Assessing Moisture in a Straw Bale Wall –

Written by Andrew Morrison

peeling paintThe most common time I’m asked the question of how to identify moisture issues is when someone buys an existing straw bale house. Moisture is not often a problem in straw bale homes as long as the house was built well. That said, there may be issues that show up over time. Here are some basic things to assessing moisture in a straw bale wall.

moisture damaged wall
Take a look around the lower corners of the windows for signs of moisture damage. This is the most common area that moisture damage occurs. Check inside and out in this area. You’ll see staining on the plaster, or worse, buckling and peeling plaster.
wall plug
Anywhere there is a penetration (electrical plugs are the most obvious), take a strong smell and see if you get a hint of damp. In most cases you won’t.

If you believe there is moisture damage, the best thing to do is drill some small holes in the plaster in the suspect areas and insert the probe of a moisture meter. Any readings over 20% are bad and can sustain mold growth. I would want to see readings around 12%. If you do have moisture problems, you’ll need to push some dry heat through the wall while it’s hot and dry outside as well, if possible. The key will be finding the source for the moisture and that will dictate how hard of a fix it will be.

straw bale moisture meter

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25 Responses

  1. Hi Andrew,

    I’ve paid the $200 dollar fee for registration for your next workshop but I’m starting to have some doubts about straw bale homes. I read the info. that you sent in your straw bale success tips about moisture in straw bale homes and mold. This is a really big issue for me and one I had not read about before. I have a lot of allergies, am very sensitive to mold and the thought of having a home that could get these mold issues is very troubling to me. I live in Washington state an,d it’s pretty wet in this part of the state. I am starting to have second thoughts about the whole straw bale process.

    What info. do you have to alleviate my concerns?

  2. Hi Andrew

    In response to the moisture aspect, would it not be wise to include damp sealing around door and window apertures, much the same way you would damp seal at the base of the walls, or would the plastering around these areas be compromised?

    I am considering a straw bale home and, in contrast to Allen Robertson, I live in South Africa, a much drier part of the world, but the area I am considering is one of the high rainfall areas of our country. I am considering a double damp course layer at the base of the walls, broad roof overhangs, the lowest level of the plinth to be at least 6 inches above ground level (gradient considering)at its lowest point. As I am considering thatch for the roof, I intend installing ground gutters to drain rainwater away (to storage tanks)as quickly as possible…. are these all sensible ways to deviate possible damp away from walls and foundations?

    Comment please…

  3. As someone wanting a “healthy” dwelling who has been considering and excited about building with strawbale, I too am concerned about the mold issue in the wet Northwest. Beyond building correctly, do you recommend that the design include overhangs/porches/raised foundation? Would that help protect the walls from potential water/ dampness infiltration and potential mold?


  4. Don’t worry Allen, if it is built well there is no chance for moisture trouble.

    I have strawbale walls in quarter of my newly renovated home, and there has been no sign of moisture or mold for almost year and a half, and weather was rainy, snowy and temperatures went bellow -15 celsius.

    It’s all about good roof that has to be extended as much as it can, so less rain would fall on wall and windows.

    And lime plaster does miracles to moisture control in house and on outside surface.

    All the best, and don’t give up! 🙂

  5. I found this article about the difference between straw bale home and clay straw homes. In it talks about the ability of the clay straw home to regulate humidity. Does the straw bale home have this ability? This is the quote from the other site:Straw bale and clay/straw construction are actually quite different in many ways. Clay/straw construction combines loose straw with clay soils and water to form a 12″ thick monolithic wall that contains both the insulating properties of straw and the mass storage capacity of earth. In the mixing process each straw fiber is coated with clay making it fireproof, vermin proof and resistant to decay. The clay/straw walls have sufficient texture to receive natural plasters directly without the need for additional netting. Due to the high clay content clay/straw walls are hygroscopic which means that they have the ability to moderate and balance indoor humidity, an important health promoting factor.

  6. If you use clay plaster or lime plaster, there is an ability to manage indoor humidity; however, this is not the same as handling exterior humidity. Again, your area is not a problem for bale construction and would, in my opinion, be served well by it. Clay/straw walls are very cool, yet are different than straw bale walls and not as insulative. They’re better suited for areas that get really cool at night and hot during the day as they release their stored cool into the room throughout the day. They are best used in desert climates.

  7. I live in a straw bale in Bend Oregon. A roof leak caused some mold in the insulation at the attic area. It was found during home inspection and had a professional remediation performed. 4 years later no problems and my son is highly allergic to mold. Mold isn’t the end of the world, just needs to be dealt with correctly. Many homes, straw bale or not will have this issue.

  8. We live in Louisville, KY where the humidity is high here and local farmers have a problem with mold growing in their hay bales. We have 50 acres in Breckinridge County, KY and are making plans to build. We were intrigued with the straw bale construction and ordered DVD’s to watch but haven’t had the time yet. My husband was convinced building a straw bale house was the way to go until we started researching a little further and realized that mold could cause a problem. We keep reading if the house is constructed right, we SHOULD not have a problem, however how do you ensure it will be built correctly? We really want to believe we should not worry, but are afraid to take any risks even in the rare chance we could get moisture. It seems with the high humidity levels, moisture would develop in the walls even if the house was built well.

  9. Hi Rebecca. You are so right when you talk about the “should be fine” aspect of building in humid climates. There is not a lot of research data to show how bale structures fair in humid climates. There is some out there, but often the research is done in a sterile setting that does not really reflect what the conditions are like in the real world. I can say that there are several houses in the South, Northwest, and other humid climates that are doing fine after years of use. Does this mean that yours would do just as well? It should! Sorry, I couldn’t help but say it. 🙂 I believe, based mostly on the experience of those who live in bale homes in humid areas and my experience building in Tennessee last year, that a bale house can do very well in humid climates if certain details are accounted for.

    I think it is vitally important to have some type of moisture removing system within the home. I know that mechanical systems are just one more thing that can fail (power loss, broken parts, etc…) but I do see them offering a great assistance to the overall longevity of the home. A heat recovery ventilator (HRV) in areas that are more cool than they are hot or a energy recovery ventilator (ERV) in areas that are more hot than they are cool is a great idea. These machines provide a constant exchange of fresh air into the home without the loss of conditioned air, allowing things to remain in flow, i.e. no stagnant air. They also remove a large percentage of “excess” moisture from the air and dump it through a drain line outside the home. This helps keep the interior air condition, flowing, and balanced in moisture. It is that excess moisture that can cause damage in the walls if it is pushed out through the natural pressure of the home into the straw as an attempt to leave the structure.

    Of course, quality construction is of utmost importance as well. Keeping moisture from entering the walls as vapor is very important. Clay or lime plasters do a great job of absorbing moisture and holding onto it until it can be released back into the air. They have a higher capillary action rate than straw and so will actually draw moisture out of the bales. Cement plaster will not do this as well. Keeping the moisture out of the wall is also necessary in penetrations and joints. Be sure to seal the wall/floor and wall/ceiling junctions in a way that eliminates the direct path for moisture. The same is true around electrical fixtures or any other wall penetrations. Gaskets around electrical plates are a really good idea as is sealing the back of the box to the bales. Using a short shot of expansion foam in the back of the box to seal the openings is a good idea.

    Hope this all helps you decide what to do. In the end, you will need to be comfortable with your decision or your house will never be a place in which you can fully relax. There’s no point in building a house you can’t relax into.

  10. Rebecca..I also live in Ky & have worried about the humidity that we experience here ..Would love to have a straw bale house but am worried about anyone having enough experience to assist with building a straw bale in Ky, We don’t seem to be too much into sustainable living around here!!! If you decide to build and would like assistance…let me know, would love to be in on working on it. (

  11. We want to convert a cinder block barn into our home. Since we live in a hot, humid region of North Carolina, we are concerned that the cinder blocks will have too much thermal mass. So, we want to massively insulate it on the inside, thinking that straw bails might work nicely. What we’re wondering is if the cinder blocks or the paint on the outside of the cinder blocks will prevent the bails from breathing and cause them to mold in this very humid climate? Do you have any other suggestions as to a better way to go about insulating our barn/house?

  12. You can certainly do that; however, I suggest you put the bales on the outside of the block. This will improve the efficiency of your home by placing the added thermal mass inside the insulation (bales) and better regulating the heating and cooling cycles of the house. This will also reduce the risk of trapping humidity in the bales which I would worry about if you place the bales inside the block wall, especially since the block is painted. This will mean more work: extendin foundation, roof, etc, but. Is a far safer way to go. If you cannot do it this way, I would not suggest you ace the bales inside the block. Good luck

  13. Two years ago my husband and I bought a straw bale home that had been abandoned half finished. The outside stucco had gaps and holes throughout as well as the interior. Straw was exposed everywhere. We live in a dry climate in Colorado but do experience occassional monsoon seasons. Since purchasing we have slowly been patching all exposed bales. We did moisture test before buying the home and all areas were dry. However there is a smell of old bales coming from places we still need to patch. Possibly at some point the bales were too wet and mold grew. My question is once we have thoroughly sealed the home will any existing mold die off and no longer be a problem. In hopes of not needing to rebuild our home.

  14. It is most likely that, once sealed, the walls will be fine as long as they are in good condition and dry before they are sealed. Even old mold growth shouldn’t be an issue as long as it is now dead and sealed behind the plaster.

  15. To those that are concerned about the mold issues, perhaps adjusting the medium would help with the mold. For example, mixing with Earth (mud.)

    There are also essential oils which have been proven to not only destroy mold, but to actively repell its return. They do require a quality diffuser (I think something professional, not your typical “hot plate” kind) in order to get the particle size which is effective in this.
    Here’s something I found in just a brief search:

    It was effective for me when I was living in a basement in the midwest. It was over 1000sqft converted basement of an old victorian-style home (not sure of the exact age, probably 1920’s), and after using it, my “mold-reaction symptoms” diminished, and then disappeared. No mold was ever visible, but I had a scratchy throat for a long time, and headaches/stuffy feeling. Typical mold reactions. A homeopathic treatment for mold also helped me.

    Hope this is helpful for you guys. Strawbale houses are totally awesome! I’ll hopefully be covering my friend’s strawbale house on my blog sometime this month. Its absolutely gorgeous and done very cheaply and with great craftsmanship.

    Take care.

  16. Hello Andrew, I leave in Lithuania in a straw bale house with earth clay, we just recently finish it and start to leave here, and I start realising that humidity in the house is always 68-73 %. And I am I’m lost in conjecture why is so high humidity. Maybe you can open my eyes on this, will really appreciate
    Many thanks

  17. Hi Sergei. It’s possible that the high humidity is because of the moisture in the clay plaster. It depends on how long ago you completed the work. If it has not been more than a month (in dry weather) then you will likely continue to see moisture levels higher than desired. It should calm down in time.

  18. Hi Andrew, thank you for quick reply, I finish with clay plaster year ago. I was measuring the humidity all the time and when the walls were drying it was about 80 %, but year after it should already drop down, is in it?
    I do have also basement 1.5 on 2 meters and 1.5 deep walls from concrete , could it be the reason?
    Many thanks

  19. The basement could certainly be adding moisture to the house if it is not protected from ground water and/or is not properly vented. It’s important that the basement area be either vented to the exterior so that the moisture can be pulled out OR conditioned to equal the rest of the interior, also having the ability to remove excess moisture. I imagine you have adequate ventilation in the house itself (kitchen vent and bathroom vents for example). If not, that will contribute moisture to the home as well. All of those things together could add to up to a problem. As long as the bales stay below 20% moisture content (this is NOT humidity but a measurement of the actual bales and their moisture content) then you are fine. You will need a moisture meter to check the bales and you can insert the probe into the straw behind electrical plugs and switches or in other areas where a hole drilled in the plaster won’t be an eye sore. Once you check those areas and determine if all is okay, be sure to seal them back up so that no moisture can get in.

  20. I purchased a straw bale home in Monticello, NM. The builder did an excellent job and I have had experienced straw bale owners and an inspector assure me the property is well built. The home is 20 years old.

    This part of Sierra County is high desert – approximately 5200 ft elevation. The air is very dry most of the year. I would like to use a room humidifier to counter the dryness for the sake of my skin. Deserts are hard on women and horses, as they say.

    Any advice? I have already tested all the walls for moisture and every reading registered well under the limit. I don’t want to start using a humidifier and risk altering the status.

  21. Hi Carla. I would not anticipate that a mechanical humidifier in the home would cause problems in the bales. After all, you are looking to make it comfortable for you, not dolphins! 😉 A little moisture will be fine.

  22. I would like to build a straw bale house in Arizona. The outside air is very dry, and thus straw bale houses are quite popular here. However, I have respiratory issues and must use a humidifier inside my home. Is it possible to use a vapor barrier on the inside of the hay bales to prevent this humidity from adding moisture to the hay bales?

  23. Hi Aaron. I don’t see the use of a humidifier as reaching damaging levels in the house. That said, it would be good to know more about how much moisture (percentage of humidity) the house will have year round. If it’s higher than I’m anticipating, that could be an issue. I would not recommend adding a vapor barrier as in Tyvek, etc. If you go that route, I’d suggest using a wall finish such as linseed oil or some other sealer to the plaster surface. This will give you the desired effect but won’t interrupt the connection of the plaster and the straw: something very important to the overall structure of the home.

  24. Hello! I am in the process of buying a 25 years old straw bale house in Quebec. During the inspection, moisture level on a pinless moisture reader was indicating 40% in the walls. another instument with small pins was used after and was indicating 15%. With these 2 different readings I want to push the investigation further and i am thinking about inserting a probe thru the electrical outlets. What are your thoughts about the right tool to use? What do you think about those moisture reader that don’t have a probe, can their readings be accurate?

  25. Hi Simon. I would highly recommend using a Delmhorst Moisture Meter with a probe to get the reading INSIDE the bales. I would be concerned that the other meters are reading moisture in the plaster and/or in the outer edges of the bales; both of which fluctuate and hold higher levels than the bale core. What matters is how the bales are doing in the middle of the bale, not the edges. Good luck.

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