Plastic Encased Straw Bales

Written by Andrew Morrison

plastic encased straw bales
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Several of you have, over the years, written to me about using plastic encased straw bales in the construction of a home. The suggested benefits of this have ranged from no need for additional vapor barrier, to ease of stacking, to being able to mortar the blocks together. Although this would seem like a good idea at initial glance, I don’t like the concept.

Here’s my thinking:

One of the most important aspects of straw bale construction is the ability for the walls to move moisture through them. I’m not talking about water, per se, but moisture in the form of vapor. The fact of the matter is that vapor will be present in your house and that moisture will find its way into your walls. What if you shower and forget to turn on the fan? Where does that moisture go? What if you forgot to turn the kitchen fan on while boiling a pot of spaghetti? That moisture has to go somewhere too.

Moisture collected or formed in the house (think about breathing house plants) will be pushed through the walls by the pressure of the home. Again, this is just reality: that homes are pressurized. Notice when you close an interior door that has a tight fit or when your bathroom fan turns on and slams your door the last 1/2″. That’s the pressure I’m talking about. If the bales are wrapped in plastic, there is no way for that moisture to get out. It WILL find its way through at whatever cost. That may mean finding a small penetration in the plastic and getting into the bales…only to find it can’t get out the other side. Now you have a plastic wrapped box of mush!

Another advantage of bale construction is the fantastic bond that is made between the plaster and the bales themselves. If the bales are wrapped in plastic, that bond cannot exist and so the overall wall assembly is weakened.  Not much more to say here. I think that’s pretty clear.

The advantages, or potential advantages of using plastic wrapped bales are far outweighed, in my opinion, by the risks that such a process would create. I suggest you stick with the bales in their natural state.

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16 Responses

  1. Also worse with plastic-wrapped bales is that in addition to ease of stacking, there’s equivalent ease of unstacking. Normal bales stacked (just loose!) three high are pretty sturdy, the top row just isn’t going to fall off without a good hard kick or being lifted. They almost merge together as all the tangly edge bits stick into each other almost like velcro.

    Two pieces of plastic aren’t going to stick to each other like that. They’re going to be more like stacking bricks together without using any cement.

  2. Inthe Midwest, could an adaptation be made on the straw bale concept for existing construction? Could straw bales, one or maybe two, be put in plastic bags and place around the perimeter of the home, or at least the rear side of he house, to provide some of the insulation for the sill box area, but from the outside of the house?? As background, in May I had a blower door infrared camera inspection of the 1976 home (I am not the original owner, builder). The inspection indicated some areas in need of improvement for insulation, etc, one of which is the sill boxes that should it was recommended that spray foam be applied. Getting someone to do it is next to impossible, including trying to find someone who understands the Focus on Energy/Energy Star requirements or knows anything (or cares) about products that do not off-gas. If someone knows how to do this themselves (which eventaully the auditor indicated is what he had done), it is a plus. This Old House had a segment that aired this Fall that included sill boxes and concrete basement walls with foam board, but it is not easy to find how to view the program on-line again.

    Any advice or feedback is helpful. I attended the Midwest Renewable Energy Association fair in June 2010, and saw a part of the straw bale construction demo.

    Thanks for any info on how some of the straw bale concepts can also be applied to existing properties will be appreciated, as well as the dream of new construction at other locations in the future.

  3. There are other major downsides of plastic wrap on bales. The global trend in plastic wrappings is improved bio-degradability. A large percentage of wrapping plastics (as opposed to architectural plastics that might be a component of a standards certified vapor barrier or damp proof course (DPC)) are now formulated using vegetable oils and vegetable derived cellulose along with other vegetable compounds to increase the rate at which they breakdown in landfill. There seems to be quite a lot of variability in the plastics used for wrapping bales across the globe, without adequate statement of formulation and experimental testing the wrap life would be unpredictable. The breakdown process occurs through enzyme, bacterial, fungal, oxidation and small insect and rodent activity as well as UV light where exposed.Want this added extra in your walls?
    If you were to make a bond of mortar, adhesive or tape to any of these plastics there is no knowing (without the manufacturers specifications and experiment) how long any of the bonds will last let alone how long the integrity of the plastic wrap structure will remain. Many of these wraps have a degradability rate shorter than 12 months. Straw, as a product, is a biochemical wonder, why spoil it with a polymer designed for the purpose? Yes, I know, many balers can autowrap bales in plastic in the field however the samples I have looked at and these were baled on days with a Relative Humidity (RH) around 20% still have moisture condensate visible on the inside of them within a few hours of baling! The plastics also add cost that will inevitably be paid for by the end user.
    The argument to me seems clear!

  4. Apologies moderator, there was an error in my last post.
    The error is in the line…
    “Straw, as a product, is a biochemical wonder, why spoil it with a polymer designed for the purpose?”
    That should have been “Straw, as a product, is a biochemical wonder, why spoil it with a polymer designed for an incompatible purpose?”

  5. No expert here, but I’ve been “imagineering” and pondering this subject for years.

    With the concept of buildings that are designed to ‘breath’ there is no active process that causes the bales to dry. Basically you just rely upon the ambient air’s humidity to be dry enough to reverse any moisture collection that may occur.

    What if we were to create a factory process where the straw could be
    [1] baled to the exact optimal tensions and dimensions for construction purposes,
    [2] using a web-like set of wrappings with pre-configured segmentations designed into the bales,
    [3] with anchors bound into the fiber of the bales that are designed to tie the bales together,
    [3] where the straw could be dried to a precise value through-out,
    [4] sterilized via either irradiation or mass application of disinfectants,
    [5] sealed inside a purpose designed heat shrinkable sleeve,
    [6] and coated with one half of a two part wetable epoxy on the joinable surfaces.

    Obviously these bales would cost more. They would also possibly ‘offend’ the sensibilities of those inclined to prefer to purchase natural or ‘organic’ products. They would be more of commercial construction type of product and would have to be distributed like other commercial products are.

    What do ya think?

    – Duece

  6. What about in a below zero climate where the moisture condenses and freezes midway through its journey through the bale wall?

  7. I have not seen this to be an issue. The key is to seal the walls around things like electrical plugs so that moisture does not move through the wall very easily. When there is easy access to the wall, not through the plaster but around it (like at an electrical plug) more moisture moves into the wall and more risk is presented. Because the straw itself is not a great vehicle for condensation, the moisture , in its fine form, is more likely to move out through the wall than it is to condense.

    I am not a moisture scientist (not sure the correct term even) so I may not have the best description of the process; however, I have not heard of this being a problem in cold climates. Perhaps someone else with better credentials can explain this to us all…

  8. Hi Duece. Sorry it took me like twenty years to finally get back to you on this. I have been slammed with new product creation and other details. I think this is an elaborate concept that could certainly work. I think you are right that it will offend some and please others. This is true for most things in life. My biggest concern would be the what ifs. What if moisture DOES get into the sealed system? What if the sleeve is punctured? What if the bales are not stacked exactly where they need to be during the process, but the epoxy has set between the two bales (or a series of bales)? This does not stop us from considering this as an option, it just bergs answers to more questions.

  9. This is a tough one as the bales will deteriorate over time if not fully protected from weather, especially if sitting on the ground. I suppose, with the right detailing, they could serve this issue very well.

  10. I like the suggestions of Deuce , #8, they seem to make a lot of sense to tame the “wild properties of a natural straw bale”.
    If you add to this a material like flax or natural fibers instead of plastic, would that not do a much better and cleaner job ?
    Other suggestions are to make this packaging breathable by makes (micro) holes of a sort of web structure.
    Or only to cover the skirt areas and leave the largest surfaces uncovered for ventilation purposes.
    How does that sound?
    greetings from Holland (the Netherlands)

  11. I am just getting my mind set into straw bale construction and have lots to learn. I have seen a couple of TV shows including it and on the surface I am quiet impressed. I will do a course soon not quiet sure where suggestions welcome. Not got a clue how a roof would stick on a straw bale load bearing building yet.

    From the TV at first glance it seems like the bales are held together with sticks and only lime render is preventing moisture and rodent intrusion this does concern me. Bales are held together with nylon string which can degrade especially in UV light is this an issue also does the render trash the string? (I suppose on second thoughts unless the wall has holes the light shouldn’t be an issue.)

    I like a lot of Deuce’s ideas but fear that one of the main drivers in the building of a straw bale building, apart from the obvious green benefits and the insulation could well be cheapness of material. If commercially made to the above standard mentioned they may be out of reach of some people financially.

    Is there any mileage in a half way house here say fit the bales together with a different method to a potentially rotatable stick, say some sort of off the shelf metal stake and paste the top and window surrounds and base or other vulnerable areas in say fibre glass? Would fibre glass be anti eco? Not quiet sure about the rodent thing maybe forcing lime into the straw under pressure with a compressor or other cheap method may help keep out the munchers out. This may also help disinfect the bales. Any thoughts additions or errors in my thinking gratefully accepted. As I say I know very little about the subject but aim to learn.

    Thanks in advance.

  12. Hi Kevin. I agree that houses built in a “cheap” way are not the best options. I have a system that I use that builds a VERY strong home and very resistant home. That’s what I teach on my DVDs and at my workshops. Making that system affordable is not always easy as you point out as some materials are hard to get in certain areas. That said, I think it’s important to build with local skills and materials as much as they are available and fit a “safe and strong” design for the home.

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