Breathability of Strawbale Walls

Catherine Wanek After growing up in Las Cruces, NM, Catherine lived in various places until she settled in Los Angeles to work as an assistant director in Hollywood film production.  Lured by the beauty of New Mexico, Catherine purchased the Black Range Lodge in the 1980s with a vision of making movies there, and developing the property into a lush permaculture landscape, which now includes an abundance of fruit trees.   Since then she has been providing a comfortable respite for others seeking rest and relaxation in a natural, healing environment.  After building a straw-bale greenhouse in 1992, she became an advocate of “natural building”, traveling internationally to research and educate, authoring books and videos, and hosting hands-on workshops at the Lodge to spread the word about ecological construction methods.  Books she has authored on these subjects include The Hybrid House: Designing with Sun Wind Water and Earth, The Art of Natural Building, and The New Strawbale Home.

Questions and Answers

Q: I would like to build a straw bale house. I have not decided yet if the walls will be load bearing or not. However, I would like to have vinyl siding on the exterior and sheet rock the interior. I like the insulation value of the straw but I don't care for the adobe look. What would be the most practical and economical way of going about this while saving time and money? Would the bales still need to have the stucco on them?

A: (Owen Geiger) Whatever you do, do not use vinyl siding or any other impermeable wall treatment on straw bale walls!  Moisture needs to pass through.  You could use many different materials such as wood siding, etc.

Q: Would it help if there was a space between the bales and the siding? And that space vented into the attic space.

A: (Owen Geiger) Do NOT create an airspace between the bales and siding.  It will create a fire hazard through a chimney effect.  Add blocking if necessary.  One option is to slip 1/4's under the twine and nail/screw the siding to them.  Coating the outside of the bales with earthen plaster wouldn't hurt.

Q: Mr. Hart writes: "Breathability is virtually essential with straw bale walls, or the risk of rot from accumulated moisture is too great... Wood framed or sheathed buildings are trickier to make breathable without jeopardizing the wood to rot from moisture; careful ventilation must be provided." Hence my "double" problem: I would like to "wrap" an existing wooden structure (a 11x16ft garage) with straw bale for insulation! My issues are: 1- painting (sealing?) the wood surface (interior?/exterior?) ecologically 2- breathability: leaving a gap between the wood wall and bales? putting up a vapor barrier? "finishing" of the bales at interface of walls, roof and windows).

A: (Jeff Ruppert) Ideally, you would be able to create a gap between plastered bales and the siding, in essence, creating a rain screen. Not knowing your location, I must assume wet conditions, and therefore recommend separating the plastered bales from the siding. If you live in a relatively dry climate, you will still need to make sure all of the exposed bale surface is plastered with at least one coat of plaster. One way of doing this is to dip the bales in plaster (on the siding face) as they are placed. The reasons for this are many. First, insulation values are preserved by the lack of convection loops on the bales themselves. Second, you keep out bugs and rodents who like straw - this is very important! The third reason is to reduce fire danger. Fire will climb along the siding and bale interface if no plaster is present.

Breath-ability - again I don't know where you live, so this must be approached climate specific. Heating climates are different from cooling climates, but having plaster on both sides of the bales will control moisture transfer considerably. NO moisture barriers are needed - ever! Don't make me repeat this, please! Good luck!

Q: I recently purchased a straw bale home built in 2000 in the Sonoran high desert of southern Arizona. We receive about 15" of rainfall a year, most of it in July and August. Lots of sunny dry weather the rest of the time! Unfortunately, the builders did not tint the concrete/stucco exterior coating; it is a rather dreary gray. I would prefer a terra cotta color in keeping with the natural tint of the soil in the area. Would it be appropriate to paint the exterior, given our relatively dry climate? What about elastomeric paint to minimize the appearance of cracks? Would you recommend another technique, possibly using the (sandy) soil itself as a tint?

A: (Jeff Ruppert) You could possibly use the soil on-site in the form of an earthen plaster, but you would need to use a bonding agent to make sure it bonded well to the cement base coats. Another option is to spray the existing plaster with muriatic acid to achieve a rust color. This is probably your cheapest option. I would suggest experimenting prior to doing it. I would lean against paint, but given your climate I don't think it would cause serious issue. I might add that another option could be to have a "color coat" of stucco applied to the currently gray surface. It sounds to me as though the stucco job was never completed. These color coats come in a wide range of tonalities. I would advise against elastomeric paint, as this would completely eliminate the breathability of the wall, which is not good for strawbales.

Q: The problem is, this will be the first straw bale building in the city of San Diego. And while they have allowed straw bale (non-load bearing) they have balked at our plans for adobe plaster, covered by a lime slip, on the outside of the bales. I've done the testing here, and the results are quite water tight (and vapor permeable), but that's not good enough for them. We need help convincing them that this a reasonable thing to do. We need literature, web links, test results and examples of municipalities who have accepted similar external coverings for straw bale walls. Any help you can provide will be very much appreciated.

A: (Owen Geiger) Here are two links that should be of help: Ecological Building Network - ecobuildnetwork.org
(They've been involved with much of the strawbale and earthen plaster testing.)

CASBA - California Straw Building Organization -strawbuilding.org (This is the main strawbale organization in California. They've also been active in testing earthen plasters on strawbale walls.

Q: This question is about real-world insulative properties of strawbale, though it may be applicable to general construction materials as well...In an attempt to understand energy efficient building designs and materials (being new to design and construction), I read that strawbale construction can have up to an R-49 rating (bales stacked on edge). I've also read that strawbale construction makes for a very breathable home, providing upwards of 3 air exchanges per hour (this with proper construction). Both of these statistics are cited in the book "The Straw Bale House."

What I'd like to know is this: how can a building that has upwards of 3 air exchanges per hour (with no mechanical assistance/technology) be energy efficient? What difference does it make what insulative values that a home has if it has a high rate of air exchanges per hour? NOTE: The authors of the book "The Straw Bale House" state that the upper limits of strawbale homes range from 1.5 to 3.0 air exchanges per hour for homes using only plaster finishing- no (non-natural) vapor barriers (which don't appear to be desirable with/in strawbale construction).

A: (Owen Geiger) The Straw Bale House is way out of date and inaccurate on some points. A new version is due out very soon. A number of tests have been done since The Straw Bale House was written. The insulation rating is closer to R-30 to R-35, depending on thickness of bales, etc. The orientation makes little or no difference ("flat" or "on edge"). Don't get too concerned about the exact ratings, because even major US labs don't agree with each other. At some point, it doesn't matter. We know straw bale walls are approximately 2-3 times wood-framed walls (twice 2x6 walls, and three times 2x4).

Air exchanges: you can make a strawbale house as air tight or as leaky as you want. It all depends on how much effort goes into detailing the gaps around doors and windows, how much ceiling insulation there is, caulking, etc. And yes, you want to use vapor permeable finishes. Another factor is internal mass in thick plaster, stone floors, etc. This mass helps moderate the internal temperature, giving strawbale added energy efficiency not shown in some tests.

C: More research has given me better insight into the insulative factors at play. I now realize that stick-frame buildings' insulative efficiencies are strongly compromised due to the studs: I recently got an infrared temperature/heat gun, it really tells the story! With regard to air exchange numbers, given a fixed rate for both a strawbale and a stick-frame home, am I safe to assume that a strawbale home's energy performance wouldn't be any different that the stick-frame's? Or, perhaps the more (with conductivity playing a part- e.g. studs acting as thermal bridges).

A:(Owen Geiger) Three ACH per hour is very poor performance. Think about it: this means all the air inside the home is replaced by outside air three times every hour! You'd have to almost open the windows to achieve this. Something is wrong. The strawbale homes I've built have ranged from .1 to .4 ACH. These ratings are based on official blower door tests performed by Energy Rated Homes of Colorado - the state-recognized organization that specializes in this type of work.

You said "given a fixed rate for both a strawbale and a stick-frame home, am I safe to assume that a strawbale home's energy performance wouldn't be any different that the stick-frame's?" It would be difficult to get a stick-frame house down to .1 to .4 ACH. Have you stood next to a stud wall on a winter day while the wind was blowing? Whoaa.

The thermal bridging point you made is true. It's also true that fiberglass ratings in the field do not match the ratings under ideal conditions in the lab. An R-19 wall actually measures closer to R-11.

Another point is internal mass. The thick plaster on the inside of straw bale walls has been shown to play a significant role in stabilizing indoor temperatures. This is the flywheel effect. Have you ever noticed how often a forced air heater kicks on in a stick-frame house in the winter? It's cycling on and off almost constantly. This is due partly to the lack of thermal mass and partly to excessive air infiltration.

Q: I am living in a load-bearing hemp bale house built in summer/autumn of 2004, in Ontario, Canada (Winter temps in the -30s, summers hot, but can be rainy). My house is not quite finished. I hope to apply a final exterior coat of tinted stucco this summer (2005), and I'm still busy finishing off the interior. I want some color on the interior walls, but prefer not to go with tinted stucco inside. Are there any paints I can use that will allow the walls to remain breathable? I don't know much about lime washes, but have been told the color rubs off if you touch it. Is this true? What do others use for color on the interior walls of their bale houses?

A:(Owen Geiger) Most commercially available paint has very high embodied energy (the energy required to produce and transport it) at around 61-93 MJ/kg. You may have heard how cement is causing about 7% of global warming due to the large amount of energy needed to produce it. Well, cement has about 5.6 MJ/kg of embodied energy. So paint uses about 10-16 times more energy than cement, plus it's full of toxic chemicals to kill mold, etc. which off-gas into the home.

Natural alternatives:
1. BioShield makes nontoxic paints, but get ready to pay $30-$40/gallon: www.bioshieldpaint.com

2. Lime wash with stucco pigments and latex: This is a low cost option that doesn't rub off much. Kelly Hart just used it on the exterior of his house, so he can explain the process in detail. This seems like an excellent choice for durability, resistance to rubbing off and for inhibiting the growth of mold. One minor drawback is the difficulty of getting a perfectly uniform-colored coat. But to me, the slight variation looks natural.

3. Clay paint or aliz: This will likely be the lowest cost option and it is very beautiful. It will rub off, however. Ideally, you would use local clay. Get a book and follow the recipes: Earth Plasters for Straw Bale Homes, by Keely Meagan. The Natural Plaster Book: Earthen, Lime, and Gypsum Plasters, by Cedar Rose Guelberth and Dan Chiras. (One option is to skim through them first at a large bookstore.)

4. Milk paint: Another nontoxic option, but some people have complained of poor coverage. It doesn't appear to be very durable.

Q: We are planning a load-bearing strawbale chicken coup addition in Southern Alberta. I would like to put sheet metal on the outside mounted on 1x4 horizontal straps. Do I need to put some type of material over the bales such as tar paper or should I leave them exposed behind the steel which means that there will be a 3/4" air space? The inside will be covered with stucco or parging.

A: (Owen Geiger) When it comes to animal pens like this, I think the typical rules for strawbale building can be greatly relaxed or even eliminated. I wouldn't worry much about the details. Why not use just bales - i.e. an unfinished bale wall? Skip the stucco and metal siding and consider it a temporary building. Or for a more durable building (and also reduced costs), use earth plaster.

Q: I am working on a plan for a strawbale home for myself. I am a general contractor and like the concept and hope that the current trend in commercial construction, i.e. LEEDs qualified, trickles to residential. My home will become a model for others to follow, hopefully! I am in New Hampshire and the climate seems viable for the construction of a strawbale in-fill home. I am planning a saltbox timber frame, with hefty over hangs and proper drainage...bla..bla..bla. I am concerned with the exterior. Stucco, earthen clay, Adobe, do not appeal to the local pallet and will not catch on. I want to seal the exterior with plaster and then put on a rain screen with wood shakes or beveled siding. What is the best approach? Can you save time on the plaster coats some how? Is there a best practice for attaching rain screens to plaster? Is the price just not worth it?

A: (Owen Geiger) Your overall design sounds good -- LEEDS standards, wide overhangs, good drainage, regional design.

Yes, you can use siding on the exterior.  Just be sure it is designed in such a way that moisture can pass through the wall.  You'll want to 1. coat the exterior of the bales to protect against moisture and fire, but they don't need to be fully finished (so you save money by not applying a finish coat -- also consider spraying on the plaster to save time and money);  2. add nailers to attach siding before plastering (1"x2"s or 1"x4"s would work -- these can be slipped behind the twine if the bales are on edge, or you can simply attach with twine if the bales are stacked flat); 3. install siding that allows moisture to pass through (wood shakes, beveled siding, etc. -- I'd use a nail gun for ease of nailing); 4. optional: apply a breathable finish that allows moisture to pass through.

Note: there will be a small 1/4" or so gap between the wood siding and the bales.  You don't want a large gap that will create a fire hazard (through creating a chimney effect), but a small gap is fine.

Q: We live in the foothills of Northern California, and would like to retrofit our house with strawbales. I've read quite a lot about adding to the outside of the structure, but we would like to add strawbales to the inside of the structure. Can we place them next to the drywall that is already existing, then plaster the "new" interior side? Would we need to do anything special to the side next to the drywall to seal it?

A: (Owen Geiger) In your area, you shouldn't have any problems.  If you have very high humidity, then there's the risk of mold growing on the paper facing of the drywall.  Provide good ventilation at major sources of humidity (kitchen and bathroom) and you should be fine.

Q: 15' porches that will encircle the entire building will be screened and covered. Pretty much water proof, in that. Can the bales be completely sealed with tyvec?

A: (Owen Geiger) Don't use tyvec or anything that can hinder the transfer of moisture through the walls.  Also, the 15' wrap-around porches will further restrict the amount of light entering the windows.  Plan accordingly so you have good daylighting.

Q: I would like your comments on a design that a fellow in Wales used to construct a family home there. (Please see www.simondale.net/house/index.htm) In particular, it appears that he used strawbales to form an earth bermed wall on the back side of the structure. If one used a breathable interior plaster, do you believe this to be feasible?

A: (Owen Geiger) When you build ultra low-cost, un-permitted, non code-compliant shelters, the "rules" (optimum building methods) start to go out the window.  There's a certain freedom in building something dirt cheap and not caring if it all falls apart in 5 years.  However, if you want your house to last for a reasonable amount of time, certain details are not recommended. How long will it be until there are tears or pinhole leaks in the roof plastic?  Of course, water will quickly rot the roof bales and turn the house into a mold-infested space.

Bales below grade (in the floor and against bermed earth) are not recommended either.  It's the same logic -- you're fighting the laws of nature and eventually the bales will succumb to moisture problems. Better alternatives include pumicecrete, stone, rammed earth tires or earthbags (possibly filled with pumice) against the bermed earth, pumice in the floor, and bags of pumice in the ceiling.

Q: One of my concerns was that of can strawbale be used in any climate conditions and it appears from the Q&A that this is the case. I have been considering using a siding - either vinyl as I saw one on tv or give it the cypress log look and to use that on the inside as well. Will this work for a good exterior instead of plaster or do you still need to use plaster? I do live in fairly humid Missouri.

A: (Kelly) The thing about strawbales is that it is important to allow them to breathe, so that if any moisture does get into the wall it can eventually escape. Vinyl and perhaps the other siding that you mention may not allow enough breathability. Also, the vinyl on the outside will provide a place where condensation may form to make even more trouble. You are much better off with a breathable plaster, such an an earthen plaster, or a stucco with a low proportion of cement in it.

Q: From what I've seen after all my research into straw bale, I still get mixed signals when it comes to the use of non-permeable interior finishes. I would like to know your opinion on whether it is important to have breathability into the house as well as out. Also if this is a straw bale no-no, what about only having such finishes on walls in high moisture areas such as the kitchen and bath?

A:(Owen Geiger) I recommend breathable finishes and high quality fans vented to the exterior in high moisture areas -- kitchen, bathroom, laundry.

Q: My husband and I have been researching and daydreaming about building an earth friendly house for over a decade, and have finally managed to save what we consider an adequate amount of money to begin building. Our final plan has centered on strawbales, (mainly because of the insulation factor and the ease of building), but with a few twists. We live in tornado country, and even when we don't have a full-blown twister spiraling through, we tend to get a lot of straight-line winds (which do a pretty good job of knocking down trees all by themselves!), so we want to cover our bale structure with ferrocement (including the roof). We actually much prefer the idea of lime plaster to cement for a number of reasons (including environmental concerns about cement's high embodied energy) but feel we really need the strength of ferrocement considering our weather conditions. Is it possible that lime plaster alone would be strong enough for this, or can lime plaster be successfully incorporated into a steel mesh framework (as with ferrocement), to produce a similar strength end product?

A: (Kelly) With strawbale walls breathability is a very important factor, and most ferrocement is so rich in cement that it is pretty much waterproof (they build cisterns this way). Either lime or a less cement-rich stucco would be better. Any of these can be placed over a wire mesh for reinforcement.

Q: I'm in the process of buying a house that, among other things, I would like to do an internal strawbale wrap. My concern is that the house is made of 100 yr old brick with a crumbling cement stucco. On the interior wall I will do a cob and natural plaster. If I remove the stucco, will this be a sufficient breathable exterior covering for the bales or do I need to create some sort of breathing system for the bales? The straw will be non-load bearing and 14" thick.

A: (Owen Geiger) Remove the crumbling plaster.  Leave a 1/2" gap between the bales and brick wall.  Provide weep holes at the base for any moisture to drain out.  Dip the bales in clay slip to coat the side facing the brick.  Use earthen plaster on the bales.  Install high quality fans in the bathrooms, kitchen and laundry areas that vent moisture to the exterior.

Q: I have a follow up question regarding the air exchanges per hour (ACH) for straw bale homes. Efficiency Vermont, an independent efficiency 'utility', has data from blower door tests done on roughly 10 straw bale home in Vermont. The 'best' result was an ACH in the low 2's. Most were in the 3's and the worst was in the 6's. You stated in an answer to an earlier post that you have achieved ACH in the 0.1 - 0.4 range. Can you give more details on what it is you are doing to achieve a much tighter home than others have apparently built? Also, is there a larger available list of blow door test results for straw bale homes? As you probably know, the benefit of large R-values in walls that are achievable with straw bale homes rapidly becomes of less and less importance if there is anywhere the infiltration evident in the blower door results measured by efficiency Vermont. Any help or resources you could direct me towards that would help me end up with a tighter home would be greatly appreciated.

A:(Owen Geiger) We went to great lengths to seal the Habitat strawbale houses for the reasons you've mentioned.  We bought caulk by the case load and sealed everything, and I mean everything: holes drilled for electrical wires, any penetration to outside walls, electrical boxes, windows, doors, etc.  The edges where walls and ceilings join is especially critical.  For example, sloppy sheetrock work may leave 1/4" gaps along these edges.  The edges are covered with drywall tape, but heat will pass right through this thin paper barrier.  Our energy rater was very helpful in tweaking the design and checking everything.  They even used smoke to help locate air leaks. 

One house was post and beam, stuccoed with cement plaster directly on the bales.  This isn't usually recommended, but it has worked for the dry climate in Pueblo, CO.  The second house was our most efficient, in part because we learned a lot from the first one.  We bermed it slightly on the north wall, used a slab on grade with insulation below, good passive solar and hot water heat.  On this house we stacked bales inside a stud wall that was insulated and sheathed with 7/16" OSB.  Each joint in the sheathing was caulked and then we added lap siding, which was also carefully caulked.  In addition, we advised the owners to check the caulk every year.  Caulk is cheap in comparison to heating with gas.

Another factor to consider is the accuracy of the tests.  This is not an exact science and three different testers will likely get three different test results.  I wouldn't worry so much about the number on the report, but instead focus on the actual efficiency of the home.  Doing everything mentioned above will be a good start.

Q: For a future strawbale home project, a two story home made from the frame of a old barn, I would like to use cast stone siding over some of the exterior strawbale walls. Perhaps 7 - 8 feet high along the first story. Would this cause moisture problems? The only information on cast stone about permeability I could find was this: "Cast Stone has a natural permeability which is approximately equal to natural limestone and architectural concrete." Is that good enough? I should mention that I live in a seasonal climate that ranges from frigid winters to hot summers. Does the changing of the seasons require the strawbale walls to breathe more or less?

A: (Owen Geiger) Vapor impermeable barriers such as concrete are not recommended for straw bale walls.  If you choose to go this route anyway, coat the exterior bale wall surface with clayey earth and leave about 1/4"-1/2" space between the bales and cast stone to allow any condensation to drip down and out through weep holes.  Vent bathroom fans and range hoods to the exterior.  Seal the house carefully to reduce air infiltration (and the associated leakage of moisture vapor.)  Use a dehumidifier in humid climates.

Q: Is there a method/mix for coating strawbales that has emerged as the most reliable/durable? I've seen lots of different opinions posted on this, but the more I dig, the more it seems that no method prevents even short term cracks from forming. While that's ok, once I have my home, I want to spend the bulk of my time using it, as opposed to repairing it... any thoughts?

A: (Owen Geiger) Most strawbale structures are plastered with lime or cement plaster, or with earth plaster for interiors or on exteriors if you have adequate roof overhangs. There's a large body of knowledge that's been built up and published in The Last Straw journal. I suggest buying one or two of their back issues that focus on plastering (each issue has a theme). Small hairline cracks are not an issue with good design and detailing, such as wide roof overhangs.

Comment: Here is a link to a strawbale dome being built in Germany, using an EPDM liner over it to protect the bales from getting wet: www.minke-strawbaledome.blogspot.com

Response (Kelly): Thank you for letting me know about this interesting project. You did a beautiful job of building this dome. And it looks like you had a lot of fun doing it.

I am concerned about the longevity of the dome however, since the combination of a straw dome with an EPDM liner on the outside, while keeping moisture from entering the structure, will likely cause condensation to form on the inside of the rubber liner and eventually rot the straw. The only successful straw dome or vault projects that I know about place a roof over the straw with an air space so that the straw can always breathe and dry out. I hope I am wrong about this, for your dome's sake...

Of course we know well about this problem. On the other hand, I don't know about a design that is really airtight inside and outside (we are going to do a blower door test soon as well) so we don't have a problem of moving air, but only diffusion of vapor. We also will have a ventilation system with heat recovery, and this will considerably lower the moisture content in the dome during winter, probably as low as 30%RH at 20°C. Even at this level there is a risk of moisture as you mentioned, but the absolute moisture content is very low, and we believe the 4-5cm thick clay plaster coat under the liner should be able to absorb and protect the straw.

In summer the hot weather and heating by the sun from the outside and relatively cold inside should let the moisture escape trough the inside. In winter the moisture content in the outer layer will definitely be high, but the temperatures very low, so there should be no risk for mold growth. We plan to open the membrane next year to check on the straw on the outside. And we have 12 sensors to follow different moisture and temperature values.

In this project we wanted to verify the structural integrity of straw bales in a dome construction and use natural materials in a setting that is otherwise close to passive house standard (air tightness, triple glazing, heat recovery etc...). The other big experiment is covering the straw with a non permeable membrane, and done so on purpose. We might have to remove the membrane next year, and do a ventilated membrane construction, but for the moment I think this is going to work out - might change my mind later ... :-)

Thanks for explaining all of this to me; I appreciate your attention to detail. Obviously you are aware of the issues involved, and this a really interesting experiment. I would love to be kept informed of how this all works out over time. Perhaps this is all fully explained at some point in the blog, and I missed it, since I did not read every page. I wouldn't want a casual reader to think that your experiment could be replicated without the controls that you mention.

Q: I am interested in building with straw bales. But, I want to use metal siding instead of stucco or clay etc. I have an idea about building 2x4 stud load bearing walls. Placing the straw bales up against the stud wall. This would leave a 3.5" gap from the bales to the outer wall. I would attach the metal siding to the outside of the studs. I have an idea I would incorporate a vented area at the top of the walls where the walls meet with the 3' eave over hangs. Do you think this would work to allow the straw bales to expel any moist and breath properly? I could also put some sort of Tyvek wrap between the metal siding and the studs which would eliminate any metal sweating that might occur from the siding.

A: (Owen Geiger) Stud framing/strawbale: This is what we did in Pueblo, Colorado for two out of three strawbale houses. We filled the stud space with insulation for improved R-value. We used OSB and wood siding on the outside. Others have used metal siding. Either way is okay.

(Kelly) I might add that your idea of providing a venting "rainscreen" effect with the metal siding is a good idea.

Q: Can you comment about sheathing (with boards) directly over straw bales, both inside and out. I'd like to not use plaster if possible for this first project.

A:(Owen Geiger) I saw this done in Colorado and it worked fine. First, you have to tie vertical 1x2s or preferably 1x3s nailers to the bale wall with baling twine or cord. You might want to add nailers to both sides. This would stiffen the wall and make it easier to attach them. Tie them together really tight using a truckers hitch knot. Use a 'baling needle' to feed the twine through the wall. Take care to keep the nailers in one plane so the final plywood is reasonably flat. Add nailers about 24" apart. 1/2" plywood is all you need. On top of the plywood add tar paper and stucco mesh. This will greatly speed the plastering job and save on plaster.

Q: I recently built a strawbale home in northwest Arkansas. I have 2 coats of natural hydraulic lime plaster on the building (scratch and brown coats), but they were not done well so I have a lot of cracking. I contacted a plaster company and the guy is recommending to use a fiberglass mesh around the entire building and then use Sto Powerflex - an elastomeric stucco. If the Sto product is only used on the outside of the building, could I prevent trapping any moisture in the bales by reducing the humidity inside the house through air conditioning and my ERV/HRV system? Or is this just a bad idea completely?

A: (Kelly) The mesh might be a good idea but elastomeric stucco is not. Strawbale walls really need to be breathable or it is too risky. You cannot rely on interior air conditioning to avoid problems that might arise.

I think we are leaning away from the elastomeric plaster (i.e., Sto Powerflex Freeform).  We are now talking about a non-elastomeric plaster that is acrylic-based.  They claim the US Perms are over 30.  Do you have any insight or opinion about that?

My understanding is that anything greater than 10 perms is considered vapor permeable and is appropriate for use with straw bale walls. I can't vouch for the product that you mention.

Q: When we realized we are likely not going to be able to get the outside of the bales plastered before winter, we decided we would Tyvec the outside of the bales. However, we were just reading in your Straw Bale Q & A to not use Tyvec. Do you think it would be an appropriate temporary solution, though? Also, we were planning to lime plaster the straw bale walls in our wood shop and wood storage area. Do you think it will be durable enough, though? Specifically, I'm wondering if we should do a metal siding over the bales in the wood storage area because I am concerned the plaster will get too dinged up and damaged. I was reading in the straw bale Q & A that it is possible to put a scratch coat on and then cover with a different siding, such as metal, but do you think this would be the better option in a situation such as ours, or would you still advocate for plaster? We plan to have large wood slabs leaning against this wall.

A: (Kelly) I see no problem with temporarily using Tyvec to cover the bales until they can get plastered; it is breathable and will keep most the moisture off of the bales. Lime plaster is quite durable and is a good choice for straw because it is also breathable. If you do decide to use metal or other siding material than plaster, then you should probably install it with a "rain screen" system that allows air to circulate between the siding and the straw.

Q: Owen talked about the house he built using stud framing, stacking the bales inside and filing the stud void with insulation and sheeting with OSB and siding with wood. Does this give enough breathability. I'm thinking of doing this and using EPS. What do you think?

A: In the design scenario you describe, your stud walls with sheathing will not be permeable, so regrettably, the answer is "no".  Over time, you would likely find some moisture vapor trapped by the (OSB?) sheathing, and that would likely condense into liquid water inside your wall.  You don't mention what climate you are in... in some rainy climates, straw-bale walls are finished with a ship-lap siding, to deflect rain, and protect exterior plaster from getting loaded with moisture.  In this case, the straw-bale walls would be plastered on the outside with a clay plaster, and there would be a ventilation gap between the plaster and siding. In a very dry climate, it's possible that you could get away with impermeable exterior sheathing, if you plastered the exterior of the straw bale with clay, and allowed some air space between this plaster and the planned conventional insulation (fiberglass?).  In any scenario, it is very important to plaster both sides of the straw bales, and if possible, even the top of the bale wall, to seal the bale from insects and the possibility of fire.

We're building in south central Montana and usually it's mostly dry except, of course, this year. We intend to do a stucco/lime plaster finish on the inside.  Would this be enough to allow the bales to breathe?

If you put a coat of stucco on one side and not the other, the bales will be structurally unbalanced.  The wall will literally want to fall over, due to gravity, with heavy plaster on one side and not the other.  With plaster on both sides, the bale wall performs like a stressed-skin panel, and is very sturdy -- and can serve as a load-bearing wall system without additional structure (depending on design, of course).

Generally, both interior and exterior plasters should have similar permeability qualities, as there are seasonal changes in pressures driving vapor through the wall, as warm air moves to cold.  In the winter, warm air from inside moves toward the outside, and in the summer, warm outside air moves toward the cooler, air conditioned interiors.  Earthen plaster is the most permeable, and actually stores moisture safely, as it is hydrophilic, and readily absorbs water vapor, without wicking it into surrounding materials.  Unlike cement plasters.  Lime/cement stucco is adequately permeable in the ration 1/1/6, that is one part lime, one part cement, 6 parts plaster sand. I really recommend that you pick up the book Straw Bale Building Details.  It really represents the state of the art, and the best practices based on the most experienced architects and builders.

Q:  Would it be advisable to put a Parex (synthetic stucco coat application on a 20 yr old straw bale home that has only a thick layer of cob? Applying it with a wire lath over the cob to help it stick? Would the walls be able to breathe?

A: I'm not familiar with this product, but if it is a synthetic (elastomeric) coating, it is likely not "breatheable".  This term is often used, but actually a misnomer, to describe the "permeability" of plasters, which is more akin to sweating than breathing. However, you might use a regular cement-based stucco mix with a one-to-one ratio of portland cement and lime, plus 5 or 6 parts sharp plaster sand (and water. Using the minimum amount of water, and some poly micro fiber, will reduce cracking).  This stucco mix has been shown to be adequately permeable in a Canadian study.  Yes, you would need a wire mesh/lath to support and reinforce this new plaster, as cement does not adhere to earth.  The wire should be securely attached to the top, middle and bottom of the wall.    You don't mention where the house is located -- there is a bit more leeway for permeability in a dry climate than a rainy climate. Hopefully you have good exterior overhangs, and interior clay plasters as well.  Clay's hydrophilic properties actually help protect the bale walls from moisture vapor.

Q: I purchased a 20 yr old strawbale home which has had a recent exterior stucco finish that needs painting..The contractor told us the home will need to be painted before the SW Az monsoons start in July..I was told best to use a primer coat and then an exterior coat. Please tell me what the best primer and paint to use for the breathability of the exterior walls..the inside walls have an Aliz finish.

A: Probably the best kind of paint would be a silicate paint. You will find a very good explanation of how a silicate paint protects while remaining permeable to moisture vapor on the website: EndeavourCentre.org/resources. There’s a section on finishes, which discusses silica paints and how to make them. There’s an awful lot of other great ecological building information on their website as well.

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