Testing Moisture Levels in Your Straw Bale House

Written by Andrew Morrison

straw bale moisture meterPeople often talk to me about testing the moisture content of the bales in their walls once their straw bale house is complete. For some it’s just a passing thought, but one they want to pursue. For others who are perhaps buying an existing straw bale home from a previous owner, it’s a must as they want to be sure the bale walls are dry. Whatever the reason, unless there are sensors built into the wall (which is rare) they need a way to access the bales without destroying the plaster.
The best way to accomplish this is to utilize the space behind the electrical boxes throughout the home. Because they are usually placed every 12′ or so along the base of the wall (plug locations) and at or near every door, both interior and exterior (light switches), they make the perfect access port for checking the condition of the bales.

What’s more, because many builders over the years have not done a good job of sealing up the electrical boxes, they are heavy indicators of moisture penetration into the bales. Warm, moist air is driven into the walls and takes the easiest path in. Since the plaster acts as a vapor seal over the vast majority of the wall surface, the penetrations in that seal (the electrical boxes), represent the path of least resistance. I suggest that people seal their electrical boxes in one of several ways. The easiest is to use vapor gaskets on every box. The cheapest way is to use spray foam in the back of the boxes. The spray foam obviously makes checking the bales at a later date harder, but it can still be done.

Here’s how to test your bales. Remove the box cover plates and pull the receptacle (switch or plug) out of the box. Use a screwdriver to pry the wire clamp or pinch strip (if present) out of the way and insert a moisture probe into the space at the back of the box. Insert the probe as far into the bales as you can. Getting the angle right may take a few tries. If there is spray foam present in the back of the box, dig a hole through it with your screwdriver and continue as above. Be sure to reseal the box when you are done.

This method works really well with the boxes we use in North America; however, I know that the boxes are different all over the world. For example, the boxes we used in the Portugal workshop could also work for this; however, they are much smaller than our boxes and so the proper angle for penetration of the bales may be hard to achieve. Give it a shot and see how it goes. It’s better than drilling holes in your walls!

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12 Responses

  1. It may be obvious but you didn’t mention to make sure the power was off before messing with anything electrical especially if you are sticking a screwdriver or metal probe in there.

  2. Walls should be around 8-12% on the meter. Mold growth occurs above 20% so the closer you get to 20% the more you want to keep an eye on things. Anything above 18%, in my opinion, reflects an issue with moisture entering the wall in some way.

  3. Great point Dale. Indeed…TURN OFF THE POWER before you work each area. I switch off each circuit at a time so that I can still have power in other areas of the home when I do inspections. Be sure to use a tester to ensure the power is off before you remove the cover plate…each and every time, even if you think a plug is on the same circuit that is already turned off. Sometimes wiring is not what it appears!

  4. A lot of people leave a “proof window” at some point in thier wall finishing. Wouldn’t a glass “proof window” that actually opens on hinges offer an entry point to check the moisture in at least that part of the wall?

  5. Hi Amy. I have always referred to them as Truth Windows, but clearly we are talking about the same thing. Yes, that could work although they tend to be placed in areas less prone to moisture issues. Plugs and switches, on the other hand, are often found near windows and doors and even under windows. They make for well located points of entry.

  6. Hi Andrew, thanks for a great article! I tried subscribing to your email list but got an error: “Unable to locate mailing list.” Please could you add me to that list?

    I’m about to build a strawbale studio up on stumps about 1m above ground level. Should I install moisture meters as part of the build? If so, where should they go, i.e. at what spacing and height above floor level? Should I also install moisture meters near plugs, switches, doors and windows?

    I could install the sensor and cable with socket to a wall plate and simply attach the sensor meter to each socket to test periodically. Is there a better way?

  7. I don’t usually install meter lines in the wall. I think it could be a cool thing to do, I just have not seen the need for it in most cases. That said, when you need it, it’s great to have it there…assuming the meters are in the right place to catch any issues. If I were to install them, I would place them near the bottom corners of windows and otherwise in the bottom course of bales, perhaps about a foot off of the ground. I might put a couple at the very top of the wall as well. It’s hard to say as they could go everywhere in hopes of finding an issue. Like I said, I have not used them as I have not seen the need to date.

    I’ve asked Gabriella to get you signed up on the list. I’m not the right guy to ask about that as I don’t know the inner workings of that stuff. 🙂

  8. The first step is to find out why the moisture levels are so high and stop the issue from reoccurring. After that, it will depend on how deep into the bales the moisture has traveled. You may need to add dehumidifiers, heaters, and fans to the space and that’s it. It could be that you need to drill access holes through the plaster and then add the machines listed above. It could be that you have to remove the damaged bales entirely. I always hope for option one first!

  9. What kind of moisture meter do you use? My meters all have different settings for different materials. What setting would be best for checking the moisture in a straw bale wall? Thanks!

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