It's all Plain to Sea

[Steve777]

I am finally moving forward with plans for a solar heated GH at my home outside Boulder Colorado, at about 8300' elevation. Mr. Hobbit might recall he helped with a site visit and suggestions on a smaller garage attached GH we built some years back (maybe 8 ) on this property. That GH has been used to capacity ever since and it seems time to expand our growing space. Anyway I have passed the first of many regulatory hurdles and it is now time to submit rough elevation drawings and finish material lists for the proposed GH to the building/zoning dept. Before I can do that, There are some questions I need to answer and was hoping to get input from the folks on this list.

First a description as it is planned now:

The proposed GH will be built into a south facing slope at the edge of what is now a drop off. The intent is that the north wall would back into this hill/drop off. There would be utility rooms along this north wall which would be un-glazed (some intended rooms are: root cellar, chicken coop, storage, work room, and airlock entry). There would be a wall separating these rooms from the glazed area of the GH which extends the rest of the way south. The west wall would be partially underground (due to a slight E-W slope). South and east walls would be glazed, with the only entrance at the NE corner into the utility room area. Approximate dimensions 42' x 30' with the glazed area 42' x 24'.

To make the building permitting easier, I had been planning on using a conventional poured concrete foundation wall on the north wall, with concrete stem walls as needed for the remaining walls. 4" closed cell foam insulation outside the walls. Conventional stud walls to separate the utility rooms. Hopefully steel tube rafters (depending upon the engineering requirements). And either double, inflated poly glazing or double wall polycarb.

Some considerations & questions:

1. There is a very high snow and wind load for this area (60# snow, 130mph wind). And they are realistically needed; there has never been a year that I have not seen 100+mph winds during the winter (18 years now). Almost all the time these winds come from the west, so having the west wall partially underground is probably good. But it raises some other concerns:

a. Would an inflated double polyethylene glazing really stand up to these sorts of winds, and if so what would it's lifespan be under these conditions?

b. Would you modify the typical rafter spans to account for these loads? (Not asking for an engineered answer, unless you happen to know it, more just experience)


2. My intended use for this GH is several fold: To extend the growing season on each end. To allow more winter crop production. And if possible to grow cold hardy citrus and fig trees year round without any or minimal supplemental heat. Any thoughts on whether or not these are feasible with a subterranean heat storage system in this locale? (I expect the first two are, whether the GH will stay above freezing inside for the citrus and fig is not clear to me.)

3. Current placement/design would have glazing on the full roof and 3 sides (less some on the underground part of the west wall). From what John and others have said this may not be ideal as there will be no sunlight from the north. Any comments on how much light and growing ability would be lost by this? It would be possible to relocate the GH, but for a variety of other reasons (access, utilities and lay of the land, this is my preferred spot).

4. Thoughts or alternative suggestions on foundation systems? I am not thrilled with conventional poured concrete, but will an 8' wall mostly underground, I am not sure how many alternatives I have, structurally, but would be interested to hear about them.

5. Other thoughts on the glazing materials? I have considered using recycled double pane sliding door units for the lower glazing. And have considered using double wall polycarbonate as well as double wall inflated polyethylene. The polycarb has held up well in the smaller GH, but it is becoming clear (or actually not clear ) that it will need to be replaced at some point. The double polyethylene may be hard to get by the zoning/planning board due to its aesthetics.

6. Anyone have any experience with engineered loads and tubular steel rafters. From the GH shops I've talked to, none seem to have detailed engineering on their tube based GH's. They are sure they'll stand up but have no hard numbers. Since I will likely have to provide stamped engineered drawings, I'm curious if anyone has done this before and what sort of spacing and spans you got for the size steel tubes you used.

Comments, thoughts, question all appreciated.

[mrhobbithhnet]

Steve,

Thanks for the great prospectus on your project. You've got a clear idea of intent, and the limitations of the site. And it would seem to me that you've got to the crux of the matter WRT to the design - glazing material. Until you sort that out, the framing requirements can't really be sorted out.

I understand you are set on a permanent, formal building, so that means working with your building officials. And is probably a good idea if you plan on all season tropical environment just stone's throw downhill from the Continental Divide in the central Colorado Rockies!

I had one chance to work with an engineer brought in help design a greenhouse in Idaho with 100# and 100 mph building demands... so I am familiar with the 'problem'. I am glad you've done your homework too... because I suppose it's do-able, but for that client, the project was unworkable. Once he got into the issues with the local officials, it was clear that the modest building he had in mind was not going to be possible. There simply wasn't any glazing material we could choose that didn't require overly massive framing to support the worst case events likely on the site. And we were looking at kind of Hollow Structural Steel (HSS) to use for the framing so as to maximize the light getting in. The problem was so perplexing for the engineer he ventured to have a chat with the fella that wrote the section in the Uniform Building Code for greenhouses... fortunately he was still alive at the time.

His response?... "Well, if you did design it to that code, you'd be the first one to do it that way with those loads..."

So, it was indeed a serious problem for the engineer. How do you frame a building when it is the glazing itself is the limiting part of the structure? IF you do get those kinds of snow loads and they don't stay on the roof, the greenhouse has to be very high for the snow to pile up around it. Every design we came up with meant that the building would essentially be buried, have no light getting in it, so would not be a greenhouse anymore, just an ice cave with decent ambient light to see to get around, certainly not enough to melt snow or grow plants. So, the scenario slowly moved to making sure he could plow all around the building all year so that the snow would slide off and that there was a furnace installed sized mainly to melt snow, not just heat the building.

So, I definitely think you will need an engineer familiar with HSS framing design, you will certainly need rigid glazing with a steep enough slope for snow removal and you will have to provide some way to ensure the snow sliding off can be removed before it buries your walls or the walls are high enough to manage through the winter.

The rest of the floor plan you suggest makes sense. Western exposes are overrated, so you can de-rate them without too much concern. Make sure your winter morning light exposures are optimal. Good early lighting is the most likely 'subtle' factor to make for a successful tropical winter time environment. The next most valuable and neglected source of light is the sky above - that is why you may have heard that limiting the northern exposures might not be good. Certainly there is not sun shining from that direction... but the thing is if you only use the sun's direct rays, there will never be enough in the winter to sustain good growth. Plants that are intended to be be actively growing in deep winter can only get enough light if as much of the entire sky as possible is allowed to shine in - as well as the sun's direct rays. Of course that raises the issue - with that much exposure, don't you have too much heat loss to keep the place heated? Well you actually you can design out some heat loss with other building design - no walls, just shallow domes and sunken, grow zones is one way. But in big snow country that's not an option. So if you need snow slide room you need walls... and you will need supplemental heat. The question is how much...

My initial take on some heat loss gambits that come to mind:

Include in your plan a forced air heater with enough BTU output to melt every bit of snow that can land on the building - just to be sure you can if you need to.

Consider a CO2 generator with appropriate CO and low O2 alarms/safeties for your normal heating loads.

Then work with the walls that can be opaque, insulating them as much as practical.

Then use the Frost Proof Shallow Foundation spec to horizontally insulate the perimeter as much as possible - to keep your enclosed geothermal mass as large as possible.

Use a SHCS to charge that mass to best advantage, to dry the daytime winter air and to supercharge the night winters with recirculated the high latent heat of moist subterranean warmed air.

Use high mass materials everywhere in the greenhouse you can.

Insulate the outside of any high mass exterior walls.

Incorporate a deep cat-walked frost trench somewhere on the south downhill side to draw the cold air off all the soil surfaces in the greenhouse.

Think about solar concentrating parabolic hydronic heat collection from outside of the greenhouse.

If you can, zone the greenhouse with some sort of winter deployable glazing curtains such that there is a central, more easily managed 'tropical' zone for a few of the more exotics, populating the perimeter exposures with more temperate and hardier perennials or parts that you can let go dormant or put to 'sleep' with ground covers.
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[Steve777]

Mr Hobbit wrote: