Here, I'll show you a few videos of greenhouse construction, with my commentary. The are good points to be made from each, and even though two of these are rather large, those principles still apply in smaller versions. The first video is the smallest, simplest, and closest to what is needed for personal consumption. (You may want to mute the sound on the first one. All the info is typed and the muzak was killing me.) Despite the fact that I sound critical of all three, each has put a good deal of thought into it, and built good structures. In other words, I looking at the hybrid of the three.
Overall, I like the simplicity of this one, and particularly how it maximizes the exposure to the Southern sun. It really demonstrates how easy it can be to build, and that you can do it in the city, unless you're in an authoritarian HOA or municipality which demands both green measures and prohibits their construction.
You can use the same principles from "Framing the Little Home-2" to build it, so don't fear the complications of framing. The difference is that you don't need 16" centers for this. Four foot centers will probably be the most you want to stretch it, but the only weight your dealing with is any snow that accumulates, and in all honesty, we want to get rid of that by internal heat anyway, which leaves only the weight of the materials, and that's pretty light. In fact, this would be easily built on the South wall of the Little Home.
The first thing I would change on this one is the foundation. Wood does not last long when it is in contact with dirt. It seems to pull the moisture out of the dirt and rapidly rot. I would recommend that concrete block be used to an area above the dirt line. Conversely, the foundation blocks are a great way of avoiding having to dig deep holes and pour concrete footings.
This homeowner is using gravel for his floor, but it appears that it is primarily to keep his feet out of the dirt. This is not deep enough to use in heat retention. He does have the option of filling it deeper later, and I recommend that he does, to capture that daytime heat to keep the nightime warm.
Notice that he paints the framing. That's a good thing, but staining it is better. Stain penetrates the wood, while paint covers it. Paint has a tendency to flake when moisture pushes out of the wood, and that is a consideration in a greenhouse.
If you stain the wood before you build the structure, it is a lot easier. Cabot stains are the best I've found, and the more transparent, the thinner, and the deeper it can penetrate. Cabot was designed for the salty sea climates of the Northeast, so it stands up well. The most important reason to paint or stain wood is to preserve the wood. The second function is to make the work look nice, and especially if you're putting a greenhouse up in the city, you want it to look nice, but just because you're living with nature, doesn't mean you have to make things look "rigged." Build things right. Take care of them. They'll last longer, and you'll have more pride in your work.
You'll also notice that he uses 6 mil clear plastic for his walls and roof. This is a very common material used in greenhouses. It works. It's a lot more durable than one might think, and he stretches it out well, using the 1x2 wrapping method, but he didn't paint the 1x2's. That would have helped preserve his wood, as well as made it look better, for him and his neighbors. Galvanized screws would be a better way of holding down the 1x2.
Building on the plastic, you'll note that he plans to replace it every 3 years. That's fairly realistic. The sun and the cold are very harsh on plastic. I would probably have wrapped the plastic around both interior and exterior sides of the wall, creating a 3 1/2" air pocket. The downside to this is that there will be moisture that builds up inside the wall on the bottom plate. If you leave an opening at the top of the rafters on the inside, it would allow the heated air out, but also decreases the effectiveness of the air pocket to near nil. An alternative to permanently venting it versus building the insulating air pocket is to put an actuall vent at the top and bottom of each air pocket to allow it to breath in the day and act as insulation at night. This means that someone or something has to open and close them twice a day. There do exist solar foundation vents which may help with this, but they would likely need to be modified to close at a warmer temperature.
A 100ft roll of 20 ft wide 6 mil plastic runs about $120. It also comes in 10 ft wide rolls, and it's a good idea to have a roll laying around. 200 sf of plastic costs the same amount as about 2 12 ft pieces of clear roofing, which is 72 sf, or about 3x at much.
If you want to avoid replacing plastic every 3 years, you can either use old windows, particularly old sliding glass doors, or use the clear plastic roofing, or both, either initially, or as it becomes available. Either or both can be used in conjunction with plastic as well. The benefit of doing it on the front end is that you can frame around what you have on hand, as opposed to retrofitting it. If you keep your roof pitch steep, you shouldn't have to worry too much about the glass breaking, unless you have a tree dropping limbs and walnuts on it. If you do, it would be well worth it to put the clear plastic roofing over the glass.
Another thing I would not recommend is putting a 2x6 between your fascia board and gutter. If you do, you'll need to put a piece of flashing in to bridge that 1 1/2" gap. If you do, make sure you take into account the probability that water will find it's way between the roof and gutter, into your greenhouse.
The next video is a larger, stand alone greenhouse. It's larger than needed for personal use, but very well thought out, and probably fairly expensive as constructed. (Unmute your sound)
The first thing I notice is that he used filled concrete blocks for his walls. This should help with the heat retention, but it would probably require a double wall to do so. Cement block is a good, cost effective, and strong material. It costs about $2 per square foot of wall (including mortar) to build with it, assuming you use your own labor. The hardest parts are getting the footer square and level and mixing the mortar. Buying a mortar mixer can be expensive, but you can also rent one, and after mixing a few wheel barrel fulls of mortar, you'll likely come to the conclusion that it's a good idea to allow a machine to do it for you. Laying block is more labor intensive than thought provoking, but you want to put time and effort into getting the corners right. Once you have the system figured out, you'll be flying by in no time.
Now, this guy is in no way trying to "get off the grid," but he has built a solid on the grid structure. Running the electric and water in an easily accessible manner was a great concept, but if he had planned his HVAC prior to building his end wall, he could have ran that without the first 90 degree turn, or losing his sun. It would simply have sat on a pad outside the wall, and that part of the wall would have been ductwork instead. Also contrast his active water heated floor with the passive force air heated gravel in video 3.
There are many benefits to concrete block, and I'd say the main reason most don't build walls out of it is the appearance. Still, if you seal and paint it, it can look fairly nice, and it stands up well to wind and other nasties of nature and man. I should also point out that anywhere wood meets dirt or concrete, it should be a piece of PTW, full of arsenic to kill the bugs. Building codes allow for it to be separated by tar paper instead, but that is a moisture barrier, not a termite killer.
He uses steel for his rafters, but doesn't paint them, because he feels the moisture would rot even redwood (and the fact that he considers redwood tells me the guy has a lot of money). Unpainted metal rusts. This will be especially true, even if galvanized, at the top and bottom of the rafters, as well as any place it drilled, ie. the joints which are most critical.
Many commercial greenhouses use PVC pipe for support of the plastic. That is a structure that could be adapted to the rafter type structure that I've talked about. To be honest, I haven't really put enough thought into the humidity levels of a greenhouse, but for now, I'll stick with stained wood. Cedar would be a good choice, and that could be clear sealed for a beautiful effect.
Cedar is bug resistant and soaks up moisture like a sponge, without rotting like pine or other woods. It is second only to redwood in those aspects, but costs only twice as much as the arsenic soaked PTW (pressure treated wood). PTW resists bugs, not water, so don't fall into that misconception. It reduces rot by preventing bugs from burrowing through it, for the water to follow. It still needs to be treated with stain, for the water itself. And it will shrink, particularly at miter cuts, making your carpentry look shabby, and it will crack, allowing rot causing moisture in.
Another thing I haven't put a lot of thought into is the airflow issue he mentions. In a smaller greenhouse, this could probably be accomplished with small fans, potentially even DC fans powered by solar. There is also a possibility of inducing a passive air flow by means of the heat generated. Still, this needs more thought than I have put in so far. One technology that should be considered is the use of solar powered attic fans. They're a little pricey, but designed for high heat environments.
The third greenhouse is a commercial enterprise of the hippie type. While there's a certain degree of rebelliousness to the lifestyle, there are some good points to be made about the methods he has pointed out. He states that it cost about $45,000 in materials alone. He was able to get a $5000 grant to pursue it. Using some of the cost saving measures I've pointed out previously, one could probably lower this considerably The prior greenhouse likely cost as much, but the owner probably payed for labor, and I couldn't figure out what he had used for roofing. There is a big difference in the cost to operate.
In this 3rd one, we see much greater use of passive solar heating to keep the structure warm at night. Still, this guy uses a dirt floor and pushes the stone below it. This means that he has to build a passive heating structure below the dirt. That passive structure, with the forced air system is a good idea, but can be improved on. If we use the stone/gravel for the floor, it reduces mud, and means we don't have to dig as deep.
In fact, if we go with a combination poured concrete footer and short (2-6 blocks high) cement block wall, we can back fill the whole thing with gravel/stone, completely ridding ourselves of the cost of backhoes and most of the digging. Forcing the hot air through that stone is an excellent idea. Heat rises and this system certainly increases the use of the hot air built up during the day.
You might note he has 6 solar panels on his roof. That may mean between 600 and 1800 watts of energy a day. You should also note that while his roof is not set to optimize sun angle, his panels are, and he has 6 batteries to store the solar energy. The sun angle in January in Minnesota is 29 degrees, which would mean walls at 61 degrees. That would be a bit steep for a 30 ft wide structure.
A question I have not yet worked out is how much "thermal mass" is needed to stabilize the temperature. There is certainly a limit to how much "thermal mass" can be heated on a 12 hour winter day, but there is a possibility that if it began the season with a certain degree of heat, this would carry over for a good deal of time. Depending on how cold things gets in your area (and the last one is in Minnesota, so it's pretty cold), and what you built into the system, you might want to supplement the passive system with some version of wood or propane heating.
When it comes right down to it, I'm looking at this as a means of saving money and being more self-reliant, not a desire to save the environment. It's more important to keep those plants at the right temperature, than to eliminate "greenhouse gases."
If one wished to add thermal mass, while working within the confines of gravel flooring, one might add drums of water as the structure to support the beds. This could in fact be used as a part of your closed system water supply. If it is, you're going to have to have very watertight fittings, assuming that you've elevated your water supply to use gravity as pressure. You'll also want to use drums previously used for edible liquids, not crude oil or other non-potables. Throw some Dawn or other dishsoap and some bleach in it to help flush out the edible oils.
Along those lines, there is another video which combines the concepts of "rocket heaters" and the thermal mass of water, to help maintain the heat in a smaller greenhouse. There are many videos about "rocket heaters" out there, some of which are highly complicated and others of which are pretty simple. I'm developing some ideas on how to adapt their concepts to other woodburning concepts, and that may be something I'll add later. The basic concept centers on copper tubing, which means moonshiners have been using it forever, but it's all about heating water.
Consider using 35 gallon drums rather than 55 gallon drums, as they are the same height, but a smaller diameter. That means there is a bigger surface area per gallon of water for heat transfer. It'll heat faster and dispense heat more readily. It'll also mean that you have less barrel taking up space.
By putting your beds on top of water drums, they'll be at a comfortable height and you won't have to bend over to weed them. On the other hand, it will decrease the space plants have to grow up. At worst, you'll have 3 feet of height above. The drums will provide a sturdy support for all that dirt you put on top. The gravel will disperse the weight below the barrels, but I'd still avoid putting them directly over your PVC heat ducts. Plants and shelving is going to cast shadow, so, you'll want the highest stuff on the North wall. You may even want the plants closest to the South wall, at ground level.
One of the things noted in the (poorly drawn) plans but he doesn't cover is that he put garage doors on both ends. Watching the video, these are barn style doors. If you're building a large structure, that is a very helpful detail. If you're building a greenhouse, it means you can open them up in the summer and cool the place down.
There is limited value in use of clear material on the end walls if you oriented your main solar wall to the South. You'll get some sun in the mornings on one wall and some sun in the evenings on the other, but you're losing the insulation factor for the rest of the day. If it were me, I'd stick to just the South wall/rafter system, for the expensive, clear materials.
One thing I'd point out about the last two is that if the sun isn't hitting the surface, there is no point in using thin, clear materials. The second one noted this to some extent in building the North walls out of cement blocks, but not the North roof out of metal. The third one uses a lot more clear material than is efficient, in my opinion. With the concept of the masonary wall, I believe it would need to be a double wall to do what he wants (heat retention). The reason is that the outside of that wall gets no sun and is exposed to cold air. That means the single wall is heating on one side and cooling on the other.
Another takeaway from these videos for me, is that this is one of those places where long and narrow is more efficient than is square. The more directly the rays of the sun hit the clear materials, the more radiant heat is transferred inside, and the less is reflected off. The point that you need that heat the most during the coldest months, so setting the rafter walls to a December or January angle will increase the efficiency of radiant heat when you need it most, and decrease it when it becomes harmful.
And all that space above what you use? That is a place for hot air to gather, but is structure that costs money to build while being unused. In fact, I'm going to say that there is limited value in using clear materials on the roof that isn't at the proper angle.
Using Tennessee as the model, the December sun is at a 30 degree angle, and January is 38 degrees. That would put the pitch between 21/12 or 60 degrees and 15/12 or 52 degrees, in order to catch the greatest efficiency of radiant sun heat in the coldest months. (In June the angle of the sun is 78 degrees, which is less than a 3/12 pitch, so a lot of the rays are going to reflect off that wall. September is 54 degrees which is a little under a 9 pitch.) With a 52 degree pitch to our South wall, a 12 foot diagonal would be only a 7'2" run, but a 9'5" rise. I started out with the diagonal because 12 feet is the only length that clear plastic roofing comes in. You have to heat every cubic foot of air inside, and the highest air is warmest.
If one used these measurements, one would have a walk area 7 feet wide, with a ceiling height on the North wall of 9 feet. One could extend the width by putting a less steep roof back down to the North wall, or pitching it back down with a ridge at that point, but that South wall is where you're getting your passive solar heat. The fact is that it is going to be more cost efficient to have shorter walls than that though, basically the height of a patio door (about 7 feet) or half a sheet of clear roofing(6 feet).
A point I would make is that if you put (much) transparent roofing on a 3 to 9 pitch, you're going to maximize radiant heat in the summer's hottest months, but much of that radiant heat will reflect off during the winter. For that reason, I would limit the amount of clear roofing to that desired for passive lighting, and insulate the rest of it. (See skylights.)
To maximize efficiency of that, it would be better to build the structure 20 feet longer (along the South wall) to add square feet of greenhouse, than to make it 20 feet wider (along the east-west). Of course, if the South Walls are perpendicular to the sun's rays, sun will also reach far back into the structure, so, even if non-transparent materials were used for a shallower pitch roof, one could go wider than just the run of the roof, with insulated walls, and still grow plants.
To re-iterate, the considerations for use or non-use of transparent materials is a consideration of cost, durability, passive radiant solar heating, and insulation to hold that in.
Once you have that heat inside your structure, your next consideration is how to stabilize the temperature, and to hold the amount you want inside. You can do this, passively, with stone and water storage, but you want to insulate the walls that aren't part of the heating. And you're going to want to record those humidity and temperature differences during your first year, so that you know if and when you need to add some active heating or venting.
You particularly want to know how much heat you lose overnight around the winter solstice. That's mostly the longest your greenhouse will go without sun to heat it. Another difference you'll want to know is the temperature difference on a sunny vs. overcast day. These best case/worst case data points are key to knowing if you need to open the doors, or turn on the heat, or when you can start your growing of seeds.
If you were to build your Greenhouse on a South facing slope, with a terraced rock floor, you would be able increase the South wall, without wasting significant space above what you can use. It would definitely require a means of getting the warm air circulating though, unless you're using the temperature differences for what different plants need.
If you were to expand the width of your greenhouse to the North with insulated walls and shallower pitches, you would still have sun during the coldest months, or you could use the other space for other things, such as storage of your tools. The lay of your land may have a bigger impact on the design of your structure than what an entrepreneur in California or a hippie in Minnesota have to say. And how far your land is from an electric pole may well determine how "off the grid" you want to go.
How much warmer you can get the inside of your greenhouse, and how much of that heat your system retains through sunrise the next day, compared to the outside temperature determines when you can start growing plants each year, and that is also influenced by how cold it gets outside. If your greenhouse is 30 degrees warmer at 3PM and it is 50 degrees outside, it'll be a balmy 80 degrees inside, but if you lose all the heat by 5AM and it's 20 degrees outside, most plants will die.
Some people use greenhouses only to get an advanced start on growing plants in the spring, while others use them throughout the winter to grow food. You'll need to decide which you want to do in order to determine how much you want to control the climate inside. And you can adapt that usage over time, if you preplan for that.
Many commercial greenhouses use non-solar heat to keep things warm at night, and vents to cool it down during the day. That "thermal mass" of stone and water helps regulate the temperature during both the day and the night.
As a bonus, I'll throw in a video of a below ground greenhouse. I have to caution you: it is dangerous to be in a pit that doesn't have strong walls, and water does roll down hill. While this is a solid concept, it would also be a good place to use concrete block walls, that are waterproofed with oil based tar on the outside.
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