Mold Spores in Straw Bale Homes

I recently heard that some, if not all, straw and hay out of the Pacific Northwest has a mold spore in it. The question that accompanied this information is how to counter act the affects of the mold on the bales so they can be used for constriction.

The answer is quite simple: keep the bales dry. Mold can only grow and therefore have an affect on the bales when there is moisture. As long as the bales are kept dry, the spores will not be able to grow and will eventually shrivel up. The walls, sealed with plaster, will not allow any of the spores to enter the habitable space.

47 Responses to Mold Spores in Straw Bale Homes

  1. Barbara Ann Wright Sat, January 26, 2008 at 6:39 am #

    Andrew is there any way to tell how moldy the bales of straw are that you are buying. I realize that buying something that is already turning black isn’t smart however I would like to know how to tell if the mold level in the straw bale is safe to use.

  2. Andrew
    Andrew Sat, January 26, 2008 at 7:54 am #

    In general, it is about appearance and moisture content. Using a moisture meter to test the amount of moisture in random bales is a great idea. you can get a good idea of the quality of the bales by checking that number as well as a visual inspection. If the bales look dirty or dull, they have probably either seen moisture or the rake was set too low while baling. If the rake was low, the chance of having dirt and other junk in the bales is high and hey probably are not the best option for you.

  3. Brian Steel Sat, January 26, 2008 at 7:59 am #


    I can’t stress enough how important it is to keep bales as dry as possible. Lung damage due to inhalation of spores is a very serious business. Here in UK there a lots of farm workers who rue the day they worked with moldy bales. A friend on mine suffered years of debilitating chest problems because he used damp bales and contracted what in UK is known as Farmers Lung. So if bales are delivered with mold clearly visible or raises a cloud of smokey dust when hit REJECT them. If you already have them and the problem is due to your lack of care then take the offending bails out into the middle of a field and set fire to them. Don’t chance years of ill health on the price of a few bales of straw.

  4. Gilberto Sat, January 26, 2008 at 1:42 pm #

    Okkkkk…let’s see if I can make this question clear. We are about to begin building…BUT…the only way for us to buy bales is to order them from a company in Colorado. We live in Texas. How to make certain that the bales we are buying are suitable for construction? It would be crazy to drive to Colorado to pre-inspect the bales…the only recourse that we have is to trust the company. Given the sad experiences that we have had with other contractors during this building and planning process, trust is getting hard to come by…so the question is…what to do to make sure that we get good bales?
    Thanks for your input

  5. Andrew
    Andrew Sat, January 26, 2008 at 9:32 pm #

    Brian: Good point indeed!!!

    Gilberto: I would suggest you ask them to send you test results of moisture readings with some kid of guarantee that the delivered bales will meet the test results upon delivery. I would also ask for pictures of the bales and the storage facility if applicable. Finally, I would ask what their return policy is should they not meet the criteria you spec upon ordering.

    That said, I would look long and hard for a nearby source. I find it hard to believe that with all the cattle in Texas there is not a local source for straw. You might contact the Texas Department of Agriculture’s Hay Hot Line at,1987,1848_5410_0_0,00.html?channelId=5410. (Yes, I know it is for hay, but the farmers will know where to find straw). You might also try the Texas Hay Exchange at I hope that helps!

  6. Julia Birky Sun, January 27, 2008 at 11:53 am #

    Hello Gilberto,

    We are in the process of building in Texas and got really nice straw bales from Texas. Email us if you would like information on our straw bale source.

  7. Bill Christensen Sun, January 27, 2008 at 8:50 pm #

    In addition to the sources that Andrew mentions above, we put together to help bridge the gap between straw sources and buyers. It’s essentially a classified ads site for straw, with sections for bales available, bales wanted, tools, and even transportation. It’s free to post. We ask for a donation if a deal goes through, but to be honest we’re not that concerned about it.

  8. Andrew
    Andrew Mon, January 28, 2008 at 9:07 am #

    Thanks Bill. I thought about this link and then almost immediately forgot to put it in the post. Thanks for posting it so others know about it.

  9. Sarah Mon, February 11, 2008 at 10:28 am #

    Hey guys. I’m no where ready to buy or build a house, but I’m looking to the future. My boyfriend and I are currently in the planning stages of our relationship and he is very supportive, understanding and even a bit enthusiastic about my ideas of living green. Being the girl that I am I’ve looked up everything that I can and we have started doing basic planning for our home of the future. The only question I have not been able to find a answer to is one that one of my friends (a particularly aggrivating type of naysayer) has posed. I would like to tell her with absolute certainty that she is wrong. She has hay fever and says that a straw built home would trigger a reaction from her. While I’m sure that over an inch of plaster is more than enough protection I just have to ask to make sure. Thanks so much. :)

  10. Andrew
    Andrew Mon, February 11, 2008 at 3:04 pm #

    You are absolutely, without doubt, correct in your assumption that hay fever would have no affect on your friend in a straw bale house. I have horrible hay fever and I am pretty reactive to the straw when I build, but once the house is plastered, there is not a hint of concern for me with hay fever. Your friend can nay say all she wants, but she won’t win that argument. :)

  11. Zoltan Fri, February 15, 2008 at 4:07 am #

    I was reading the questions & replies here. I was shocked when read about the intention of importing bales from Colorado to Texas! this is tipically welfare stuff: people have got more money than sense! How can somebody believe that moving bales over great distances like that is still “green”?! I live in the UK, but got a farm in Slovakia, where the house is literally next to the fields from where I could get my very own bales that are apart from the fuel & string costs for free. Unfortunately it is not possible to build houses from straw yet so I have to give up for now and rebuild from “conventional” materials, but I can still make it as green as possible, using more natural & local materials. If I could do it when bales are laying on the ground after harvest just a few yards from the house I want to rebuild, than you could try that as well when the transport is a many hundreds of miles away!

  12. Andrew
    Andrew Fri, February 15, 2008 at 5:34 pm #

    I agree with you about the transporting of bales. I always encourage people to buy locally if possible. If not, then it is important for people to figure out the cost (both financial and environmental) for moving bales. Does it balance or out weigh the benefits of building with bales? If it is more impactful than the savings created, then other options should be considered.

  13. Maggie Miller Tue, February 26, 2008 at 9:01 pm #

    We are starting a school in our local community and wish to build it using straw bale construction however some of the land is flood prone. Is traw bale construction ever been done on stilts?

  14. Andrew
    Andrew Wed, February 27, 2008 at 12:36 pm #

    Bale houses can be built on stilts or standard raised floor foundations with no problem.

  15. Gav Fri, June 27, 2008 at 2:21 am #

    I’ve been trying to locate locally a farmer that will square bail. Not just square bail but also do it right – moisture and density etc. Impossible and I’ve been looking for three years. So I have no choice but to look further afield. So I think it alittle unfair to suggest no concern for the enviroment. We don’t all own next door farms. I can’t see any other alternative that’s so beneficial. If I build out of other so-called enviro products they wont be manufactured locally either – so the same issue applies – maybe digging a cave is the best option of them all but…. And I live in Ireland, small green and full of farmers but I still cant find locally!

  16. Andrew
    Andrew Fri, June 27, 2008 at 7:09 am #

    I can see how that would be frustrating. In such a case, you would have to look further as you suggested. I totally agree. No matter what you decide, I can tell you are trying your hardest to do it right and that is all anyone can ask.

  17. jennifer pope Sun, September 7, 2008 at 2:04 pm #

    hi gav,
    we are in the process of building our straw bale home and we had a small fire due to lightening. there was quite a bit of water used inside the house to put it out. the fire only damaged a small part and that is being rebuilt with standard construction due to the way we built the house it would have been impossible to replace the wall with straw without redoing the roof as well. i am afraid that some mold may have set in where the water was used. i don’t want to replace those walls with standard construction because we have worked so hard on this for several years. i am hoping that we can plaster over the walls without any problems. what advice can you give to us? thanks jennifer

  18. jennifer pope Sun, September 7, 2008 at 2:05 pm #

    im sorry i should have addressed that comment to andrew.

  19. Andrew
    Andrew Sun, September 7, 2008 at 2:54 pm #

    Hi Jennifer. The key with the wet bales is to check where the moisture content is now. Buy yourself a bale needle if you don’t have one already and stick the probe into the wall where you are worried. If the reading is below 20% then you are okay. If it is above 20% or close to it, then drill some additional holes in the plaster and set a fan near the all to drive out the moisture. Once you get comfortably below 20%, say near 12-14%, close the walls back up and finish the walls. Assuming the construction of the house is such that the walls are well protected from future water infiltration, you will be fine.

    I am curious as to why you cannot rebuild with bales. If it is a load bearing house, you can still yank bales out and add new ones. Let me know if there is some other reason should you need to replace these bales as I believe there is likely a way to replace the bales without having to go conventional.

  20. Peder Thu, January 8, 2009 at 9:43 am #

    Maybe Borate could be sprayed on Straw before its baled? Fire retardant and Enzyme function in mold inhibitor. The Cocoon Cellulose website reported a cellulose wall being soaked for 2 or 3 months and no mold growing at all. I did huge research a year ago on Moisture in Wall Assemblies. Posted on Design Community Forum. Architecture Site. The title of the thread is as follows. A lot of people contributed to prevention of rot and mold.





    Borates are toxic to fungus species. Borates also affect the enzyme system of fungi and the active ingredient is a contact toxicant to fungus.
    Borates demonstrate efficacy against bacteria, mold, fungi, and insects. Borates are low in mammalian toxicity, low in environmental impact, easy to handle, and easy to apply.
    The organic components remain close to the surface to provide protection against mold, while the borates, when exposed to moisture, will penetrate through the entire thickness of the panel, providing interior protection against both mold and decay, and at higher loadings, termites too.
    Metabolizing Effects – In certain organisms, borates can inhibit metabolic processes. This makes them useful in controlling insects, bacteria and fungi in everything from construction timbers to cosmetics.
    Borates are durable. They do not break down into ineffective byproducts. They do not evaporate. Although they are water soluble, they do not move far in the environment, especially in soil. If placed in solution, the borates recrystallize and keep working when the water evaporates. When properly applied for insect control in homes or other structures, their insecticidal efficacy lasts for years and years. The same property of durability applies to diffusible borates used for wood preservatives.
    The borates are essentially nonvolatile. This means that they do not evaporate ;from the site where applied. This characteristic prevents environmental degradation from chemical trespass from borate vapors. Additionally, indoor air quality is not adversely affected by borate-preserved wood. This is also true for borates (boric acid or disodium octaborate tetrahydrate (DOT)) used for management of cockroaches, fleas, ants, silverfish, bed bugs, or other household insect pests. Most of the other products available to the householder or professional cannot truthfully make this claim.
    The EPA has also determined that boric acid and its sodium salts do not show any evidence of carcinogenicity for humans and no evidence of mutagenicity. However, boric acid and the borate salts are lethal human poisons by ingestion at levels of 5 to 20 g. Exposure through the skin, eyes, or inhalation is minimal, so minimal protective measures are necessary. Expensive specialized protective gear is not needed.
    The environmental advantages of using diffusable borates for wood protection are many.
    They can be summarized as follows:
    * Very effective in protecting wood against termites, carpenter ants, wood-boring beetles, and staining and wood rot fungi;
    * Invisible when properly applied, most useful for protecting historic wood artifacts;
    * Not volatile, so does not evaporate or pollute indoor air space;
    * Odorless;
    * Durable, providing wood protection from insect and fungus attack for years, eliminating repeated treatments;
    * Reduce susceptibility to fire; and
    * Have low risk to humans and other non-target organisms.

  21. chris & dana milani Sun, October 4, 2009 at 11:04 am #

    Hello, we have constructed a strawbale home in SW Texas. The outside plaster is lime over 2-3 coats of adobe. We have recently started the mudding process inside the home with some help from a local plaster guy. Dana went to the home this weekend to find what appears to be mold all in the mud of the 1 1/2 walls that were completed. It is usually very dry here. To our knowledge the roof does not leak. What should we do? We didnt have this problem when doing the outside. If we leave the doors/windows open and drill holes in the wall will it dry out? Do/should I use Borates? Any help or suggestions would be helpful. Thank you in advance Chris and Dana

  22. Andrew
    Andrew Mon, October 5, 2009 at 7:14 am #

    Bummer. Has the weather been moist while you’re plastering? If so, that can cause the plaster to dry slowly and the bales to wick up extra moisture and start to mold. If you are still misting the walls, that can also add the necessary moisture. The chances are that this is not a long term issue. It is not uncommon for the walls to get over-wet during the plastering stage and start to show signs of mold. I would suggest letting the space dry out entirely and then seeing how it looks. I bet it will go away. As soon as the moisture is gone from the room, any mold will die off. You can spray a borate/water mixture if you want. That will work as well; however, I would wait until the space dries out first so you can see if that solves the problem. If it does, then you can spray the walls with the borate as a secondary measure of precaution. I don’t think you’ll need to drill holes in the plaster. Again, let it dry out and see how it looks. Good luck.

  23. Jennifer Pope Mon, October 5, 2009 at 11:24 am #

    The reason that we did not replace our walls that caught on fire with the straw walls is because the roof went up first. In order to replace those straw walls and drive the rebar into the bale as was done on the other walls, we would have to take that portion of the roof off. Too much trouble, so we went with conventional walls on that corner of the house.

  24. Andrew
    Andrew Mon, October 5, 2009 at 11:29 am #

    I see. For future reference, you can build load bearing without driving rebar. I don’t use it other than as staples in the corners for load bearing construction. I use the welded wire mesh to achieve the same results as the rebar pinning. In fact, I get better results and more uses out of the mesh. I understand now why you built the way you did though. Thanks for clearing that up.

  25. Carrie Siemens Sat, October 10, 2009 at 9:04 pm #

    Hi there, I recently posed this question to Andrew via email but am interested in the opinion of anyone who has experience with straw bale construction in cold climates. We are building a straw bale house in Saskatchewan, Canada, and are getting pretty close to closing in the walls. We had planned on stuccoing the outside first as usual, but due to a sudden drop in temperature we are not sure that we will have time before winter sets in. My question is whether it would be all right to stucco the inside of the bale walls, tarp the outside, and live in the house through winter. We don’t have much extra money and can’t afford to pay rent and our mortgage costs through the winter…any thoughts? Thanks for any input!

  26. raqueluca Tue, November 24, 2009 at 11:50 am #

    We are building a little dormitory type of thing out of straw bales and wood. We are raising the floor of this 5×5 metres construction with stilts. I konw the strawbales have to be kepts dry and if built on rock or concrete they need to be on a bed of wood chippings for the damps to filter through. Do we have to create something like that too? Even if the construction is going to be raised about half a meter from the ground?
    Thank you

  27. ASHLEY Mon, January 18, 2010 at 6:07 am #


    We are considering building a strawbale house in Northeat Georgia. I have read the humidity and moisture are problematic in our area. I know we would have to keep the bales protected from rain during construction, but is there anyway to combat the humidity within the bales?

  28. Alain Belanger Thu, February 4, 2010 at 6:41 am #

    We are planning to build a hybrid straw bale cob house on a small island on the coast of B.C. It is very humid climate, fog and rainy. Mold grows here as easily as grass. We don’t see the sun much for a few months in the peak of winter and our site during that time as sees only 2 hours of sun rays(if it’s shining) hitting it. We were planning to build large overhang to keep rain away from our earth or lime plastered walls. I am wondering about if the humidity content in the air would slowly creep through the plaster into the bales overtime? Any comments will be well received, thanks in advance!!

  29. Andrew Morrison Thu, February 4, 2010 at 8:46 am #

    Hi Alain. Humidity is indeed a problem for straw bale homes. If you have a time period where the house can dry out, you’ll be fine; however, if the weather is such that you really don’t get a drying period, straw bale may not be the best choice for you. Sorry to say, but it’s better that you be disappointed now than once your house in built! The key is discovering whether the house would have a chance to dry out during the year. It’s the constant humidity which is dangerous. Rain, as you alluded to, is not a problem as long as you design the house well. Best of luck.

  30. Alain Thu, February 4, 2010 at 6:22 pm #

    Thanks Andrew for your prompt response!! And also thanks for the E-course I have been enjoying it. I contacted someone that as done several project in the islands in my neck of the woods and assured me that none of them have had moisture and or mold problem, thanks to the wood stove heat!
    I think between wind and the odd dry spellwe should be able to make it happen…
    Thanks again to be so available.

  31. Andrew Morrison Fri, February 5, 2010 at 12:11 pm #

    You’re welcome. I’m glad you got some good local input as well. Best of luck to you.

  32. Tim Fri, April 9, 2010 at 10:47 am #

    I live in eastern Idaho, a reletively dry part of the state, but also very cold in the winter. I am concerned about the movement of water vapor into our bail walls and condensing at the dew point, wherever that may be, inside the wall. Can I use a latex masonary sealer and caulk to seal the lime plaster inside the house? Also what are your thoughts about a heat recovery ventilation system to remove humid inside air with fresh outside air in the winter. Any thouhts would be appreciated.

  33. Andrew Morrison Fri, April 23, 2010 at 9:44 am #

    I suggest you seal up the electrical outlet boxes in the back of the box with a fire rated caulk (the electrical inspector will want to know it’s fire rated). In addition, any other wall penetrations could be sealed up as well. I would not recommend sealing the plaster itself if you can avoid it. I love HRVs and think they are a great option for removing the moisture laden air from the home.

  34. Brenda Wed, September 8, 2010 at 8:50 pm #

    I built a straw-bale bedroom addition in my Seattle home in September 2008 (The first permitted strawbale construction in Seattle!). While the straw was a lovely golden color when I picked it up in Easter WA, and the moisture reading indicated its moisture content was less than 20%, it turns out that psocids hitched a ride. Psocids emerged in the first summer (9 months) after it was constructed. Summer 2009 I’d kill approximately 50 bugs a day. For that first summer, I now realize, this was a small infestation of tiny insects that just crawled but did not bite that I had NOT at that time identified as psocids. After about a month of smooshing them, they went away on their own, or so I thought. (My architect told me that it was common for bugs to emerge after a year because, as he explained it, the residual grain left in the straw was consumed in the first year by hitchhiking bugs and that the creatures emerge because they are looking for a new food supply.) I accepted his explanation and that was that, or so I thought.
    This year, 2010, beginning in early May, the psocids returned and EXPLODED. If I vacuumed or smooshed 500 in one day, within an hour I’d have to vacuum and smoosh another 100. Needless to say, I vacuumed and smooshed psocids all summer. Exterminators came and left, perplexed about how to rid my home of them. So I kept smooshing and vacuuming while I researched and tried to contact those in the know.
    All of the research I’ve done on the internet and from speaking with a few entomologists informs me that psocids thrive on microscopic mold, that they cannot be exterminated, and that they can be eliminated only if I change the environment in which they thrive. Specifically, they cannot live in relative humidity (RH) below 50%. It is speculated that the humidity (moisture) in the bales resulted from our very moist Seattle spring and June, combined with my turning off the radiant-heated floor. It is further speculated that the moisture entered through the electrical boxes and hairline gaps where the windows meet the stucco. My contractor came to fill all gaps with calk and spray-in foam that hardens and forms a seal. In addition, I’ve run a dehumidifier for the past four weeks non-stop in the straw-bale room. )I hate the noise it makes and the electricity it uses.) These two steps, however, have eliminated major outbreaks in the straw-bale room. (I do not know if the dormant eggs will survive though.)
    The problem now is that the critters are emerging in the traditionally-built hallway that connects the straw-bale addition with the rest of the home. My contractor thinks they are traveling through the vented box-ceiling into the adjoining ceiling pocket. Some have even appeared in the bathroom, kitchen, other bedroom, and living-dining rooms (usually about ten or so appear, not the massive numbers that daily emerge in the hundreds in the hallway).
    The straw-bale room was off-limits all summer as it was overtaken by the psocids. My biggest fear is that over this approaching winter, the eggs laid this spring and summer will lay dormant and with the next wet spring when I no longer turn on the floor heat, they will come back with either the same ferocity or even worse. I’m hoping the calking will thwart them.
    With that back-story, do you have any suggestion for how I may rid my home of these psocids or can you please point me in the right direction for help. Have you heard of this problem before? I’M RATHER DESPERATE! Thank you. Finally, if they do indeed have a microscopic mold food source within the walls, why are they coming out in such large numbers?

  35. Andrew Morrison Thu, September 9, 2010 at 7:54 am #

    I have seen this happen once before and I know how horrible it can be with seemingly millions of little white spec bugs walking around. I originally thought they were termites when I first encountered them, and I strongly recommend that you have an expert confirm that they are indeed not termites you are dealing with. In general, psocids are very resilient to fumigation and tend to survive even the best attempts to kill them (entirely kill them that is). The best way, as you noted, is to change the environment where they are living. You will need to lower the moisture level of the house and dry things out. This is a good idea anyway as you don’t want your bales to be that wet either. You may need to increase the heat and install a dehumidifier in the structure if you don’t already have one. In my experience, once the moisture was removed from the walls, the psocids went away. I don’t know why they come out of the walls either, but they sure do.

    Best of luck and please keep me informed of your progress.

  36. Brenda Thu, September 9, 2010 at 3:36 pm #

    Thanks for your response. Yes, they have been confirmed as psocids by two entomologists and one green exterminator, only these psocids are brown not white. (Yes, the do come in different colors.) As I had mentioned, I have been running a dehumidifier and a fan in the room for almost a month now, non-stop, which has practically ceased outbreaks in the straw-bale room. (They’ve moved to other parts of the house, which is heartbreaking.) I guess I rambled on so much in giving you the history that my real question got buried. So here it is: For how long do I need to run the dehumidifier to kill the psocid eggs laid in the straw walls? Can you please put me in touch with the other folks who had this similar problem so that I may compare notes and learn how they got rid of theirs once and for all. Thank you.

  37. Andrew Morrison Fri, September 10, 2010 at 9:35 am #

    HI Brenda. Thanks for the clarification of your question. Here’s what I know about these little critters. They have a short life span, which is good. In all, their total lifespan from hatching to death is somewhere between 24 and 100 days. Females are usually able to lay eggs within 24 days of hatching themselves and those eggs take roughly 3 weeks to hatch. If I take this information and extrapolate it out, I would say that if you can keep the space dry for 100 days or maybe 4 months, you should have removed the ability for the psocids to reproduce and thus, you should kill off the infestation. The key is being able to keep the environment dry enough to kill off the adults while not providing a viable environment for the eggs or newly hatched insects.

    As you mentioned, the bugs are moving to other parts of the house. This means what you are doing in the addition is working; however, it also means that they may take up refuge in other areas of your house. They are known to feed on the glue/bindings of old books, for example, so keep an eye out for them in your book storage areas. You may have to add some more fans and dehumidifiers to your home and run the entire house dry for 3 months or so to make the kill off successful.

    The owners of the other home that I mentioned have moved and are no longer available for me to ask about their story. That said, I believe it was about 4 months of drying out of the space that worked for them. I hope that helps.

  38. Will Sat, October 23, 2010 at 4:59 am #

    Hi Andrew, We are building some straw bale retreat cabins to do a traditional 3 year silent solitary meditation retreat. We have been preparing for this for years and we go in on Dec. 30th. If for some reason we cant get a certificate of occupancy by then the house has to be abandoned for 3 years while others do the retreat.

    That said we just discovered lots of what turns out to be black mold throughout the cabin. About 40% of the bales are saturated in it. Plaster is up on the outside. The bales are all dry (we live in southern Arizona) and we are not sure what to do.

    Some have suggested just finishing the plaster on the inside and the spores won’t come in, others have said they will, some have said to try and pull out what bales you can and replace them and others have said that will only stir it all up and fill the air with black mold. We have bought some fungacide for this purpose but some have told us this won’t kill the spores themselves and we will still be living with all those spores.

    Any suggestions or what is your opinion. We are running out of both time and money and have so much invested emotionally and financially already that it is almost impossible to imagine just abandoning the project. Thanks for your time.

  39. Andrew Morrison Wed, October 27, 2010 at 2:09 pm #

    Hi Will. That must be very disconcerting for sure. Can you tell me more about the mold you are seeing? Sometimes there can be small flecks of black (in color) mold on the stalks of the straw. This is usually from the straw getting wet before it is cut and is in place before the bales are created. The easiest way to determine this is to look up close at the straw. Is the mold on the stalks or does it cross the stalks? In other words, did it form after the bales were made and thus crosses the individual stalks or did it form before the bales were made, thus it is only on individual stalks and does not cross over?

    The next thing to look at is whether the mold grew after the bales were installed or before. Same concept as above, only now you want to look at whether the mold crosses bale boundaries, i.e. from one bale to another, or does it stop at bale boundaries. If it does, you can assume it formed before the walls were stacked.

    Now, what to do about it. If it is post construction, you first need to determine where the moisture is coming from that feeds the mold growth. Find it and stop it. That’s job number one! If there is no source for the moisture, i.e. the bales were moldy before they were installed, then you can skip the fix step since there is no problem that created the mold in the building. If there is a large amount of moisture in the bales and they feel wet, they need to be removed. Use a bale moisture meter with a bale probe (Delmhorst makes a good one) to check the center of the bales as well. You should not have readings above 20% moisture content.

    As long as you are below 20% moisture content throughout the house, you could technically simply plaster the walls. This is, of course, assuming that there is no structural problems causing the mold as outlined above. I say this because mold cannot survive without moisture. If you take away the source and the bales are fry, then there is no mold to contend with. It may still be staining the straw, but it cannot grow. You could certainly spray the bales with a fungicide or bleach or a lime wash just to make sure if you were so inclined. I probably would myself just for more peace of mind.

    I suggest you use lime plaster as well as it has a natural mold killing property to it. I hope this helps you move forward. I wish you well on your inner journey too.

  40. Tracey Sun, January 2, 2011 at 10:32 pm #

    Dear Andrew,
    My husband & I have been interested in renewable architecture for many years. That being said, we are being moved by my husband’s work to Houston in the next month & we have been wondering would a straw bale house work in this climate as the humidity in Houston is rather extreme. We both love Spanish architecture (we currently live in Los Angeles) but the houses we have seen make no sense in sustainability or cost. Please advise. Also, if you know of someone or where we need to find out what the building codes are, we would be very grateful.
    Many thanks in advance,

  41. Andrew Morrison Mon, January 3, 2011 at 9:55 am #

    Hi humidity areas are certainly harder for straw bale construction, but they can most definitely work there in most cases. I would suggest talking to folks in Austin, Texas about the codes as there are a lot of bale folks there. Frank Meyer immediately comes to mind. You can find him at

  42. sarah leamy Mon, January 3, 2011 at 7:30 pm #

    hello Andrew and everyone! I’ve bought an old strawbale home in NM that I’m told has mold in the walls. I know that there are places where the roof drips onto and into the plastered walls. I’ll fix the drainage and the outside. Should I replaster inside? Or spray lime or something? What do you recommend for inside? the walls have a one coat plaster but I think i need to redo it. Any suggestions? Thanks!

  43. Andrew Morrison Wed, March 16, 2011 at 12:36 pm #

    Hi Sarah. Sorry for the long delay in responding to you. The key is going to be fixing the leaks first and then checking for damage. If there is water damage to the bales, you may need to dig some of the straw out of the wall and rebuild that section. Sounds horrible and it is in deed a pain in the butt, but very necessary to make sure the walls stay healthy for you. With large amounts of water (or even small amounts sustained over long periods of time) in the wall, the bales can end up not only moldy, but rotten. As they rot, they heat up and start to create the perfect conditions for further rot in the bales around them. It’s best to stop this cycle as soon as you can.

    Here’s What I Would Do (And Have Done in the Past)

    1.) Fix the leaks.
    2.) Drill small holes in the walls near the damaged areas and insert a moisture meter probe (use a hay meter with a long probe)
    3.) blow hot, dry air into the walls to dry out whatever moisture is present.
    4.) If the moisture levels are above 20% consider removing a small area of plaster to visually inspect the bales
    5.) If black, rotten bales are present, continue to remove plaster until you find healthy bales.
    6.) Remove the rotten bales and insert fresh bales and stuffing to repair the wall (chances are that the bales you remove will not cause the bales around them to fall into the open space as they should be held in place with mesh and/or plaster.) If you see the bales slipping or collapsing, use some temporary framing (2x4s with plywood) to build stilts for the remaining bales.
    7.) Complete any mesh work being sure to overlap the new mesh onto the old mesh by at least 6 inches and then tie it to the old mesh with wire every 6-8″ on center
    8.) Sew the wall as usual to secure the mesh from one side of the wall to the other.
    9.) Replaster the wall to patch the repair.
    10.) Apply a finish coat of plaster over the entire wall surface to hide the patch.

  44. Laine Mon, July 18, 2011 at 8:18 am #

    Hi Andrew,

    My husband and I (with the help of countless others!), built a straw bale home in Prince Edward Island, Can. I didn’t even realize how humid our climate was until we did.

    We also built during record breaking wet summers. Our top coat of lime/sand stucco failed spectacularly during an immense downpour and water poured through the walls. Our build was delayed and interior stuccoing continued through the winter in a barely heated space. We ran dehumidifiers constantly and did all we could to help the plaster dry quickly. But we had white mold growing on the earthen-lime base coat. We sprayed it with vinegar which seemed to make it disappear and did a lime sand finish coat overtop. Our exterior stucco suffered from freeze/thaw damage and we decided it would never hold up to our winter climate so last year we dug through points in the stucco, put cleats on the studs, spray-foamed around them and added sheathing and shingles.

    We have psocids all year long. Though they do lessen in the winter, they bloom after every rainy day. Along with them is a distinct moldy smell. They show up along the base, and around windows. We have a dehumidifier, but it really sucks living in a house that you have to keep at desert conditions. Never being able to open windows and let the fresh air in. We’ve taken off the baseboard and dug inside a few of the gaps and the straw seems dry.

    We’re confident there’s no actual rain penetration from the outside now. The inside is earthen/lime, then sand/lime, then silicate paint. The walls are constructed with an 8″ plywood chase around the base for wiring and I know there are gaps in that chase. There are a few cracks in the stucco that formed as the house settled or dried. The interior partitions were built before we stuccoed, so where walls hit exterior envelope there is straw exposed to the interior partition cavity. Also where the walls terminate at the ceilings needs to be caulked in some places.

    We’re planning to take off all the base and ceiling trim and seal those transitions really well and fix the little cracks in the walls. Do you think we need to open up the interior partitions as well to get at that weak point where it meets the outside wall? Should we make more holes and measure the moisture content of the bales everywhere? Is there anything else we should do? I only want to do this fix once and do it right. It’s going to be a huge depressing mess.

    Great forum by the way!

  45. Andrew Morrison Tue, July 19, 2011 at 8:22 am #

    Hi Laine. What a bummer. So sorry to hear that you have this stress to deal with. I would seal the walls wherever you can. My biggest concern is that there is still large amounts of moisture in the walls. After all, psocids need moisture to survive. The fact that they bloom every year and whenever there is a large rain fall makes me think that there is water getting into the bales, not just moisture vapor. You also mention that you smell mold when things get wet outside which also makes me nervous. If it were my house, I would drill some small holes, just large enough for my moisture meter to get into the bales, and test them in several spots. Start with the areas you are most suspicious about due to the smell of mold and the location of the psocids. You might also consider taking readings below some windows as those are obvious potential failure points.

    If you built several years ago, which it sounds like you did, I would be surprised if you are still feeling the effects of the weather during the build, i.e. the wet summer. That said, if there is moisture trapped in the walls, it may be causing all kinds of long term problems. When you added the siding, did you add a rain screed or is the siding tight to the bales? It’s a really good idea to have a ventilation space between the scratch coated bales and the siding to allow them to breathe.

    Once you determine that there truly is not a bigger issue at hand, then I would strongly recommend that you seal all of the penetrations to the walls from the inside. In other words, you should not have gaps that enter into the bales around partition walls, baseboards, ceilings, electrical plugs, etc.. All of those areas need to be sealed so vapor cannot pass into the walls. It’s amazing how much moisture can be driven into a wall assembly through such areas. In fact, you should probably use the moisture meter probe to check those areas as well to see if they have high levels of moisture.

    I hope you find success in solving this issue and that the home tightens up to be the dream I imagine you hope it to be. Good luck.

  46. jenny Wed, January 18, 2012 at 1:58 pm #

    hi Andrew, I’m not sure if you are still responding, but I’d love your thoughts. We built a strawbale bedroom addition onto our home 1 1/2 years ago. We live in NM. The bales all seemed great (and hopefully they are) when we built, and the walls were covered well with tarps for the couple days they were up before they were stucco’d in. I’m not sure if our builder-friend did a moisture test on the bales right before closing them in. He mudded the walls and created a mud floor and used BioShield products to seal the floor. He did not put an impermeable barrier between the strawbales and the chicken wire and wall mud coat. (I’ve gotten mixed thoughts on yes, one is necessary to prevent mold from mud-moisture entering the bales, and no you shouldn’t so the walls can breathe (out any moisture). Anyway, the room had a lingering smell for about one year…it seems to mostly be gone now. The builder said it probably took awhile since the room has only 2 windows and that made it harder to air out (vs a big house where you can open everything up). The smell seemed kinda linseed-oil-like at first, but I kept describing it as “musky”. Like i said, i don’t smell it nearly as much, but we moved a dresser on the north wall last week and there was evidence of teeny bits of mold on the floor where the four wooden legs sat. And now, we are kinda nervous once again that moisture is trapped in the floor or could be the walls? it is our bedroom and we have a 2 yr old and of course are REALLY nervous about finding out way later there is in fact mold infestation in the bale walls.

    Do you have any thoughts? Are there any signs we’d notice? Do you suggest we do an invasive wall or floor test? Do you know a mold specialist in NM? It really feels like a sacred space. But i guess if we had to tear down all the walls, we could turn it into a barn and get horses and move our bed back into the living room. Sigh. I forgot to add, the main part of our house is rastra-block and tight. We get a lot of condensation in the main part of the house on windows when the winter nights are negative.

    thank you for your time and thoughts and experience.

  47. Andrew Morrison Fri, February 3, 2012 at 11:44 am #

    Hi jenny. Sorry for the delay. I think the most important thing is to get air circulation in the space. The amount of water that was introduced into the room during the plastering and the floor creation is high. That water needs to be pushed out and the only way to do that is to provide a lot of circulation of DRY air. With limited windows, it’s probably stuck in the room. It’s not a dead cause though. Simply add some fans in the space and air it out (hard in the winter, I know). It should be that as soon as the outside temp warms up, you can open things up and dry it out. Hope that helps.

    ps. I’ve seen this a couple times before and things DO dry out when approached with this direction.

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