my searching I hadn't found any backward sloping roofs on greenhouses, and most were trying to increase heat absorption rather than limiting it.

First off, you're already doing great.  A few thoughts for ways to boost your temperatures this winter would be:

Try to hold the snow on the roof during the coldest part of winter when the sun is low.  Unless it blocks a bunch of light, the insulation would help hold in a ton of your heat.  If it would block too much sun, maybe still try to insulate the North half.  Of course you could try removable insulation that you put on in winter and remove in summer instead of relying on a layer of snow.

Digging down the 6" around the perimeter and adding some underground insulation would help keep the cold ground outside the greenhouse from transmitting under and into your greenhouse.  I'm putting styrofoam in a "Swedish skirt" arrangement so it goes down a foot and then horizontally out 3 feet.  I have a 4' frost depth in my area so that will keep it from encroaching under my greenhouse.

I like the compost idea and will be trying it myself.  I think the recommended minimum size for it to generate good heat is 3' by 3' by 3'.  I'm guessing a smaller volume would still work but need more tending and quicker addition of new material.  One key is for the compost to be in a lump/ball/cube/pile.  If the same volume is spread around the floor 4" deep, while it may be the same amount of compost, it won't be able to feed off its own heat to get composting.

Mike Jay wrote:First off, you're already doing great.  A few thoughts for ways to boost your temperatures this winter would be:

Try to hold the snow on the roof during the coldest part of winter when the sun is low.  Unless it blocks a bunch of light, the insulation would help hold in a ton of your heat.  If it would block too much sun, maybe still try to insulate the North half.  Of course you could try removable insulation that you put on in winter and remove in summer instead of relying on a layer of snow.

Digging down the 6" around the perimeter and adding some underground insulation would help keep the cold ground outside the greenhouse from transmitting under and into your greenhouse.  I'm putting styrofoam in a "Swedish skirt" arrangement so it goes down a foot and then horizontally out 3 feet.  I have a 4' frost depth in my area so that will keep it from encroaching under my greenhouse.

I like the compost idea and will be trying it myself.  I think the recommended minimum size for it to generate good heat is 3' by 3' by 3'.  I'm guessing a smaller volume would still work but need more tending and quicker addition of new material.  One key is for the compost to be in a lump/ball/cube/pile.  If the same volume is spread around the floor 4" deep, while it may be the same amount of compost, it won't be able to feed off its own heat to get composting.

I think we got three good snowstorms last year, so relying on the snow for insulation wouldn't work. The space was noticeably warmer after a snowstorm, though. During the winter the sun doesn't really hit the roof at all, so the question would be how to insulate it without affecting runoff. The water from the roof feeds into my nascent forest garden behind the wall and it needs all the water it can get.

So the panels won't be sufficient insulation as far as the cold ground? I just had a thought, though--last year the spaces between the foundation blocks were completely open to the inside. With the compost and soil added, there's an air buffer. Not completely sealed, but I'm not sure it would have to be. The inside "ground" level is lower than the outside by that 6 inches, so as at least a stopgap the air buffer might be sufficient. I'll have to think about the underground insulation idea, figure out how it could be done.

The compost is a layer of wood chips and pine needles, covered with 2 - 4 inches of garden soil up to the top of the blocks that make up the foundation. Since I want to actually grow in this space it was essential to improve the soil (straight sand and rock) and this seemed the simplest. Any additional heat will be a bonus, but I can hope. The greenhouse really isn't large enough for a traditional compost pile or bin. In future years I may do a rotating compost pit, filling it with everything organic I can find and covering it for the winter. Not this year, though.

Since this is my first full winter with the greenhouse, I'm still learning what it's capable of.

Ok, so it looks like removable roof insulation could be the ticket.  The issue is that for it to work best, it would be nice to somewhat seal it to the roof panel.  The waves at the ends make that a challenge.  Does the place you got the wavy panels also sell foam wavy insulation pieces to seal the ends with (Like This)?  If so, you could pick up some of them to seal the ends.  Then put some 2" sheets of pink styrofoam on top of the roof.  That way the styrofoam would give you an R10 (instead of the approximate R1 you currently have).  The sealed air gaps would add some R value and the panel would still give its R1 so you'd be up around R12.  

The downside it that it would look really funny.  The other common styrofoam product is expanded polystyrene which is made up of compressed white beads but critters like to eat/tunnel in it.  Maybe that's not a problem in the winter?

Or you could go with the white and hold it on from the inside instead.  In either case you'd only need to cover as much of the roof as you want.  More is better but it might feel like a cave.

Another seasonal option would be to get some greenhouse plastic or bubble wrap and wrap that around the roof (and sides?) to give you a second layer of glazing.  That would bump up any surface you put it on from R1 to R2 (doubling you heat holding capacity).

I suspect the panel underground isn't actually blocking much heat.  Most materials get their R value from the air that is stationary right up against the material.  So a single pane window is R1, mainly due to the film of air on either side of the glass, not due to the awesome insulation value of the glass itself.  So when you bury it and eliminate that air layer against the material, that insulation goes away.  Styrofoam on the other hand, has the air trapped within it (so does bubble wrap, fiberglass insulation and other insulation materials).  If you buried a piece of pink styrofoam (white isn't recommended underground) it would definitely hold the heat out.  It could be put inside or outside the wall as long as it's quite close to the wall.  Maybe even easier (and better?) would be to get that same wiggly foam seal stuff for the panels.  Dig away 4" of dirt around the greenhouse (4" deep, 2' wide).  Put a piece of the foam stuff in the wiggles of the panel and then lay a 2' piece of pink styrofoam in the bottom of the trench.  That way you'd have 2' of insulation, including in the waves of the buried panel.  Cover it back up with the dirt.  Now the insulation goes 2' out from the greenhouse (instead of down).  It still holds the frost away because now the cold needs to travel under the insulation and 2' before it even gets to the edge of your greenhouse.

Thanks. Any roof insulation would have to be on the inside because of the wind. You've given me a lot of ideas that I'll have to think through before I make any decisions. I appreciate your help.

Can you direct me to anything online that deals with the backwards roof? I'd like to see how it works for others.

I can't think of any online sources for backwards sloping roofs for greenhouses.  I'm sure they're out there but I don't know of them off hand.  I think I've seen it in books, maybe check your library?  

Good idea. Thanks again for your help.

Anyone done the math on the effectiveness of water jugs for regulating temperature? (Or has some empirical data to share?)

Here's a thread on the topic of thermal mass: Thermal mass for hoop house

I don't have any data to share but I've read a fair bit about it.  From what I recall, drums collect and distribute heat over the course of days, 5 gallon buckets do so over the course of a day or so and gallon jugs are over the course of hours.  So the time span that they need to be able to help your greenhouse partially dictates their size.  Painting them black or a dark color helps them absorb heat, putting them in the sun also helps.

Jerome Lee wrote:Anyone done the math on the effectiveness of water jugs for regulating temperature? (Or has some empirical data to share?)

I only have last winter's measurements, and no real comparison to when the greenhouse didn't have the waterwall. The greenhouse went up toward the middle of November and the waterwall was completed toward the first of December. Before that point the average pre-dawn temperature difference (remembering that for part of this time the glazing wasn't secure) was around 7 degrees. After that point the difference ranged but the average was 12 through the end of the year. Temps were variant depending on the outside temperature--the higher the outside temperature the lower the difference between inside and outside. Down into the low thirties, the pre-dawn difference was generally around 10 - 15 degrees for this period. Get up into the 40's and 50's and it was around 7ish.

On 12/6 (the day after I finished the sealing) the pre-dawn difference was 15 degrees. It remained in the 11 - 15 range, inside temperatures hovering between 29 and 32 degrees, for the rest of the winter, with the exception of the nights when the outside temps got into the single digits. That was when the temperatures dropped below 29, and 26 was the lowest I measured.

I can see that you sprayed a good bit of foam in the roof, but I would triple check for air leaks.  It is amazing how much heat can escape like a chimney if you have a tiny opening in the floor and roof. Light up a bunch of incense inside and see what happens.  Good work!

Mike Jay wrote:Here's a thread on the topic of thermal mass: Thermal mass for hoop house

I don't have any data to share but I've read a fair bit about it.  From what I recall, drums collect and distribute heat over the course of days, 5 gallon buckets do so over the course of a day or so and gallon jugs are over the course of hours.  So the time span that they need to be able to help your greenhouse partially dictates their size.  Painting them black or a dark color helps them absorb heat, putting them in the sun also helps.

I just got around to reading the post you shared above. It looks like most people's experience with greenhouses is very different from the results I'm getting. 3 - 5 degrees above outside seems really low based on my experience last  year. According to the University of Missouri calculations in the article (https://bradford.missouri.edu/passive-solar-greenhouse/) mentioned in that thread, I should need about 500 gallons of water for season extension (area of glazing * 2.5, or 2.5 gallons per square foot of glazing. Double that if you want to grow year-round) and I have about 250 total including the two big water barrels which weren't there last year. About 150 gallons last year, mainly in the gallon jugs.

Which makes sense of some questions I had with the patterns developed by my records last winter. The water in the greenhouse wasn't enough to keep the temperature up into the 40's or 50's when outside temps were below freezing, but it was enough to keep it right around freezing (between 29 and 31 degrees) until the outside temps got down into the mid teens. Too cold to grow most things, but enough to extend the season by two months last spring and also to make it a better environment for some of the things that don't survive outside. With almost double the amount of water, I expect that the temps will be even more stable this winter.

Because of the backward slanting roof the temps inside pretty much matched the outside temps until late August, when the sun started hitting the front wall again. I assume that is also in part because of the water mass.

If I can work it right and get the right setup in the greenhouse, I should be able to grow until November and start again in March.

Nice design! I have a few comments, haven't read all the others in detail, just skimmed them, forgive me if I duplicate info...  

I gardened in New Mexico for 30 years, and never did that kind of greenhouse, but have a good grasp of how heat mitigation is more important sometimes. The backslant roof is lovely! I'd question if your roof water run off is going someplace you don't want it, check for whether is ending up down your back wall, making it weirder to deal with heat loss on the north wall. The north wall could be totally solid, no panels at all (which gives you swap out panels for later.)

You might be able to snag some ideas from this concept, and maybe some of their math Chinese Passive Greenhouse

The water in the greenhouse math... I asked a related question here at one point and got some good responses Graduated thermal mass in greenhouse including some data from Tony Paul Martin (about halfway down the thread) that he has in a PDF. You might find that interesting.

And one more thought: I'm a dumpster diver/recycler type. The bottles you have water in are going to sun damage fast in your climate. If you don't want the floor wet by them when they leak, make arrangements for where the water will go. I really wish they made those out of better plastic for those of us who reuse them, they don't, so mitigate the damage that they might cause. That thread of mine above had a suggestion to put the small thermal masses right by the plants, that may be a useful thought for you too. I'm using Folger's coffee cans for my smaller thermal masses, they are tougher plastic and stack well.

Lovely work! Looks beautiful!! Keep us posted on how it does this winter :)

Milk jugs with water will eventually spring leaks possibly!

Twin wall & triple wall polycarbonate panels are also available. Increases solar gain horsepower.

source 1

source 2

Pearl Sutton wrote:I'm a dumpster diver/recycler type. The bottles you have water in are going to sun damage fast in your climate. If you don't want the floor wet by them when they leak, make arrangements for where the water will go. I really wish they made those out of better plastic for those of us who reuse them, they don't, so mitigate the damage that they might cause. That thread of mine above had a suggestion to put the small thermal masses right by the plants, that may be a useful thought for you too. I'm using Folger's coffee cans for my smaller thermal masses, they are tougher plastic and stack well.

Lovely work! Looks beautiful!! Keep us posted on how it does this winter :)

Somehow I missed this comment.

There is a 1.5 ft overhang on the back of the greenhouse roof, which theoretically puts the water far enough from the wall, but that is something I'm keeping an eye on. The back wall is block, filled with cement. It's actually significantly warmer toward the back wall than the front.

I check the bottles about every week and any that start to show leaks are replaced. Those that are leaking go by the plants for additional water and then I cut the bottoms off and store them for plant covers in the spring. Some of the plants have bottles around them for extra protection and a few (I'm attempting to over-winter sweet potato) have a cloche.

Temperatures are ranging lower this year than last, which I think is primarily due to lower temperatures outside and more overcast days, but interior temperatures are still staying above freezing the majority of the time. Based on the existing data, I'm seeing a possible March 1st last frost date inside the greenhouse. Soil temperatures are apparently staying at least ten degrees above the air temp, because some of the seeds I put down are sprouting (cilantro, which is supposed to sprout when soil temps are between 45 and 50 degrees). The lowest temperature measured in the greenhouse to this point (20 F when the outside temp was 4) puts it in approximately zone 8.

   My father has a greenhouse indoor year around for 60 years.  He tore it apart showing a yellow foam. With screen at the bottom and of course the wooden frame.  

    Does anybody have any idea what that foam is, where do purchase it, and what it is used for? Google does not have anything I can see.   Thank you for looking and your knowledge.

Tammie