SHCS vapor barier question

steve333_gwNovember 4, 2013

For those of you familiar with the SHCS design, I have a question regarding the vapor barrier that might go between the subsoil heat storage layer and the upper planting soil.

John C mentions putting such a vapor barrier under paths and walkways, to limit night heat loss in those areas. Simple enough. However in at least one of his youtube videos, he places such a barrier over the entire subsoil layer.

I am curious if anyone on this forum knows the finer details of where to use or not use this vapor barrier in a SHCS GH.

Thank you for reporting this comment. Undo
curlygirl(5-6 Massachusetts)

Hi Steve333,

I tried to find the video you described to refresh my memory but could not track it down. If you could send a link, that would help. However, I am pretty sure that layer of plastic is not a vapor barrier. If I recall, I think he drilled holes into it while it was folded several layers thick to create a ton of drainage holes. You need the holes or otherwise you will have a drainage problem if you are planting in soil directly above. I think that layer of plastic is some sort of protection but I cannot remember what for.

Currently, we are building a greenhouse addition with a SHCS. We used stone instead of subsoil because we have a high water table and we were concerned that overtime, the water going into the tubes might bring soil into them. We did not use a vapor barrier because we poured a 6" concrete slab over the heat sink which we figured was enough of a barrier. The slab has drains in it for the raised beds to shed excess water.

What kind of greenhouse are you building? What are you going to grow in it?

    Bookmark   November 7, 2013 at 8:01PM
Thank you for reporting this comment. Undo


My confusion stems from this statement on John's website

about 7/8's of the way down, he says:

"Use a vapor barrier under your pathway surfacing. By limiting how much evaporation of moisture occurs in the pathway at night, you limit the uncontrolled night time migration of heat from the soil to the air. Uncontrolled losses this way will dissipate heat unnecessarily. Typical pathway sizes could mean you'd be preserving as much as 20% of the heat loss from the entire exposed soil surfaces. 6 mil poly sheeting under your paving will stop the vapor migration from deep below and keep them dryer too. Make sure you perforate the plastic to allow for water to move on through. You can drill holes in the roll before installing. You shouldn't need more than one or two holes per square foot."

But in this video

John show a vapor barrier (cheap tarp with holes every 4"OC) being placed over the entire subsoil bed. At least that is what it looks like to me from watching the rest of this series and seeing where the beds eventually go.

If you know which of these applications for the vapor barrier is the right one to use, I'd sure like to know. It seems to me that unless one was only growing annuals, you would want to allow your trees and other permanent plants to send their roots down into the subsoil.

As to my GH, it is a 34' x 15' lean-to on the S side of a barn. It is being built under a fairly strict budget (but is running over as all building projects seem to do ;-). I am using standard steel arches from a commercial GH with some additional structural changes (my area requires 50# snow load and 135mph wind loads, 8300' above sea level in the Colorado Rockies).

I think the final design is a fairly cost effective as well as sturdy and long-lived design that will meet the structural loads we need. But I suspect that the project would have cost 1/2 as much in a less demanding environment.

My intended use is mixed perennial and annual growing. There are quite a few plants which cannot be grown to maturity outside in this climate, as well as seed propagation. And I have some dwarf citrus which are taking over the livingroom and need a more permanent home. Additionally, the barn on the N side of the GH should be able to use some of the GH's "waste" heat, as well as insulating the N wall of the GH. It will be interesting to see if it all works according to the plan.

    Bookmark   November 8, 2013 at 2:07AM
Thank you for reporting this comment. Undo
curlygirl(5-6 Massachusetts)

I think he was trying to control too much heat evaporating through the soil while still having drainage. So, it is a vapor barrier but obviously it allows some vapor to come through the drainage holes.

I think he is also using it to keep the soil from clogging the stone he is using on that job. David Roper's YMCA greenhouse in Blacksburg, VA only put plastic under the pathway (I think). It sounds like you plan on backfilling with subsoil -is that true? That might be part of it. The video shows stone, Roper's greenhouse uses stone and I am using it as well. We won't have the benefit of roots being directly warmed by the SHCS's tubes because of the heat sink storage medium is stone. But if you are using subsoil, then yes, I think you are right, the plants and trees will benefit from sending roots down into the subsoil, where the warm moisture will be. And, it probably is not as necessary to control heat losses when using subsoil because there is so much more soil for the vapor to go through that by the time it gets to the surface it is minimal. -Stone has a lot of air spaces for the vapor to travel. One concern I had when I was planning on using subsoil in the heat sink with my raised beds being open to it on the bottom was that some of my trees with aggressive root systems might gum up the tubing. Back when Sunny John's forums were up, I saw some discussion on it and Sunny John believed that those roots would be "air pruned". In another forum discussion he was less sure. The air pruning makes sense to me but, as you know, we ended up going in another direction with our design, making the point moot.

Your greenhouse sounds amazing! Can't wait to see pictures. Ours is built more like an addition with big windows and several skylights. Our building department is requiring a lot of extra structure which is adding $$$$ but hopefully will give us piece of mind. We are also going with closed cell spray foam insulation which adds 30% rigidity. Before we bought our house, we were planning on building a lean-to GH to save on money but we could not find a property where a lean-to would work while also getting enough light. Our greenhouse is much more expensive than we planned but having to build the way we did, gave us a lot more latitude in space and design. -We tried to make the most out of it.

Looking forward to hearing updates. When will you build your SHCS?

    Bookmark   November 10, 2013 at 2:26PM
Thank you for reporting this comment. Undo

Thanks for your input. I think you've got it.

My GH is up, or at least most of the way, still needs some caulking and corner trim but basically the shell is all there. Recent flooding has made getting topsoil/compost delivery next to impossible, but hopefully as things return to normal that will happen too. Will try and get some pics.

Regarding the roots getting into the air tubes: Jerome (at CRMPI) tells that when he was disassembling his burnt out SHCS GH, he did not find any evidence of roots growing in the tubes, and as I recall he did not use a sock.

    Bookmark   November 12, 2013 at 12:31PM
Thank you for reporting this comment. Undo
velda(z5-6 MI)

Curlygirl and Steve333,
I'm reading with great interest your posts about SHCS. I've been studying various websites on the subject. My husband and I are having a new house built this spring and want an attached greenhouse. SHCS makes sense but it is still all just theory to me. However you both are putting it into practice, so I hope I may ask a few questions. First, Curlygirl, why did you opt for a 6" concrete slab over the heat sink? Will your planting beds be independent from the heat sink? How is your GH construction coming along? Steve 333, what is the configuration of your planting beds? Sounds like they will be in direct communication with the heat sink. Thanks to you both for your answers and also for any wisdom you can impart!

    Bookmark   December 22, 2013 at 10:58AM
Thank you for reporting this comment. Undo
curlygirl(5-6 Massachusetts)

Hi Velda,

Good luck with your project! Ours is still underway but is going well. We think it will be done by the end of January.

The six inch concrete slab has to do with our interior raised bed design and nothing to do with the SHCS. We designed our greenhouse to work well as a Passive Solar greenhouse where most of the windows are on the south side, very few on the east and none on the north. The west side is connected to our house. We have three skylights on the south pitch of the roof and one tiny one on the north. To be sure the fruit trees got enough light, we stepped the raised beds so that the raised bed on the north wall was the tallest, the middle one a little lower, and the ones on the south side the lowest. That way, none of the trees are blocking low winter light from reaching other trees. Because of this, the raised beds need a lot of structure to hold the weight. Originally, we wanted to have the beds open to the heat sink below but we just could not make it work structurally. We do have 10" x 12" drainage holes in the slab that will be filled with gravel so that if something went wrong (like a hose malfunctioning and filling the raised bed), the water could drain into the heat sink. We don't anticipate a lot of drainage going into the heat sink, however. We plan on being conservative with our water so I think most of the water will be soaked up by the trees.

Some encouraging news . . . our SHCS is not up and running yet, however, the Passive Solar design of the greenhouse is delivering! In mid November, we only had plastic sheeting up over the insulated framing and 20 degree weather outside and it did not dip below 49 degrees inside! Even with a serious winter storm whipping at the plastic, the temperatures stayed steady. Now our high performance windows are in and just this past week the outside temps were as low as 7 degrees and inside it did not get lower than 46! The construction workers leave the windows and doors open all the time while they are working because it gets too hot so I am expecting that once they are done, we'll leave the windows closed, the SHCS fan will be hooked up and storing heat and the greenhouse will be performing even better.

I am still really excited to see how the SHCS performs but relieved that the design so far is working so well even without the SHCS. Because our construction won't be done until January, I won't know how well the SHCS works for another year + since the summer is when you bank the most heat. The Passive Solar design is only going to make the SHCS run more efficiently and if things got bad -say an ice storm knocked our power out for two weeks- I am pretty confident our fruit trees would survive. By then, the heat sink would have built up a nice store of heat that all the raised beds are sitting on and the thermal mass with the southern exposure would keep things safe until the power was back on. So, my recommendation (and I think Sunny John would agree if he were still around), is to design your greenhouse to have as many Passive Solar properties as you can (high thermal mass, oriented south, heavily insulated, etc.) if you can.

Good luck! Can't wait to hear more!

    Bookmark   December 22, 2013 at 1:05PM
Thank you for reporting this comment. Undo
velda(z5-6 MI)

Hi Curlygirl,
So kind of you to respond so quickly! Interesting you should mention an ice storm….we had one just last night: accumulation was only 1/4" and thankfully we did not lose power. One thing we want for our new house is a backup whole-house generator. Speaking of which, the builders are drawing plans for our house and greenhouse so that it will make maximum use of low angle winter light. I'm hoping to use the GH to help heat and humidify our home. It will face south- southwest with windows also on the east and west. I'm looking at double-glazed glass as the best compromise between light transmittance and R-value.

I'm so pleased to hear the Passive Solar design is working for you already! To me it is a gamble of sorts to put theory into practice; lots of time and money are invested in the hope that it does actually perform.

Thanks for the explanation of your planting bed scheme. How deep will your beds be? What kinds of fruit trees do you have? I'm already thinking of a strategies to optimize light requirements for all the different plants I want to grow. I have bougainvillea in the house that will just take off when I put them in the GH and they will likely shade other plants if I let them. I hope to grow some citrus; nothing like the aroma of citrus blossoms!

What component(s) of your GH provide thermal mass? By that do you mean the heat sink specifically? I see where you have also used spray-in insulation. We plan to use that as well.

Again, thanks for your quick response!

    Bookmark   December 22, 2013 at 1:49PM
Thank you for reporting this comment. Undo

Hi Velda,

My project is still a work in progress, so it's too soon to draw any final conclusions. But here's what I've seen so far.

My GH is enclosed, more or less. The polycarb twinwall is up however all of the edge seams are not caulked/sealed yet, as the weather has been too cold for the caulk material to cure. And the upper 16" of topsoil that goes over the SHCS subsoil layer is not there yet either. Basic electricity is wired in, and so I have put the fan on the SHCS plenum, just to see what it can do (at present it is just on a timer for daytime running, no thermostat).

On a sunny day, the GH can easily get to 120F with no cooling or venting. Since I've turned the SHCS fan on, the max temp seems to stabilize in the low 90'sF on sunny days. I also am opening the doors to the barn some on warm days, so some heat is going there as well. I've measured the air temp coming out of the SHCS tubes, and it is roughly a 40F drop in temp (although this is with no summer heat stored, e.g. cold ground; if the SCHS system had been working all summer I would expect the temp drop to be less this time of year as the soil would be warmer)

On cold nights, the GH seems to stay at outside temps +20 to +30F. Not bad given that there are still open holes to the outside along the seams, the soil heat sink is not all there and wasn't charged up during the summer, and no insulation on the N wall yet.

I have not measured sub-soil temps at the various levels in the SHCS layer, but since they started at outside temps (below freezing) when the GH was first enclosed, I don't think the full potential warming effect has been reached yet. From what I have seen so far, I think my expectations of the GH staying above freezing without supplemental heat on all but a few extended cold, cloudy periods is still very possible.

I do plan on placing the topsoil in direct contact with the SHCS layer, over the entire surface of the GH floor. The SHCS tubes are backfilled with local sub-soil (mostly decomposed granite here, very minimal clay) and should be usable by plant roots. My planting "beds" will be moveable for the annual plants, and fixed for the few perennial plants which will be there.

At this point, the logistics and use of the GH is not defined enough for me to feel comfortable putting in permanent beds. Perhaps after a few years of using the GH I will have a better idea if I want them and where. I also have a smaller GH which has fixed raised beds in it. They work well there, but in this larger space, I wanted to leave the planting space open enough to use power tools (tillers, small tractor, etc) as needed.

As discussed above, I do plan on putting a vapor barrier down under the paths in the GH. But since this will be just under the path "paving" it should be simple to move if/when the bed configuration and paths change. FWIW I have seen humidity moving into the GH from the soil, even in the short time I have been running the SHCS fan. On mornings the GH humidity is 60-70%, where our normal outside air (and house) humidity is typically 20-40% in our dry climate. There are no plants or water usage in the GH yet, so this moisture must have come from the ground, moving up into the air overnight, when the fan is off.

Hopefully the GH will be fully operational in the next month or so. But I don't think the final results of the SHCS system will be in until a full winter after a summer's charging are in.

    Bookmark   December 22, 2013 at 3:39PM
Thank you for reporting this comment. Undo

Has anyone tried using wet decomposed granite as a heat sink. I was thinking about burying a grid of pvc pipe in the decomposed granite and circulating hot air through the pipes with a solar air heater and fan.

Any input

    Bookmark   January 17, 2014 at 5:49PM
Thank you for reporting this comment. Undo
curlygirl(5-6 Massachusetts)

Hi Bob, Velda and Steve333,

I lost track of this thread and did not see the replies until just now. -Glad to see the conversation is still going!

Bob: I think decomposed granite would work well as a heat sink but I would let the phase change of the hot humid greenhouse moisten the granite. If the heat sink is wet but not warm, I wonder if the phase change would transfer the heat as effectively. Maybe it will -I don't know- but I would investigate that. David Roper (YMCA SHCS greenhouse in Blacksburg, VA) could give you advice on that since he is a Physics professor.

Why do you want to use PVC pipe? I seem to remember hearing about someone who used it instead of the ADS pipe and had problems. If you do use it, make sure you tilt it so that water cannot pool in the pips. Drilling enough holes in the pipes would probably be too much work to be worth it. How big is your greenhouse? For ours, we estimated that we needed 45,000 holes for a 20 x 25 space. We got the slotted ADS pipe but then we expanded the holes to a quarter inch on David Roper's recommendation to get a better heat transfer. It was a HUGE job! We had over 40 volunteers and it took several weekends.

Velda: It is exciting to hear how your house plans are evolving. Have you considered putting in a root cellar? It would not cost that much more as a part of a larger construction and then you would have electricity-free food storage even if you did lose power. We included ours in our plan and are anxious to use it!

Our raised beds are 5.5', 4' and 3' tall. We plan on growing citrus, avocado, mango, black sapote, miracle fruit, cacao, star fruit, ice cream bean, dragon fruit, vanilla, cinnamon, black pepper, sugar cane, neem, pomegranate, barbados cherry and passion fruit. That is the plan but we'll see how the heat sink goes. We may just order the hardier fruit trees to start and then get the sensitive ones later once we see how the SHCS performs. We plan on having planter trays sitting on top of the beds with veggies growing at different times to maximize space.

Steve333: Any chance you could upload some photos or send us a link to blog about your project?

Looking forward to hearing more about everyone's projects!

    Bookmark   January 17, 2014 at 7:36PM
Thank you for reporting this comment. Undo

Bob, I think (hope) the decomposed granite should work as a heat sink. That is mostly what we have as subsoil here, and it is what I used around the tube grid in my SHCS layer. The granite should have a fairly high specific heat value, and will conform well to whatever piping you use. And it drains very well.

Not sure why you'd want to use PVC pipe vs the more common corrugated black drain pipe (which I think is polyethylene). The drain stuff is much easier to handle and shape than rigid PVC would be. And has quite a few more holes in it They space the holes every inch at several locations around the drain pipe. The perforated pvc I've seen (like for septic drain fields) has far fewer holes and the holes are much bigger in size. With clean DG you might get away with unsocked black drain pipe, because the holes are so small. But with the bigger PVC holes, you'll most certainly want a sock barrier to keep the subsoil out of the pipes.

I am curious what you mean by wet decomposed granite. :The DG would drain and dry out wouldn't it, especially if it is under cover and not subject to a lot of rainfall. If you mentioned wet because your GH is in a low spot subject to a high water table (seems unlikely with DG), then that might be a concern. I would be a bit worried that a high water table would move the heat in your SHCS out of the GH subsoil layer regardless of what subsoil type you have. Especially if there is any movement to that water.

    Bookmark   January 17, 2014 at 9:57PM
Thank you for reporting this comment. Undo

Hi Y'all,

I have been reading about SHSC for about a decade now and I am finally ready to go ahead and build. this is a somewhat complex project as I am planning on putting our new house in the greenhouse envelope as well as a seasonal thermal sink, and a few other features (solar hot air collectors to heat up the air even more and be able to store more heat, A trombe wall on the north side of the GH, possibly integrating some phase change material (parafin tubes), I would love some input on everyone's experience with these advanced SunnyJohn based solar systems. I have a ton of questions but am a bit at a loss on where to start.
Has anyone done a proper heatloss and gain calculation ? What is a reasonable soil temperature to expect at the end of summer when the thermal mass is fully loaded?
Someone mentioned 20-30 higher than ambient is that achievable in the north east (I lived in colorado and NM for a while and yes there I can see that)
How fast does the soil temp drop when you start pulling heat out at night? What air temp do I get in relation to the soil temp when running the system at night?
I would like to have a sub-tropical environment at min 50F just like curlygirl.
The main structure is a gambrel steel frame 80' x 40' the house (39x38) sits at the eastern end. and has a flat roof
for more growing space. the entire structure is 30' tall.
The GH part (40x40) will have 3 levels about 1-2ft steps up each time. The north wall of the entire structure is highly insulated with thermal mass/trombe wall in front (what material?) the hot air will circulate around the house and make the house which has vapor diffusion open walls a true passive house (i.e. no motorized ventilation needed). The ideas is borrowed from a German concept (Bio solar haus - google it).
Thanks for any ideas or comments.

    Bookmark   February 11, 2014 at 8:36PM
Thank you for reporting this comment. Undo

A vapor barrier reduces the amount of water vapor coming directly from the soil which can be an issue at night in the winter months. It raises the greenhouse humidity without affecting the temperature.

    Bookmark   March 21, 2014 at 12:12AM
Sign Up to comment
More Discussions
Largest greenhouse in the world...
I recently returned from a trip to SE Asia and spent...
Greenhouse building advice
I live in zone 7/8 and I am planning on building a...
Greenhouse Bubble insulation
We have a glass 8x12 attached Solargro greenhouse....
temp in your greenhouse, and temp/weather outside, right now?
I'm curious as to what kind of passive solar gain you...
Outdoor Kitchen and Greenhouse together
My idea was a brick oven built over a wood stove and...
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™