Hi
Theres no easy answer, it depends on many things.

A good place to start: How much heatloss you expect the gh to have, how much sun you get, how you plan to get the heat into and out of the water (surface area plays a part here).

To give you some idea of how much water, heres a comparison; the top 12" of your gh floor (18x26 ..assuming its soil) will hold (or lose) close to 11kw of heat per 1 degF of temperature rise (or fall).
The thermal mass of the floor is roughly equivilent to 40 x 55gal barrels full of water :)

Sarah,
As hex mentioned, you first need to figure out your heat loss. The attached link has a useful calculator. Once you know your heating needs, you can calculate how much thermal mass you would need. A very general "rule" given in several solar heating sites recommends 4 gallons of water for heat storage for each square foot of floor space. That would be about 468 gallons for your structure. But as mentioned earlier, the needed amount would depend on sun, etc. I started with only one gallon/ ft., which did very little. Then 2 gallons, now I am up to 3 gal. and am finally seeing a little difference.

Here is a link that might be useful: heating calculator

haha I just emailed that link to my dad. that ought to keep him busy for a while.

From reading past posts it seems that the answer isn't the thermal mass but how it is managed. People who have used water containers and measured temperatures admit that most of the stored heat is released during the early evening hours, when it is not needed. By morning the water temperature is often at outside temperature with the exception that will "resist" freezing once 32F is reached.

So if you want to maintain a higher temperature when you need it in predawn hours you will need to hold the warmed water in insulated containers until that time. This would usually require some type of heat exchange device, possibly as simple as a pump and a loop of pipe.

Ideally the tank wants to be in its own heavily insulated building :)
Even a well insulated greenhouse loses a fair amount of heat at night so you`ll definitely need a big tank.
The problem is a large mass takes a lot more energy to charge.
There are only so many hours of sun per day so whatever you use to collect the energy has to be pretty big too (area wise)..otherwise you won`t be able to store enough energy in the mass to make it through the night :)

I tried the thermal mass thing when I built my little 10x12 and after one winter I abandoned the idea.

For it to make any real difference you need to fill the GH with barrels of water and as has already been pointed out that will result in over heating in the early evening hours and doing nothing in the early morning when it is needed.

I decided that I wanted to use my small GH to grow seedlings not store water so I abandoned the water idea.
The only way this would be worth the effort would be with a very large tank and a heat exchanger but it would require a lot of sun to replenish the heat in the water.

Thermo mass dependence is difficult in a small GH.

Most GH thermo mass calculations do not take into account one important factor, ie wind speed (Reynolds number).

dcarch

The best idea is to use glazing only when its light and R30 insulation in its place at night.
In the dead of winter there may be 6 hours of daylight, glazing is completely useless for 18 hours a day except for losing large amounts of heat :)

Hex, Following up on your glazing/ R30 idea, here is a photo of using straw mats to cover the glazing at night. The citrus greenhouse are 2 meters below ground, covered with a poly roof and that is covered with straw (if you add enough, it would be R30) during the night.

{{gwi:286455}}

Hi cuestaroble
Thats probably the cheapest way to do it,if you don`t mind digging :) Ideally you`d want full bales over the top at night to get the ~R30 value.
It would be quite a challenge to retrofit a normal gh with reconfigurable night insulation of a similar value..
6" styrofoam or 4" iso type rigid board.

Cutting the gh heating bill by maybe 80% is a pretty good incentive to try ;)

Insulation at night?

All I am going to say is "Bubbles" and not bubble wrap!

That would be my number one priority experiment if I could get my act together.

:)

Bubbles would work and have a lot of fringe benefits too but you`d have to make certain it was a 100% reliable system.
If something broke down or you had an extended power outage on a winters night your plants would be in a whole heap of trouble :)
Conventional insulation wouldn`t give you the fringe benefits of bubble insulation,(or the ongoing running costs) but having no moving parts.. it is completely reliable.

The wow factor of bubble insulation is hard to beat though :)

How about this idea?
"Pellets Pumped Between Walls"
"For most climates, over 75 percent of all supplemental greenhouse heating is required at night. Further, studies in Ohio and Japan indicate that a polystyrene pellet nighttime insulation technique could reduce greenhouse nighttime energy requirements by 80 to 90 percent. Five inches of pellets are pumped between the walls of a double wall greenhouse at sundown and removed at sunrise for a nighttime insulation value of R = 20."

Here is a link that might be useful: insulating greenhouse roofs

Amazing how many novel approaches there are :)
I think R20 is perhaps a little optimistic, I wonder how they pump them in so it fills the entire cavity to the apex of the roof.
Storing the polystyrene balls would be fun, just one wall 12ft x 8ft would need a 1 cubic metre storage bin..about 64,000 polystyrene balls per bin :)

I think I am just going to scrap this idea! It seems like a lot of work, for minimal results. I have insulated my greenhouses north wall and roof with reflective insulation. I don't really want to use bubble wrap on the whole house because it is very visible to the public, and the bubble wrap can look sort of "unpleasing to the eye"!!! Is there anything else i could use? Like clear plastic sheeting? I lose a lot of heat from the greenhouse, being it all single layer glass, and very old at that! We are going to reglaze it this summer, so i am hoping that will help for next year!
Sarah

Adding a single layer of 4 mil polyethylene plastic sheeting inside the greenhouse ceiling, with a 2-4 inch air space between the glass and plastic, will reduce your heating requirement by about 40%.

Glad to see the insulation ideas kicking around again, specifically in regards to bubbles. I started building my own last fall and hope to keep going and get her done this summer. I spent years looking at the alternatives and really there is nothing else that works. Flowable styrene pellets. removable batts, automatic curtains... they all have that drawback of storage.

I don't have the link handy, but there is a group at the Appalachia university that built a prototype last year and won a $70K grant to expand the trials this year. I am waiting to see their results over this winter.

My greenhouse has a 24" space between the kneewalls and up to 36" at the peak, at the "reported" R-1 per inch it should do me nicely... if it works.

I also am planning to add the thermal storage mechanism. I liked the Subterreanean Heating & Cooling system (SHCS) design (see the SunnyJohn website) except that there's a lot of expense and work to that method. I have come up with my own design and as i refine it i'll start with descriptions and pictures.

I simply can't afford to heat a large greenhouse in Z5. There has to be a workable comprehensive design based on all aspects of shell design, insulation, thermal mass, sustainable energy, low cost and if it's a babe magnet that'd be nice too! *grin*

Poppa

Hi Poppa,
This might be of interest to you and others, SHCS vs Solaroof vs a control greenhouse.
http://www.sare.org/reporting/report_viewer.asp?pn=GS06-052&ry
=2007&rf=1

The technical issues seem to be the main problem with bubbles. The shcs proved reliable, maybe cheaper to install and run judging by the figures. It greatly improved the growth rate of plants vs the others.

The only one you wouldn`t want is the control greenhouse LOL
I guess it really needs a longer period of testing than just a few months to draw any real conclusions.

Hex, that's exacly the group i was refering to. I tried ocntrating them and got a message from the professor advising the team but nothing from the team itself. I am hoping to find the website where they document actual data. That has been the sore point of the whole bubble concept. No one has any real data. I keep hearing about the "theoretical insulation value" and saw one data graph from 15 years ago... the rest i am flying on intuition.

I think the issue and expense that they are seeing regarding the bubble generator is, if i get this correctly, are using a compressor with some type of "airstone" to generate the bubbles. You run into a lot of expense running that compressor, i think.

The bubbles are key as well. Big bubbles vs. foam? Easy to destroy in the a.m. vs. a better working insulation? Lots of variables to play with when i get this going.

The is another site, google "lively Up" and it'll take you to a guy in ontario who's been doing this for years. It's his word that most people quote when they talk about the 80% savings in heating. Unfortunately, like everyone else i have followed up with and been able to track down, they no longer operate the greenhouse. Doesn't bode well but i am hoping it's just a matter of determination.

Poppa

Hi Poppa
It may be the same story for a pioneer over here (Harvey)I know he was conducting his own tests on his tunnel but hasn`t updated his site in a long while (www.solarbubblebuild.com)
AFAIK, the lively-up gh used two pelton wheel water-driven blowers to generate the bubbles, quite expensive but much safer with no mixing of electricity and water. Only a 90psi water pump needed if memory serves.

My take on things is that without sufficient winter sun, you`re pretty much doomed no matter what systems are in place.
SHCS is perhaps the cheapest and most effective method of collecting/storing the energy and bubbles are ideal for insulating/preventing loss of that energy.

The hard part after building it..is keeping it running :)

Hey Hex!

I traded emails with Harvey for a while. The last report i got was he gave up as he saw no benefit. His wife is currently using it as a conventional greenhouse for their herb business. I think his failure was a direct result of not having the supporting data. He built his GH to the nebulous specs out there and didn't have the desire to have to do the science beyond a certain point...

Yes, Ross of livelyUp is a big believer in spending $$$ on the right (if you ask him) equipment. We have a large difference of opinion on that aspect. Ross thinks a $40K green house is the only way to go, me, i want to do this for less than 5K. The only thing i can offer is that at last contact the lively up greenhouse was down due to lack of funds for repairs.

The design i am considering is not going to use his $6000 water driven generators. I may use $15 water proof DC fans driven by a solar panel.

The drawbacks of the SHCS as i see it is the cost of excavation. You need to remove all your earth down to 3 feet or so and lay out your suberranean ductwork, then cover it over. There was some discussion (and sorry i forget the guy's name over at sunny john) and Rck Nelson of the Solaroof design... one of the concerns with the SHCS design is a possible problem with mold. I have never been able to contact anyone who has used that system... my guess is that they have all gone the way of the wind.

Boy, the more i type, the more i am depressing myself! *grin* Why am i taking this route?

It just makes sense to me.

I thought that was the case with Harvey`s gh..its a shame really as he did put a huge amount of time, effort and cash into the project.
I don`t think mold would be such a big issue with shcs, most greenhouses are probably just as attractive to mold with similar conditions.
Just dig narrow trenches where you need the tubing to go, don`t go digging out the entire gh floor..its a lot of work :)
Looking forward to seeing your bubble insulated gh up and running. Just keep chipping away at the various issues and you`ll be done before you know it ;)

Just a link to the announcement of the $75K award...

http://es.epa.gov/ncer/p3/press/05_07_07_asu.html

With all the funding you`d expect a lot more dependable information to be available.
Another possible avenue for information/data is Nottingham University,UK. They have a bubble insulated inflatable geodesic which is quite nifty.

Just in case you weren`t aware, alledgedly car radiator fans can make a good basis for the bubble generator.
You may need 15amps at 12v (160w)for some types though.
Being battery powered its also ideal for off grid use.

Yep, those are another alternative. I would use that if i went with a rapid refill. I am considering a constant flow of bubbles rather than Wham! Bamm! Not sure which would work better. Therein's the playing part.

BTW, you sound like you've done your research as well... Been hanging out at Solaroof?

Hi Poppa,
I visited solaroof maybe 3 or 4 years ago. Like yourself I did a lot of research before finalizing my greenhouse design and its still evolving. I do tend to be drawn to the more unusual stuff though LOL
I did some practical experiments along the way, including generating bubbles. I also tested a protoype greenhouse frame to destruction by placing concrete blocks on it.
I finally settled for a replica of the Eden Dome, smaller than the origonal of course but its unique as domestic greenhouses go ;)

cuestaroble,
It is good to know that i can use the plastic sheeting! It will look so much better than the bubble wrap I think. Thanks!
Sarah

Hex! Are you using tefzel and the pillow design as well? Someday when i hit the lottery...

Hey Sarah
Sorry to keep hijacking your thread and going off on a tangent :)
A light gauge poly on the inside will almost invisible compared to bubblewrap. It can also be quite thin as it doesn`t have to cope with the the external wind and weather.
Make sure its well sealed so you don`t have any air movement between the poly and the glazing otherwise you`ll lose most/all of the benefits. Openable vents can be framed around to isolate them if necessary :)

Poppa
Initially i`ll not be using Tefzel (ETFE) as it is super pricey but i can always retrofit it later as finances allow..its "on the list" :)
It does have pillows and uses the same geometry as Eden which is slightly different to a standard geodesic layout.

Thanks Hex. No worries about the hijacking! Hahaha. I know its easy to go off on tangents!
Sarah

How much thermal mass would I need to heat 500sq ft?

For an 18x26 greenhouse, water barrels are usually one of the most common choices for thermal mass because water stores a lot of heat and is easy to manage.

There isn’t really a fixed formula that works for every greenhouse, since it depends on your climate, insulation level, glazing type, and how much temperature fluctuation you want to reduce.

A good starting point is to place several large water containers along the north wall or areas that receive less direct sunlight. Dark-colored containers can absorb more solar heat during the day and release it slowly at night.

Also, make sure you don’t block too much sunlight or airflow. Sometimes better insulation and sealing gaps can improve nighttime temperature more than adding extra thermal mass.

I’d be interested to know your location and whether your greenhouse is heated during winter. That usually makes a big difference.