Sunday, March 2nd, 2008 | Popularity: 67%
For straw bale homes, the push has always been to provide the bale walls with a vapor permeable finish to allow any water vapor to escape the building. In most cases, this translates into an earth based plaster like clay or lime. These plasters have the ability to release vapor and thus allow the bales to dry out when the weather permits. Other plasters that are synthetic or cement based have limited ability to transfer the moisture away from the bales. In some cases, again depending on the weather or climate, the bales end up soaking up moisture from the environment which can cause decay in the walls.
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Vapor barriers often create more damage than they prevent in straw bale houses. Why then are they required in straw bale building codes? The answer is not complicated; however, the impact of vapor barriers on homes of all types is.
For straw bale homes, the push has always been to provide the bale walls with a vapor permeable finish to allow any water vapor to escape the building. In most cases, this translates into an earth based plaster like clay or lime. These plasters have the ability to release vapor and thus allow the bales to dry out when the weather permits. Other plasters that are synthetic or cement based have limited ability to transfer the moisture away from the bales. In some cases, again depending on the weather or climate, the bales end up soaking up moisture from the environment which can cause decay in the walls.
So if it is true that the bales need to breathe and that the plaster that is applied over the surface can influence the vapor transfer away from the bales, why would anyone use a vapor barrier at all? So here is the simple answer: because that is what people do on conventional homes. Because much of the code language for straw bale buildings is based on codes for conventional construction, some things just never get tested before they become requirements. I have built many straw bale homes and many conventional homes. In both cases, I find items within the code that really do not make sense for the individual situation; however, the code is not designed to address individual situations but rather large, blanket situations. As a result, I have had to fight for what I believe and then provide some type of proof or performance guarantee from an engineer. Although expensive, an engineer’s stamp is cheaper than a failed wall system.
I do not use vapor barriers on my bale walls unless I really have to. In some cases, where the bales may be exposed to rain splash or snow drifts, I may utilize the vapor barrier material. In dry climates, I do not use the material at all because I believe the ability for the building to release moisture is more important than the attempt to keep it out. The picture above shows an example of a vapor barrier in the form of roofing felt applied to the bottom courses of bales. In this case, the home inspector required it on the exposed portion of a wall where wind driven rain rain was a concern. I was sure to NOT wrap the vapor barrier under the bales. This allows any moisture to drain free of the bales into the gravel at the base of the wall. Do not confuse the need for a vapor barrier with the need for roofing felt over wood exposed to plaster. They are very different and the plaster protection is definitely needed to avoid cracks in the finish. The stripes on the wall of roofing felt are wood members covered before plastering.
I have learned over the years that moisture WILL get in to your house one way or another. Believing otherwise is like believing I can stop it from raining when I want a sunny day. In light of that, it makes more sense to build so that moisture can escape once it gets in. This is a simple answer to a complicated question. Indeed, there are many people and companies out there that spend countless hours and currency researching the impact of vapor barriers on construction projects and the results of those studies point to the inclusion of vapor barriers in conventional construction practices. Because there is limited information about the impact of those barriers on bale homes, we, as builders, are left to use our common sense and what information we can find. One thing we know for sure is that moisture can cause irreparable damage to a straw bale house. Knowing this, it is imperative that you do whatever you can to protect your walls from water AND moisture vapor build up. Exactly how you do that will depend largely on your climate, your construction materials, your mechanical systems, and your design.
Although not specifically related to straw bale construction, the University of Georgia has provided an interesting and relatively complete explanation of vapor barrier use in homes and how to find the right balance of moisture in your home. This is a good jumping off point for the vapor barrier discussion in any home. Here is a link for the information: http://www.fcs.uga.edu/ext/pubs/html/B924.html
Popularity: 67%
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Andrew Morison is a licensed contractor specializing in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs. You can check these out at http://www.LearnStrawBale.com.
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March 15th, 2008 at 5:52 am
We’re looking at doing a wrap on an existing home this summer. My wife doesn’t like the adobe look and would prefer vinyl siding (I don’t understand, either). I suppose one way to do this is to blow on a traditional mix, put a wood lattice over it (with a couple inches of air space) and attach the siding. Is there an easier way? Is there a semi-permiable vapor barrier that might work in this situation? As you said, there’s no way to keep water out but the siding would deflect precip if not water vapor. BTW, we are out in the country and don’t have code to deal with.
Thanks, Cor
March 17th, 2008 at 7:52 am
I would hate to see you use vinyl siding as it is such a nasty product in terms of environmental impact. There are other options available for siding if you don’t want to go with stucco. If your wife likes stucco, but not the rounded look of adobe, you can still go with the bales and plaster and simple cut sharper edges on the exterior.
If you do decide to use vinyl or some other siding, you would likely want to use a ventilation layer between the bales and the siding. I suggest you plaster a coat on the bales to fire proof them and then attach a mortar net system (www.mortarnet.com) and furring strips as you suggest. The problem is the attachment of the furring strips to the structure of the house. You will likely end up spending way too much time and effort to make this work well. It may not be worth doing in the long haul.
My best suggestion is to build with bales only if you like the total impact of that. Again, the plaster can be finished so it looks like a conventional stucco house and not an adobe structure. Sometimes, it is simply not worth trying to make bales look like anything else and lap siding is one place where that is often the case. Sorry for the bad news.
March 18th, 2008 at 4:23 am
Not bad news so much as fuel for my opinion that we should stay away from vinyl siding. What’s the quickest (reasonably economical) way to apply the mix?
March 18th, 2008 at 6:09 am
The easiest way to apply the mix depends on you. I prefer to hand trowel it on after mixing it in a mortar mixer. Some folks like blowing it on with a machine and then coming back to trowel it on. That goes much faster, but you have to work REALLY fast while troweling to keep up with the spray operator. Also, the machine can be hard to find and expensive to rent. The least expensive option is to use local soils as an earth based plaster and hand trowel it on. That takes time and patience and requires local soils to be good enough for use as plaster. You need high clay content, but not too high, to make that work. I suggest you pick up a natural plastering book for more description on how to tell if your soil can be used as plaster.
March 25th, 2008 at 4:37 am
Hi Andrew,
Have recently purchased your videos for infill and plastering and found them to be very motivational and interesting. It is my hope to finally realize the dream of building with straw bales in Prince Edward Island. In my research I came across a suggestion that tar paper not be used against wood especially on the thermally active exterior. During deconstruction of some walls, mold was found only between the TP and the lumber. Moisture had not been able to escape. I don’t know if the lumber was totally wrapped or not.
In Canada we seem to be VB crazy even though for half of the year the VB is on the wrong side of the wall. R2000 houses have proven to be full of mold when opened up.
Just out of curiousity, in your dvd you had drywall slip taped and meshed up for plastering. Inside and out? What up with that?
Regards,
DWD
March 25th, 2008 at 6:53 am
Dave,
I can understand how moisture would get trapped behind the roofing felt if it was wrapped around the wood; however, if placed over the surface with ample area for moisture to dissipate around the edges, I would be surprised if build was a problem in this scenario. I have not seen examples of it being a problem in any house I have cut into or torn down (only one torn down to date!).
I apologize for the confusion in the DVD. The drywall was only on a small section of wall where I did a recessed area for a special piece of furniture. The drywall is on top of studs in that area. Wherever there are bales, I plaster directly to them. More on plastering is given in the plastering DVD. Again, sorry about any confusion.
March 28th, 2008 at 4:10 am
Hi
In the picture the posts look to be quite close together. Are you using 4×4 posts? It almost looks like you’ve done a standard 2×4 frame wall with 600mm centers and notched the bales into it… I guess it’s cheaper timber to do that…
Cheers
Monte
March 28th, 2008 at 9:12 am
Monte,
The wall in the picture uses 4×4 posts 4′ on center. This uses smaller pieces of wood and means that the beam can also be a smaller piece of timber (4×6 with a 2×4 plate nailed to it). This speeds the raising of the frame in a big way and often works well with window placement for minimizing the use of excess wood.
March 28th, 2008 at 2:59 pm
Ah… thanks. It does make me seriously wonder about the use of light frames with strawbale construction. If the studs were at 21 inch centers then a bale would only need a 2×4 notch in the middle of one side and a push into the other two studs. It would certainly be much cheaper and less timber. Do you know of anyone doing this?
March 28th, 2008 at 3:04 pm
I tried that on a house a few years ago and it was a pain in the butt. I like to keep my walls in running bond and so that meant I had notches all over the place by the end of the job. We ended up turning the bales on edge and stuffing between the studs because that was easier…and I hate stacking bales on edge for entire lengths of walls! Anyway, it did not work for me the way I had hoped it would.
March 28th, 2008 at 7:16 pm
Hmm… I was thinking that if you had the right stud centers then it would work. If your bales are 900×450 then 450 centers for the studs should mean you only notch the middle of each bale. In practice I bet it’s not quite that neat and perhaps it a serious headache to notch every bale.