Many people have mentioned an interest in building a straw bale greenhouse. I have seen several different approaches to this idea over the years. I have two different column in which these projects have fallen. Some of the greenhouses have been absolute disasters, while others have proven to be beautiful and successful. Just how to move forward and how the design is realized will affect the column into which your project lands.
Let’s start with the “how to” column. The above picture is from the Round Mountain Institute in Colorado. The large building behind the tipi is a straw bale greenhouse. This building is very large and looks to hold a lot of plant life. One key to this building’s success is the style in which it is constructed. The frame itself is used to support the glass and the bales are used to insulate the end walls and the North wall.
The bales are not used within the glass of the building itself. This is, as I pointed out above, a large building and so the concept used here can also be shrunk down to a more personal size. In the photo below, the same type of construction was used and the scale was simply reduced to make the structure more realistic for the average home owner. You can read about the actual construction process for the below building at http://www.appropedia.org/Kiva%E2%80%99s_straw_bale_greenhouse
The advantage of this style of building is that the simplicity of the frame makes the entire structure easy to build and sturdy for the long haul. One comment I hear a lot though is that a small structure should really be load bearing because a LOT of wood is used to make the in fill greenhouses.
It is true that a lot of wood is used for such a small structure; however, the balance needs to be met. The problem with a load bearing greenhouse is all the glass. The roof and solar walls can be lightweight if plastic is used; however, they can be very heavy if using glass. Either way, the ratio of open wall to bale wall quickly becomes so high that any structural strength in the bale wall is lost. This is fine under calm conditions, but I have seen more than one greenhouse blow down under extreme conditions. I personally want to feel confident that my building will stand up to the worst of what my local weather has to offer, not just to the good days.
It is possible to build a hybrid system in which the bales are load bearing for the majority of the building and the glass walls are the only framed portions. This may cause uneven settling of the plaster, but that is easily remedied with some long term maintenance. This is probably the best option available to you for your greenhouse. Your foundation will have to support the bales and a frame on the solar wall. I suggest a rubble trench foundation with an earth bag cap for the load bearing portion and a series of small piers for the framed section with earth bag foundation in between the piers to support the walls.
Now, what not to do. Do not leave the bales exposed. Be sure to plaster them and protect them from the weather. In the photo below, from a project at Slippery Rock University in Pennsylvania, the bales are left exposed and will eventually (sooner rather than later) rot. In fact, you can see the top bale is already growing grass from the top. This is a prime example of what not to do.
Other than covering the bales in a quality plaster job, and protecting them from the weather, you should consider adding operable windows to your greenhouse to allow for the release of moisture which will inevitably build up in the structure from the plants. The ability to adjust the moisture levels in the greenhouse will help you in your gardening and will help protect the bales in the long term.
Below is a beautiful version of what I suggest. Used in a standard greenhouse as the end walls, Harvest Build Associates, Inc. designed and created these beautiful walls to insulate the ends of the greenhouse and allow for ventilation through the windows. You can visit their site at www.HarvestBuild.com fore more on this greenhouse and other projects.
So building a greenhouse can be simple or it can be elaborate. If you take the time to decide what you really want out of the greenhouse first and then design the structure around those needs, you will likely end up with the best building possible. Build smart, and build for function first. Protect your bales in the short term and the long haul. Use bales only as needed, not just for the sake of it. Provide the right foundation and weatherization. Remember that your food will be growing in the space you create, so be sure to add a large handful of love and gentle comfort in the design.