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the first wofati greenhouse design

 
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Here's a sketch of what I envision, not at all to scale but just to show induced convection during daytime. Night time convection reverses once trombe releases its heat...

Borehole directly under glass, framed in such a way that air can pass through the frame but loads passed to borehole casing. Dry stack, square bottle bricks, insulations, etc to face the outer portion of stub wall foundation (you all are way ahead of me in what this should be). Smaller pipe inserted into top of borehole feeding into trombe wall at front of front bed. Nitinol spring operated louvers at top end of glazing for overheat dump. Heat pipe based condensation collector at the rear (would work better if heat pipes were stuck into a high mounted water tank...). Air and splashed water sinks into cold trench, compost or worm bed absorbs some humidity and first chance reheat. Air that stays cold drains on an angle forward to borehole. Warm air rising through borehole and sinking coldest air don't have to pass by each other due to pipe in pipe effect.

Greatly reduces the chance of people, flip-flops, or keys going down the borehole. Always a chance for warm and cold air to pass by each other segregated in the borehole...
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David Haight
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David Haight wrote:

Josiah Kobernik wrote:

What if the "fog harp" was actually made of angled heat pipes stuck into the rear mass(?)



David, I woke up thinking about heat pipes this morning. I was trying to figure out how heat pipes from the mass could cool the fog harp, but I think you nailed it by having the heat pipe itself be the condensing surface.

Anyone have design ideas for DIY heat pipes that are filled with non toxic material?

Earlier in the thread, Greg mentioned using water. Do you think that would suffice in this scenario?



Any working fluid can be tweaked to the right temperature range by getting the internal pressure right. For water, this means pulling a vacuum to get the pressure way way way down so that the water is boiling at room temp. Means either pulling the vacuum as you seal it up, or having a service port were the the vacuum can be pulled after sealing the pipe and re-pulled as leakage occurs. Ammonia and propane are two other possible working fluids but as someone else mentioned the toxicity potential, especially in a closed environment, might be a no go. Ethanol and Methanol might also work, I'll have to consult my reference to see if they will work with copper or aluminum pipe material...



I forgot acetone as a possible working medium also...

If methanol in copper pipe passes the group's "not toxic enough" test, that pulled to .6atm partial vacuum is what I would go with for the condensation medium / heat pipes. The more I think about it, the more the complexity increase from having a water tank up high and the heat pipes stuck into the tank is worth it, in my opinion. Second bonus is gravity-fed water to the beds if you wanted to do that, con is having to get the water into the high tank, maybe a fill port from the outside on the roof hill?
 
David Haight
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I liked the innovation of the de-stratification pipe in the deep temperature well. Looks like it should help move air at the two times it needs it the most. However, we all know air is a fairly slow and low effectivity medium for moving heat. A heat pipe can bring up ground heat and radiate it into the greenhouse space, with less of the heat being transferred to the leaky air mass. Basic vertical heat pipes have to be "gravity assisted", aka only move heat well upwards away from gravity. But this also means you could take excess summer heat from the interior of the greenhouse and lift it up into the dry umbrella soil or water mass as well. Loop heat pipes and oscillating heat pipes have a better, though not unlimited, ability to push heat down against gravity, though some designs require taking some heat away in a gravity assisted direction too. Here again, having thermal mass above the growing area makes this possible.

So my question to Paul and the other designers is, was a heat pipe (basic vertical, loop, or even oscillating) considered for this greenhouse design? If it was ruled out, how come?
 
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I’m new to this. Seems like the primary purpose of the heat pipes isn’t to move heat up (that happens naturally). It’s more to help circulate the air and move heat down (not natural) to prevent greenhouse from becoming too hot. Likely will do some of both.

I believe for heat pipes to function they will need to be dry. Might be problem in my soil. During wet springs soul is saturated and I get standing water in a 1-2 foot hole. Perhaps the umbrella would prevent this.
 
David Haight
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Kelly Pridgen wrote:I’m new to this. Seems like the primary purpose of the heat pipes isn’t to move heat up (that happens naturally). It’s more to help circulate the air and move heat down (not natural) to prevent greenhouse from becoming too hot. Likely will do some of both.

I believe for heat pipes to function they will need to be dry. Might be problem in my soil. During wet springs soul is saturated and I get standing water in a 1-2 foot hole. Perhaps the umbrella would prevent this.



It sounds to me like you are using "heat pipe" like I would use the words "thermal borehole". I'm taking about this completely closed pipe (which would actually work better in wet soil with more thermal conductivity): https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.oa75lEKrkjwIbwKf6z6A5wHaE3%26pid%3DApi&f=1
 
David Haight
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Kelly Pridgen wrote:I’m new to this. Seems like the primary purpose of the heat pipes isn’t to move heat up (that happens naturally). It’s more to help circulate the air and move heat down (not natural) to prevent greenhouse from becoming too hot. Likely will do some of both.

I believe for heat pipes to function they will need to be dry. Might be problem in my soil. During wet springs soul is saturated and I get standing water in a 1-2 foot hole. Perhaps the umbrella would prevent this.



I also wonder if a dry thermal borehole would move heat up better or worse than the same size borehole filled with water. Might have to go look for scientific research on that now...
 
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Just ran across a design from homesteaders with 40 years experience, I don't quite know if this is meets all the design criteria but it may, I'll have to look at it more closely.  Just pasted it there.  It was on Facebook.  Maybe some useful food for thought?  They're running a pipe for geothermal 10 feet down, appears to cool and warm passively.

https://selfsufficient-backyard.com/my-book/?utm_source=facebook&utm_medium=cpc&utm_campaign=t11-interest-l1-conv-pur&utm_term=l1-int-tiny-house-us-25up-mf-all-mix-072820&utm_content=img-illustration-v1-copy-v6-h6-138899287767182&fbclid=IwAR3duFZ4kCig496pfAgr1RqlwkrOT53wqOxFxnDA4nI4tk-Iq4Btg1XzfZw

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There’s several (maybe dozens) of YouTube videos along those lines. What most designers have found is they need several air exchanges Per hour for it to work effectively, which of course requires fans and power for the fans, ie- not passive. But still a great design!
 
David Haight
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Sorry for having to go quiet at the end of the meeting, I was not able to keep the coughing at bay... Here's some more detailed reactions, ideas, ravings, and possible gibberish...

Bathtubs: I like the idea of separating moist growing soil from structural and insulative soil. I am a bit worried about the bottom of the bathtubs going anerobic. Consider a full blown wicking bed setup in the bathtubs? Also, the outside of the bathtub may have "sweaty toilet" syndrome in the warm and humid period of the year, so having a plan to harvest that condensation would be good. Also, harvesting condensation from the windows straight into the wicking bed reservoir would be passively awesome.

Gravel around posts: Nailed it. They do the same thing on fence posts here in wetter clay soils and even though they get rained on periodically, the bottom of the posts rot at least 2-3x slower to non-gravel protected ones...

Earth tubes / cooling tubes / ventilation tubes: Yes, the more the merrier. As Reximus Prime said in the meeting chat, you can always cap them off initially or to reduce the number of variables that you are testing. They are hard to retrofit in later, but easy to install in the initial build. If nothing else, they are cheap insurance against some of the potential achilles heels we have been discussing. Folks living in earthships do cover/cap these in the winter to not draw cold air through the berm or into the space. Vent up high to create stack effect to pull new air into the space through the ventilation tube is a must.

Instead of building a greeenhouse v0.7 then 0.8 then 0.9 ... I think it makes a lot of sense to be able to use this structure to test various configurations from one build cost. Could multiple thermal boreholes be drilled, one under the cold sink and one up front? Earth contact tubing between them that could be opened or closed?

That being said, I think it would be smart to come up with a thesis statement for this one to guide the initial setup of what is opened vs closed off (i.e. "a single thermal borehole and destratification pipe is enough to heat and cool an earth sheltered greenhouse in Montana"), a set of potential problems and negative results (humidity, mold, low CO2 levels, overheat, etc), potential mitigation steps (like opening the upper vent, opening the upward sloping ventilation tube, etc), and trigger points for each mitigation (interior reaches 99F the first time, mold is sighted on plants, etc). Then, the final design for this build can have these mitigation elements in place or at least the structure where the mitigation will go already installed...
 
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I think a thesis is a good idea. here is my first crack at it.

"A combination of:

    An 8 foot deep cold sink

    Two well casings extending 19 feet deep below the cold sink fitted with passive air circulation units

    Dry earth thermal mass on the roof, as well as the North, East, and West walls that is disconnected from surrounding soil by a polyethylene membrane “umbrella”

    Inflow of household greywater at or above room temperature

    A south facing glass wall measuring 10 feet by 5.5 feet, sloped perpendicular to the angle of the sun at solar noon on February 1st

Will be sufficient to keep a greenhouse with interior dimensions of 10 feet by 9.5 feet above 50 degrees and below 92 degrees year-round in Western Montana at an elevation of 3200 feet above sea level."
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For wooden beams, what about charring them before installing with gravel?
 
Josiah Kobernik
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In critique of the strategy of capping things off for later testing, I will relay two things that Paul has told me.

thing one, instead of testing each innovation independently with controls, paul likes to heap ten or more innovations into one experiment and then if the experiment is successful, you can successively divide the innovations in half to sort for relative influence.

thing two, the annualized thermal inertia aspect of wofati structures takes years to test. It may take 2 or more years for the mass to be fully charged and operating in semi-stable seasonal temperature fluctuations. So capping and uncapping earth tubes within the first 5 years muddies the results of the thermal inertia. That being said, If it takes several years for the greenhouse to start working, then it's not very attractive as a design solution.
 
David Haight
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Josiah Kobernik wrote:I think a thesis is a good idea. here is my first crack at it.

"A combination of:

    An 8 foot deep cold sink

    Two well casings extending 19 feet deep below the cold sink fitted with passive air circulation units

    Dry earth thermal mass on the roof, as well as the North, East, and West walls that is disconnected from surrounding soil by a polyethylene membrane “umbrella”

    Inflow of household greywater at or above room temperature

    A south facing glass wall measuring 10 feet by 5.5 feet, sloped perpendicular to the angle of the sun at solar noon on February 1st

Will be sufficient to keep a greenhouse with interior dimensions of 10 feet by 9.5 feet above 50 degrees and below 92 degrees year-round in Western Montana at an elevation of 3200 feet above sea level."



Excellently specific! The map is not the territory and the measurements are not the thing measured... BUT... having facts and figures  and temperature numbers does help those of us who are not in your specific spot adapt your design to our context, compare why it worked in one context and not another, etc.

List of what counts as failure, negative result, type 1 error, etc next?

And would you please verify the time for the next zoom meeting today? Thanks
 
David Haight
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Josiah Kobernik wrote:In critique of the strategy of capping things off for later testing, I will relay two things that Paul has told me.

thing one, instead of testing each innovation independently with controls, paul likes to heap ten or more innovations into one experiment and then if the experiment is successful, you can successively divide the innovations in half to sort for relative influence.

thing two, the annualized thermal inertia aspect of wofati structures takes years to test. It may take 2 or more years for the mass to be fully charged and operating in semi-stable seasonal temperature fluctuations. So capping and uncapping earth tubes within the first 5 years muddies the results of the thermal inertia. That being said, If it takes several years for the greenhouse to start working, then it's not very attractive as a design solution.



I agree that changing too many things too fast is not helpful for comprehension, invites instability through over-management, etc. Having the discipline to not touch the controls until a certain point is just as hard as having the discipline to go check on it every day. Hence why I think having defined trigger points for intervention by someone in the group of observers / managers / recorders of this thing makes a lot of sense...

What's the hoped for lifespan of the building beyond the initial annualized thermal inertia testing? 50 years? 100? more? Climate could do funky things in that time frame for 100 different reasons. For our greenhouse here, I will try to plan in enough passive, active, etc buffer for things to change say +2 USDA zones and -2 zones in climate. I will be watching with interest to see what you all decide for this one.
 
Josiah Kobernik
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Tonight's design meeting is at 6 p.m. mountain time
 
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Here is a more complete first draft description of the greenhouse experiment, I think the language is specific enough and also vague enough to lend itself to making the mitigation decisions with better data when the failure mode arises.

"the WOFATI greenhouse 0.7 experiment posits that a combination of:

An 8 foot deep cold sink
Two well casings extending 19 feet deep below the cold sink fitted with passive air circulation units
Dry earth thermal mass on the roof, as well as the North, East, and West walls that is disconnected from surrounding soil by a polyethylene membrane “umbrella”
Inflow of household greywater at or above room temperature
A south facing glass wall measuring 10 feet by 5.5 feet, sloped perpendicular to the angle of the sun at solar noon on February 1st

Will be sufficient to keep a greenhouse with interior dimensions of 10 feet by 9.5 feet above 50 degrees and below 92 degrees year-round in Western Montana at an elevation of 3200 feet above sea level.

Possible failure modes and potential mitigation steps include:

If the internal temperature of the greenhouse drops below 50 degrees while all vents and doors are appropriately sealed, then we will begin installation of either an active thermal curtain to reduce radiative heat loss from the glazing, or an active air circulation system.

If the internal temperature of the greenhouse rises above 99 degrees with all the vents and doors appropriately sealed, then we will begin installation of either a passive heat dump ventilation system, or an active air circulation system.

If condensation is occurring in a location that it cannot be appropriately harvested and removed from the structure and is thereby threatening the longevity of structural components of the greenhouse, or productivity of plant systems is being diminished by excessive levels of humidity, then we will begin installation of either passive dehumidifying units (fog harps, and or heat pipes angled up into the north wall mass), or an active air circulation system."


Each of the mitigation steps listed above could help the greenhouse function in the event that the annualized thermal inertia is either insufficient, or insufficiently charged.
The active air circulation system that I am referring to is something like a solar panel and fan that pushes air through the well casing to help store that thermal energy. This is something that Paul is expressly opposed to, however, I think that it is worth consideration if it can be installed as an automatic failure preventative and therefore not an essential aspect of the greenhouse thermal regulation.  
 
Julie Reed
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Josiah Kobernik wrote:  instead of testing each innovation independently with controls, paul likes to heap ten or more innovations into one experiment and then if the experiment is successful, you can successively divide the innovations in half to sort for relative influence.


What if the experiment isn’t a success? How do you then decide which innovations to delete? Same system or? And if it is a success, how do you know which of the ten innovations is doing what? Or how much? How do you even know that any of them except maybe one or two is doing anything? That seems really un-scientific to me. And you also now potentially have unneeded things interacting (for better or worse) with needed things. I’m not criticizing the logic, just not fully comprehending it.

Dry earth thermal mass on the roof, as well as the North, East, and West walls that is disconnected from surrounding soil by a polyethylene membrane “umbrella”


A poly membrane creates a moisture barrier, but not a full disconnect. How thick is the layer of surrounding soil? For the dry earth to act as a thermal mass, it needs to be below frost line and separated from any source of conduction. So the barrier will prevent moisture migration, but not thermal transfer. If the barrier does not also include insulation, the surrounding soil would need to be much thicker than whatever frost depth is, no?
 
Josiah Kobernik
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Julie Reed wrote:  


What if the experiment isn’t a success? How do you then decide which innovations to delete? Same system or? And if it is a success, how do you know which of the ten innovations is doing what? Or how much? How do you even know that any of them except maybe one or two is doing anything? That seems really un-scientific to me. And you also now potentially have unneeded things interacting (for better or worse) with needed things. I’m not criticizing the logic, just not fully comprehending it.



Julie, yes, the same system could be used if the thing is a failure. It is worth noting that in any scenario, standing in the completed structure and observing would give a hint regarding which innovations function to which degree. This approach is suited more for gaining experience in uncharted waters than for refining well understood techniques. We are in effect, trying to invent outliers.

Julie Reed wrote:  


A poly membrane creates a moisture barrier, but not a full disconnect. How thick is the layer of surrounding soil? For the dry earth to act as a thermal mass, it needs to be below frost line and separated from any source of conduction. So the barrier will prevent moisture migration, but not thermal transfer. If the barrier does not also include insulation, the surrounding soil would need to be much thicker than whatever frost depth is, no?



That sounds correct. A layer of dry duff such as sawdust has been used between the umbrella membrane and the dry soil beneath it to help insulate, but even with that technique the main insulation is simply the sheer thickness of the mass. So not much of a disconnect.

I'm not sure to what extent this factors in, but the membrane is usually 5-10 layers of repurposed billboard tarps. What I have seen is that even after the mass has settled, the membrane still has cushion to it as not all the air is expelled between the tarps. Their creases prevent them from laying flat together.
 
Josiah Kobernik
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here are the drawings that will be the basis for Kyle's new 3D model.
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Josiah Kobernik wrote:Tonight's design meeting is at 6 p.m. mountain time



I've gotten emails with links to the zoom meetings that contain just the link. Also, it seems that the links are invalid after the end of the meeting, so I've got no clue when it even was? I'm lost.

Would it be possible to add the planned DATE and TIME of the meeting into the SUBJECT of the email (from permies special events), to be able to see at a glance if I'm able to join in? or if I've missed it? (or lead off the BODY of message with date and time, followed by the link?)



 
Joshua Myrvaagnes
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Wow, amazing discussion!

I'm wondering about the existing designs (that have a fan, but also 10' boreholes)--could those people be involved in running some tests to get some data to inform the current project?

If it takes 3 years to charge but then works for 50, that's a valid investment.   If you have to cheat with photovoltaics for those first 3 years but then can remove them and pass them on to a neighbor building their new greenhouse, then there's minimal loss.  

It makes it harder to judge the effectiveness of the experiment and is less kickstarter-attention-span compatible, but it is permaculture. Permanence.

However, let's say the power needed in year 1 or 2 from photovoltaic is less than any competing greenhouse design in the same latitude, that is a good result to show for the experiment.

It would be great to know what the 10' borehole design needs for power in year 1, 2, 3, even without deliberate thermal mass involvement--useful information and also a point of comparison to know what number to beat to show progress.
--
I see almost no downside to making extra boreholes and capping them as options for the future.  
 
David Haight
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Josiah Kobernik wrote:here are the drawings that will be the basis for Kyle's new 3D model.



Looking forward to seeing that model, what the engineering numbers indicate the log sizes need to be, etc. When is it due? When are you all trying to start construction?
 
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Joshua Myrvaagnes wrote:Wow, amazing discussion!

I'm wondering about the existing designs (that have a fan, but also 10' boreholes)--could those people be involved in running some tests to get some data to inform the current project?

If it takes 3 years to charge but then works for 50, that's a valid investment.   If you have to cheat with photovoltaics for those first 3 years but then can remove them and pass them on to a neighbor building their new greenhouse, then there's minimal loss.  

It makes it harder to judge the effectiveness of the experiment and is less kickstarter-attention-span compatible, but it is permaculture. Permanence.

However, let's say the power needed in year 1 or 2 from photovoltaic is less than any competing greenhouse design in the same latitude, that is a good result to show for the experiment.

It would be great to know what the 10' borehole design needs for power in year 1, 2, 3, even without deliberate thermal mass involvement--useful information and also a point of comparison to know what number to beat to show progress.
--
I see almost no downside to making extra boreholes and capping them as options for the future.  



Also if you test the new design with no inputs, someone can still build it and use an input in years 1-3 if they need to.  It seems best to keep the testing of the new design 100% passive for clarity and best information gathering
 
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Josiah Kobernik wrote:In critique of the strategy of capping things off for later testing, I will relay two things that Paul has told me.

thing one, instead of testing each innovation independently with controls, paul likes to heap ten or more innovations into one experiment and then if the experiment is successful, you can successively divide the innovations in half to sort for relative influence.

thing two, the annualized thermal inertia aspect of wofati structures takes years to test. It may take 2 or more years for the mass to be fully charged and operating in semi-stable seasonal temperature fluctuations. So capping and uncapping earth tubes within the first 5 years muddies the results of the thermal inertia. That being said, If it takes several years for the greenhouse to start working, then it's not very attractive as a design solution.



All the more reason to put in all the extra tubes, and leave them capped (or leave them uncapped) for the first three years.  Then you can fiddle around with variables in year 4 without having to restart from the beginning.
 
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EDIT--I completely screwed up my math, mixed up units by 2 orders of magnitude.  Oops!



OK, I finally thought of another piece of math that I believe should at some point be considered:  what energy does the 1"-black pipe actually carry? what's the limiting factor at least (hard to know what it will actually carry but the best-case scenario is at least a point of reference.). I figure the engineering folks have already worked through this but I didn't see it in the thread, so this is more for us laypeople.

The 1" black pipe will get about 2500 watts maximum per meter.  

1" = 2.5 centimeters = .025 meters
x 1 meter = .025 square meters meeting sunlight
sun energy is a _MAXIMUM_ of 1,000 watts per square meter (no cloud or other obstruction)

So .025.5x 1000 = 25 watts if 1 meter of pipe is exposed to sunlight.  (if it's closer to 2 meters then 50 W).

2 2m 1" pipes would be 100w; 10 would be 500w.

Comparing to fans that operate in a greenhouse in the non-passive designs, a cursory look shows wattages of between 120 and 450 watts.  I don't know how many fans they're using in these greenhouses (these are exhaust fans, not stratification fans).  But how it plays out in real life is still a question.  

Now, this assumes that everything around the black pipe is completely reflective...i.e. perfectly white or mirror-shiny, bouncing all the energy away and not heating that air at all, or else is perfectly transparent, allowing all the light to pass through it without leaving any energy behind in the greenhouse.  If everything around the pipe is in fact closer to leafy green, then there's less contrast, but it's still considerable: the leafy greens will take up some of that energy in photosynthesis (a few percent), and then absorb and radiate heat somewhat, and reflect a bit back out of the greenhouse.  Some will pass by and strike the thermal mass behind the plants.


 
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Even distribution of temperature laterally is another consideration.

“Often, these systems do not homogenize the climate across the entire growing area as well as they do vertically, near the fan,” Becker says. “Additionally, conflict or buffers between fan patterns can cause inconsistency across the crop.”

Horizontal fans, on the other hand, help humidity move evenly around the greenhouse facility, while also promoting transpiration.

(from a website on greenhouse fans https://www.greenhousegrower.com/technology/heating-cooling-ventilation/four-keys-to-optimal-air-flow-in-the-greenhouse/)
 
Joshua Myrvaagnes
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Another brainstorm--what about "cool pipes" for air down-flow?  In other words, pipes that are minimum energy absorbance (so, white colored, lighter than the plants or surrounding colors), or maybe shaded by a white-painted piece of wood? how much difference might that make?  would it make up for the lack of differential between black pipe and green plant color?  
 
Josiah Kobernik
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The build will start one week from today, Monday August 17th. We were planning to start this week but the Kickstarter funds have been delayed and so some crucial material has not been purchased yet.

Kyle's Model is coming along beautifully. I will wait to post pictures of it until he is done.

Paul and I decided to completely redesign the wing walls. One of the benefits of building a new structure right now is that several of us have been actively working on the older structures and are aware of their design flaws. We have the opportunity to refine our building techniques, while also testing something completely new.

We combined two techniques from framing the berm shed, the corner cell and the attic cell, into the new wing wall design. I'm still chewing on the specific details but here is the general design


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top down view of the wing wall
top down view of the wing wall
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Paul and I got out clay and sticks to play with 3D geometry
Paul and I got out clay and sticks to play with 3D geometry
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Josiah Kobernik
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I calculated that the total weight of the roof with three feet of soil soaking wet above the membrane is 12,660 lbs.
Each of the 8, 8 inch posts can support more than 22,000 lbs. So that's cool.
 
David Haight
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Josiah Kobernik wrote:I calculated that the total weight of the roof with three feet of soil soaking wet above the membrane is 12,660 lbs.
Each of the 8, 8 inch posts can support more than 22,000 lbs. So that's cool.



13x safety factor!?! Hopefully it's not an unlucky number.

What is the purlin size needed to transfer loads laterally to the posts?
 
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Josiah Kobernik wrote:I calculated that the total weight of the roof with three feet of soil soaking wet above the membrane is 12,660 lbs.
Each of the 8, 8 inch posts can support more than 22,000 lbs. So that's cool.



Could we see your calculations, Josiah? The numbers I find say wet soil averages 3000 lbs per cubic yard. If I recall the greenhouse dimensions correctly, 10 foot by 9 foot, 90 square feet times 3 feet deep =270 cubic feet. 27- cubic feet divided by 27 cubic feet in a cubic yard = 10 cubic yards. 10 yards times 3000 equals 30,000 lbs. You still have a HUGE excess load capacity!

Edited for my atrocious spelling!
 
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Joshua Rimmer wrote:

Josiah Kobernik wrote:I calculated that the total weight of the roof with three feet of soil soaking wet above the membrane is 12,660 lbs.
Each of the 8, 8 inch posts can support more than 22,000 lbs. So that's cool.



Could we see your calculations, Josiah? The numbers I find say wet soil averages 3000 lbs per cubic yard. If I recall the greenhouse dimensions correctly, 10 foot by 9 foot, 90 square feet times 3 feet deep =270 cubic feet. 27- cubic feet divided by 27 cubic feet in a cubic yard = 10 cubic yards. 10 yards times 3000 equals 30,000 lbs. You still have a HUGE excess load capacity!

Edited for my atrocious spelling!



In a timber structure like this the vertical posts won't present the major problems because they are in compression. The real concern is with the sizing of the purlins since they are taking the load in the shearing direction. You need to figure the size of the purlin based on the span distance between posts and the load coming down from above.
 
Joshua Rimmer
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Alan Booker wrote:

Joshua Rimmer wrote:

Josiah Kobernik wrote:I calculated that the total weight of the roof with three feet of soil soaking wet above the membrane is 12,660 lbs.
Each of the 8, 8 inch posts can support more than 22,000 lbs. So that's cool.



Could we see your calculations, Josiah? The numbers I find say wet soil averages 3000 lbs per cubic yard. If I recall the greenhouse dimensions correctly, 10 foot by 9 foot, 90 square feet times 3 feet deep =270 cubic feet. 27- cubic feet divided by 27 cubic feet in a cubic yard = 10 cubic yards. 10 yards times 3000 equals 30,000 lbs. You still have a HUGE excess load capacity!

Edited for my atrocious spelling!



In a timber structure like this the vertical posts won't present the major problems because they are in compression. The real concern is with the sizing of the purlins since they are taking the load in the shearing direction. You need to figure the size of the purlin based on the span distance between posts and the load coming down from above.



Agreed! I was mostly checking the info I found for wet soil weight. I've built several above-ground pole buildings, and never had an issue with the poles, but my early work had to be redone, because the door headers sagged badly.
 
Josiah Kobernik
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I've been looking unsuccessfully for trustworthy sources for information on beam load capacities to plug into my calculations. There is lot's of data on 2 inch wide lumber and glued laminate beams, but I'm having a hard time coming up with numbers for raw timber beams, anyone have a source for that they feel good about?

 
Josiah Kobernik
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here are my calculations for the weight of earth on top of the roof. The number I gave earlier included 1246 lbs for the 8 inch rafters and purlins as well as the roof pole sheathing.

Sample weights:
Soil 1 cubic ft. weighs 70.44 lb. when dry and 87.6 lb. when saturated to the point of runoff.

Area of triangle (A)
1.4 x 8.7 / 2 = 6.09 sq. ft.

Volume of (A)
6.09 X 11.5 ft. wide = 70.04 ft.^3

Weight of (A)
70.04 ft^3 x 87.6 lb (wet soil) = 6135.5 lb
Or
70.04 ft^3 x 70.44 lb (dry soil) = 4933.6 lb

Area of triangle (B)
1.5 x 8.7 / 2 = 6.52 sq. ft

Volume of (B)
6.52 sq. ft x 11.5 ft wide = 74.98.ft^3

Weight of (B)
74.98 ft^3 x 70.44 lb (dry soil) = 5281.6 lb

Weight of soil on roof (A) + (B) when (A) is saturated to running off.
11416.5 lb.

The next step is to determine how much of that weight will be on each roof pole, purlin, and rafter so that I can size them appropriately.
wofati-greenhouse-roof-load-diagram-side-view.png
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Julie Reed
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This only shows 4 species- spruce, hemlock, birch, cottonwood- but it will give you some helpful guidelines. Go to cespubs.uaf.edu and type HCM-00752 into the search box.

The two concerns are deflection and shear. The front of the greenhouse appears to bear hardly any weight at all, but the center and rear would need sturdy rafters/purlins. One other consideration is that in the spring, you could encounter a time when not only would you have 100% saturation of the soil, but a significant additional wet snow load.
 
Joshua Rimmer
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Josiah Kobernik wrote:here are my calculations for the weight of earth on top of the roof. The number I gave earlier included 1246 lbs for the 8 inch rafters and purlins as well as the roof pole sheathing.

Sample weights:
Soil 1 cubic ft. weighs 70.44 lb. when dry and 87.6 lb. when saturated to the point of runoff.

Area of triangle (A)
1.4 x 8.7 / 2 = 6.09 sq. ft.

Volume of (A)
6.09 X 11.5 ft. wide = 70.04 ft.^3

Weight of (A)
70.04 ft^3 x 87.6 lb (wet soil) = 6135.5 lb
Or
70.04 ft^3 x 70.44 lb (dry soil) = 4933.6 lb

Area of triangle (B)
1.5 x 8.7 / 2 = 6.52 sq. ft

Volume of (B)
6.52 sq. ft x 11.5 ft wide = 74.98.ft^3

Weight of (B)
74.98 ft^3 x 70.44 lb (dry soil) = 5281.6 lb

Weight of soil on roof (A) + (B) when (A) is saturated to running off.
11416.5 lb.

The next step is to determine how much of that weight will be on each roof pole, purlin, and rafter so that I can size them appropriately.



I see where the difference comes from,I clearly didn't have a good grasp of wofati construction!

I wish I could help with roundwood strength calculations, but I am nowhere near to being a civil engineer!
 
Josiah Kobernik
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I'm definitely not an engineer either. I need to study up on deflection force. I did a bit more math today and I think that learning to see the structure in terms of loads, compression, tension, etc. will make me a better builder.

Determining weight on roof poles

Break (A) and (B) up into smaller bits to determine the volume of soil above specific areas of the roof. In this case above the roof pole span (C). The weight of a2 + b2.

a1 + a2 = (A) and b1 + b2 = (B)

Area of a1
4.7 x 0.7 / 2 = 1.65 ft.^2

Volume of a1
1.65 x 11.5 = 18.92 ft^3

Volume of a2
70.04 ft.^3 - 18.92ft^3 = 51.12 ft^3

Weight of a2
51.12 ft^3 x 87.6 lb = 4478.11 lb

Area of b1
0.8 x 4.7 / 2 = 1.88 ft.^2

Volume of b1
1.88 x 11.5 = 21.62 ft.^3

Volume of b2
74.98.ft^3 - 21.62 ft^3 = 53.36ft.^3

Weight of b2
53.36ft.^3 x 70.44lb = 3758.68 lb

Weight of a2+b2 = 8236.79 lb

Roof poles used in the berm shed have a diameter of 3.5 inches spaced 3.5 inches on center giving an average of 34 poles per roof cell.

8236.79 lb / 34 roof poles = 242.25 lb. per pole
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