House wrap often creates more damage than it prevents 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 in straw bale houses 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 house wrap 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 house wrap 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 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 house wrap 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 felt 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 house wrap material 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 house wrap 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.
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115 Responses
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
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.
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?
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.
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
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.
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
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.
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?
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.
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.
Could you use fiberglass resin on the outside instead of cement? I don’t think it would be as fireproof as cement but just a thought.I’m not a builder but find your articles very interesting. Thanks Cliff
Cliff,
I have not used the fiberglass, so I cannot give any feedback on that. I appreciate your comments about the articles and hope you continue to enjoy them.
Hi Andrew, We are now really looking into what we are going to do to the outside of our house. It still has two coats of clay , sand and chopped straw but now that the spring rains have come the areas under the windows are erroding away….all the more reason to get our final coat on. Which we still haven’t decided on. After talking with others we think that a natural lime plaster might still be too heavy and not allow enough moisture from inside the house to escape through the wall completely. It might set up a zone within the bales that is constantly wet (sorry can’t remember the term for that area) any way it would then be prone to mold; especially in our winters. Would you recommend a silcone based paint to apply over the clay layer? It would be light and permeable to air but not rain. Or should we go with a lighter whitewash sort of layer and reapply every so often? We are hoping to have a low maintenance exterior and we now wish we had gone with a cement stucco. Now that our interior walls are all finished and coated with a natural clay though, we think the vapour would be trapped on the inside of the bales.
Sorry, also meant to mention that we did use the aformentioned many studs and many notches method and it did drive us crazy!
Maryann,
I would suggest you go with the lime plaster finish coat. The permeability of the coat is good enough to allow the moisture to move through. If you are looking for low maintenance, clay plaster exterior finish is NOT the way to go. That will require a lot of follow through year after year to keep it up. The lime is much more durable and should allow for the moisture to move through easily. Silicate paints are also an option, but not before you use the lime finish in my opinion.
Andrew
Has anyone ever used straw bales in the foundation to insulate. Then use cob on the straw to make a adobe floor. I’m not sure if it would need a vapor barrier are not. I’m going to be building a home in Alaska and it gets down to -45f there
I would not recommend putting bales in the foundation as insulation. They will rot under the ground and/or exposed to snow drifts and will become a mess to deal with. Bales must be kept away form moisture so only above ground applications are a good idea.
I’m sorry I said in the foundation I should have said on the foundation. The straw bales would be about a 18″ above the ground.
i plan on using tyvec on the bottom floor of my north wall which is 2 stories. also planning on 14 guage welded wire . will mixing/ set times be adjusted to account for different rate of set on straw vs tyvec ( a breathable vapor barrier)??
any problems with my plan? ( 620 bales in place)
John,
I don’t personally like using Tyvek or any other material to completely cover bales in a house or on a story of a house. That said, if you decide to do that, make sure you have enough “tooth” for the plaster to hang on. The plaster will not adhere to the Tyvek. I assume you have no overhang on this side of the house to protect the first floor, thus the Tyvek. If you can add one to the design without messing things up, I would suggest that over the Tyvek. Of course, that may be impossible or impractical at this point.
If you have a good backing for the plaster, you can move forward as is and the set times will be similar in the end. The straw will take a little longer to cure, but not by much. You will need to mix the mud a little heavier (drier) so it can hold on the mesh with no straw backing. Again, for an entire story, I would use a different approach if possible or perhaps a different material. If the mesh is not structural (for the frame) then you might consider using stucco wire instead as it will give much more “tooth” for the plaster. Good luck.
i have a good size eve, but expect driven rain , and ice in the winter, on the lower story. its a post and beam infill,. and Im told that the 2X 4 wire is best to support against ” racking” of the frame..perhaps Ill add some smaller wire over that for more tooth on the lower floor. thankyou
Would it be possible to use straw bale construction on a mostly underground home? We had planned to use concrete blocks, but like the sound of the straw bales. We already have plans for drainage behind the back wall using drains and wondered if this might be enough with a good coating over the bales to be waterproof.
Jeanine,
No. That is the short answer. Bales under ground are not a good idea, even with adequate drainage. If you want to avoid concrete block, there are several “green” alternatives you can choose. Rastra block is one and there are others that currently skip my mind. A Google search will probably get you there. Good luck.
Andrew,
I’m currently designing a timber frame with straw bale wrap home in the Adirondacks NY. I plan on using lime plaster inside and out with no metal lath or re-bar. What are your thoughts on the “no metal” theory and do you recommend a vapor barrier between the top of the foundation wall and the underside of the bales when the bales are sitting on a 2×4″ toe-up?
Also, have you used bails for the roof insulation?
Thanks,
Dave
I do recommend the vapor barrier beneath the bales if you are building directly on a slab. although the rock and/or insulation would isolate the bales from the concrete, it helps to keep the moisture level of the toe up space low. I don’t recommend bales in the attic or roof assembly for reasons discussed in other posts on this blog. Non metal mesh is fine although not as strong as metal mesh for structural purposes.
Have you experimented with not using any mesh and applying the plaster directly to the bails in a non-load bearing application?
I have and find that with all the shaping that takes place around windows, building corners, doors and niche, that I prefer to simply mesh the whole building in one swoop rather than piece together the mesh in small areas.
Hi! I have a quick question. Say I am up to plastering the walls on the inside and I want drywall inside is it still a good idea to first plaster and then add drywall or to just put up barrier and then drywall?
If you plan to add drywall to the inside face of the bale walls, you will need to have framing to support that drywall. You should also add a scratch coat of plaster to the bales prior to adding the drywall. I STRONGLY recommend against using drywall inside. It is a very stale look compared to the plastered bales. More over, dealing with the window and door wells is very difficult with drywall because of the natural curve of the bales. If you box out the windows and doors to accept the drywall, they will look cave like and very boring. I recommend you shape and plaster the bales and forgo the drywall.
Also, the full wall vapor barrier is something I would not recommend as you are likely to trap moisture in that wall somewhere. It is much better to allow the walls to breathe as much as possible.
Dear Andrew,
I am a small builder. I visited Europe several years back to research strawboard sips. I have never built strawbale but always found the style architecturally appealing. My question, have you or anyone you know ever utilized polyols-soy bio foams as exterior finish in lieu of plasters? It seems to me that there are numerous closed cell soy foams that meet the key technical requirements, preventing moisture penetration, while still remaining breathable. The obvious advantage being the speed of application, and the ability to “DIY” with minimal labor and laborers, and the disadvantage being material costs. Any thoughts or insights?
Hi Bill. I have had another man mention the same material with the same question. I do not know much about it, other than what I have been told. I am certainly interested to learn more. Do you have any websites you can post a link to here to get us all started towards learning? Thanks.
Hi,Iam still at the stage of gatering information as much as possible from you before i start with my project, a small lapa ( I think you call it a place where you have your barber que.) where i will have my braai. It will have walls on only two sides while the other two sides will remain open. The one wall will be 3 meters long and the other 3.5 meters long with a hight of 2.5 meters. My question is. Will it be wrong to stack the bales side ways up instead of flat? I will have wire mash (grit) both inside and outside with upribht beams every 1 meter. I will make shure that the mash and beams will be propperly intact and tightened to the bales in order to give strengh to the walls. The Reason why i want to do it this way is because the walls is a very short distance and not very high. The framework will carry the thatch roof. I will also have irion round bare cemented in the foundation to the hight of the wall to help support the bales. I will also put flat bar on top of the bales to pull the bales downwards with the round bar.
Andrew,
I just came across this post, although it is a bit old. I notice that you talk about vapour (vapor) barriers on the exterior, where I think that you mean moisture barriers – especially in your climate (Oregon). By this I mean moisture coming from outside in, not vapor diffusion from inside out.
I agree that the philosophy of “breathable” walls – which means vapor open, not air leakage as it suggests – is the correct method of construction, for ALMOST all strawbale house locations. However, I believe it is misleading to suggest that they should never be used. Just like why you believe in a good toe kick for helping with burst pipes, I believe that vapor barriers are a necessity for some locations – e.g., steam showers, green houses(?), areas of high humidity and use of natural ventilation (i.e., no fan assistance), etc. I have often hummed and hahhed about would I use strawbales in our bathroom with an exterior wall shower. If I had a great fan and knew that everyone (kids included) would use the fan – maybe it would be o.k. – but the conservative side of me says that a vapor barrier would be a safer bet.
As for moisture barriers, I think that it could be possible to use wire mesh over a moisture barrier and should pass the fire rating – although I have no data on that – only what I’ve seen on woodframed stucco walls.
In my opinion, I believe in the “good hat and good boots” method of construction in cold and wet climates. Therefore, I believe that the base of the wall should be either very well protected, or overclad with a moisture tolerable cladding – of course leaving a draingage/drying gap for the bales to “breath”.
Anyway, didn’t mean to blah, blah, blah, but wanted to clear something, as I understand it – great site, great ideas and great dialogue.
Thanks again,
Jamie (Vancouver, B.C. moving to Ottawa, Ontario to build our first strawbale house)
Dear Andrew,
As your probably well aware, spray-on closed cell urethane foams have been used as a roofing material on flat roofs for decades. Urethane can be sprayed in various densities which create different finished psi strengths. The foams are sprayed on with “propriatorial”, heated and very expensive spray rigs. The petroleum based polyurethanes have been around since the 40’s.
Enter soy products in the mid 80’s. Besides the eco-factors there are some other distinct advantages…bio-based means water based clean-up, hugely important when maintaining expensive spray equipment, also much safer for the sprayer. The biggest soy bio-based companies are owned by the petroleum based companies. Consequently little incentive to promote the much greener and eco-friendly soy products. Well, until the last crazed trader-mad oil price jump. But the fact is soy poly’s production costs have drastically reduced in the past few years with a new, far less costly processing development.
Soy foams can be made into formulas suitable for interior between the cavity spray-in applications, pour-in “old-construction” apps, roofing apps, truck bedliners, and structural spray-on apps.
The big petroleum based manufacturers, “the usual suspects” if you will include, Dow, Monsanto, Cargill, and Bio-based/Bayer. There are a lot of smaller based manufacturers as the technology gets more into the public domain.
One of the smaller guys that you might want to investigate is Soythane,soythane.com/, they have developed a particularly ingenuous low tech spraying mechanism suitable for small volume DIY’ers.
as soon as the weather clears, we are ready to apply a lime based stucco. is there any “shooter ” or quicker application method other than slap and trowel? the requirement to mix the day prior to application causes us to lift all of the stucco at least 3 times, i am not looking forward to this!
and whats this soy based material? tell us bill!
andrew, we await your reply!
Some great conversations here. Thanks everyone for joining in. I will address a few different comments here.
Barnie,
Yes, that will work fine. I don’t like building on edge for several reasons, but none that appear to exist in your situation so I think you will be fine proceeding as planned.
Jamie,
I hear you. I do see where moisture barriers can be used and I still avoid them. I would rather build to not need to rely on a moisture barrier all together. For example, I don;t build bales on my exterior walls behind showers. I use a conventional wall with water isolation walls on either side to the bales. Much less risk. I build homes with large overhangs and gutters to avoid rain splash altogether. On multiple story homes, we use intermittent roofs to create the same protection and I also use hedges and other landscaping to minimize wind driven rain. I totally agree with the good hat good boots concept. I just think that vapor or moisture barriers can create more problems than they save.
Whoops. I hit the submit button too soon.
Bill,
Thanks for the details about this system. I know several people have mentioned interest in getting more information.
John,
You do not have to mix and slake the NHL. That is a “best practice” idea. You can mix the plaster for a minimum of 20 minutes and apply it directly onto the wall. It will still do fine, just a little less workable than if you slake it. Most people will not notice a major difference between slaked and non slaked plaster. You can use spray guns to apply the material. Contact the folks at de Gruchy’s Lime Works. You can see the gun on this page: http://www.palimeworks.com/lwus/default.asp?page=tools.html Let them know I sent you and they will ask you for my vendor code. You can get a discount on your purchase by using the code. If you plan to move forward and purchase the gun ($250 minus my discount) contact me offline for my vendor code.
great information !! i thank you all for the input and related comments. i also have questions concerning placing shower/kitchen on bale walls, and am planning on using limited amounts of ” wall board” or concrete drywall where sinks and showers are going to be placed. tell me more about spacing for venting. i was planning on placing one or two 4X8 pieces to protect a corner shower, (both walls bale). i am open to your recommendations.. . how does one contact you off line? feel free to send me an e mail. again thanks for the info
Andrew,
It appears that you place your post and beam structure to the outside of the bales. Is this a preference or does it just work best for the roof, window, and door structures? I thought that I would place them on the interior to expose them (the look) and not have any differential strinkage problems on the exterior – interior is not subjected to the same temp./humidity fluctuations. Other opions are to hid them in the middle, but I assume that is a pain to knotch. Or, place them, knotched, on the inside and outside, like a ladder (I can’t remember the technical term).
Thanks again for the advise,
Jamie.
Hi John. I have a whole blog entry about this subject that I think will clarify your questions for you. Do a search for “Water isolation walls” at the top of the home page making sure that “strawbale.com is checked before you search.
Jamie,
I prefer to put my frame to the outside for the reasons you named; however, it is very possible to place that frame to the inside so it can be exposed. Don’t try the center option as it will drive you nuts. There are some other posts in the blog about placing your timber frame on the interior. Take a look and you will find some important details about how to accomplish this well.
Andrew,
Have been wanting to attend your workshop, but wanted to make sure that building with strawbale is feasible in my area (Mississippi)due to temp and humidity. It is on my farm, no permits required, just hot sticky days in the summer, cold wet during the winter. Figured large overhangs, positioned correctly, slab….. The zip code is 38901, any concerns or advice?
Hi Brian,
Do you ever get any dry weather where you want to build? I would be concerned about the moisture issues inside the walls if there is never a chance for them to dry out. As you may have read above and in other places in the blog, figuring out how to translate relative humidity into moisture content is difficult. A test wall might be a good idea. Build it and let it sit for a year. You can then monitor or tear down the wall and look for signs of failure or success. Hope that helps.
Hi Andrew. Thanks for the answer. Please give more detail about the reasons why you dont like building side up with the bales. Please, what is the difference between lime for plastering and a good mix of cement, lime and sand for plstering. I have come accross people doing it that way.I am not sure about amount of sand, lime and cement they use. Can it work if I have a problem to get the right lime.
When on edge, the strings of the bales are in the way for notching around posts or creating decorative niche. The face of the bale that is exposed is more slippery and does not hold plaster well. The bales are off center, i.e. taller than they are wide, so they are not as stable. These are the main reasons.
I prefer lime and sand only because it is more flexible and breathes better than cement based plasters.
My wife and I are building a 1200 sq ft cabin/loft. We want to incorporate straw bale into the home, but without many “straw” experienced contractors in our area and even less experience between ourselves, we have decided to go with a framed straw-infill home. We want to have cedar siding on the home and use the strawbales on all interior walls that do not have plumbing. I was planning on having the contractor frame, roof, and put the cedar siding on the entire house, and then we will do the majority of the interior work ourselves.
Would we be able to get the same insulation advantages and sound proofing on the interior walls, albeit not to the extent of 18″ thick bales, if we used say 4-6″ chainsawed bales inside the framing instead of blown in insulation and drywall?
Also for the interior walls that are parallel with the exterior cedar sided walls, would it be sufficient to just lime plaster the back sides of the bales and stuff them in place and then finish plaster the interior facing walls? Would I need to leave an “air gap” of a few inches between the cedar and the “insulating” straw bale walls? Any advice or suggestions would be greatly appreciated.
Hi Justin. This is a tough one. If I read your post right, you want to have interior bale walls, like between say a bedroom and the living room. This takes up a lot of space. Then I see you are considering using chainsawed bales to make them more space efficient. I think this would be a waste of time for you. You can get a better insulation value out of standard insulation at that thickness. In terms of sound, it travels through the studs so if you want to eliminate it, you need to build two walls separated by an inch air space or use sound proofing materials.
For the exterior walls, I think you will find it easier to install the bales into the frame (with the roof on) and then plaster the exterior face of the wall with a scratch coat, After the scratch coat has dried, then install the cedar. You can use a rain screed installation with a vent between the bales and the siding for best results.
I d like to get a domme like this one ! More posts like this?
Hi Andrew. I live in the upper chehalis river valley in western Washington. I have 2 problems. I have some oak logs which i want to use to build a simple 2 story gable barn style home with, but no one around here is certified to grade hardwoods. Secondly, Strawbale appeals to me over clay-slip because of the economy of simply stacking and pinning the bales together. But, high humidity is common here for most of the year, so spontanious combustion is a concearn for me, as it is for local farmers who store it in their barns and use it for their animals. What would you say to all of this. Thanks.
Allen
Hi Allen. I would suggest talking with local engineers about the lumber grading. I imagine someone will know of a certified grading company. You could also talk with the larger local lumber yards or even go direct to the mills nearby (if there are any).
In terms of the humidity, I we talking rain or humid? I know of several straw bale homes in Seattle and Whidbey Island, and those are doing fine. If you are similar in weather to that, you will be okay. I would suggest that you use a whole house dehumidifier in your design to help keep moisture moving out of the home.