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Community and Q&A

Embedding Data-Logging Sensors in Walls

Vlad Shpurik | Posted in General Questions on

I am building a 1000 sq . ft. single story house in marine zone 4C. The wall is 2×6 with R23 mineral wool bats, plywood sheathing, 3″ exterior mineral wool boards, 1.5″ rainscreen, lap siding. I am contemplating whether there is any value in embedding humidity and temperature sensors  to help evaluate wall performance before everything gets covered with drywall. For a couple of hundred bucks I can build a monitoring system using several dozen BME280 and DS18B20 sensors located in a few “interesting” places in the wall and connect them to a central location with old CAT5 cable that I have laying around. Something like Raspberry Pi can be used to collect the data. I am interested in the group’s wisdom and opinions on whether doing this will be of any value in residential construction.

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  1. Mike Theis | | #1

    I think there is tremendous value in embedded sensors. Something predesigned and packaged for dummies like me to use. I would pay what a window costs. So cheap compared to failure of the assembly. Presently it's like flying an airplane with no instruments. Leave the system in the building envelope forever. Collect data forever.

  2. Expert Member
    Zephyr7 | | #2

    If you have the skills to build the system, then it’s not a bad idea to put the sensors in now while you can. I’d put some in the attic, basement, and living spaces too, and at least one outside in a shelter. This way you have data on the conditions exterior to the wall itself so that you can make comparisons. I’d keep lots for everything.

    Keep in mind that humidity sensors especially are famous for being unreliable, so expect to have some sensors die after a while. You may be able to mount some near electrical boxes so that you can get enough access later to replace them if needed.

    Projects like this can be interesting. I’ve actually been ordering some of the parts needed to built an R value test set for rigid foam that I described on here several months ago. The goal is to be able to measure samples at whatever temperature differential desired, to see how much performance impact cold weather can have on polyiso. Should be an interesting project, and it can answer some real questions that are asked here periodically.


  3. DCContrarian | | #3

    Sounds like a cool idea.

    The sensors are only a couple of bucks each. To address reliability, put two sensors right next to each other. As long as both are reporting the similar numbers you're reasonably confident both are working.

  4. Matt F | | #4

    I have been thinking about doing something similar.

    Reliability of humidity readings is definitely a concern. Sensirion has a nice collection of I2C sensors for environmental monitoring. They have VOC and Particulate sensors as well.

    I haven’t dug too far, so post whatever you come up with.

    1. Vlad Shpurik | | #14

      Matt, I think all capacitance based humidity sensors suffer from the same issues, even if some less than others. Based on what I've read so far, SHT71/75 or the newer ones SHT85 sensors are much better than the rest, but also about 10x the cost of BME280. Maybe I'll use a few Sensirion sensors as a reference ones, and the less expensive ones elsewhere. The temperature sensors in these probably will be useful for much longer though.

  5. Expert Member
    Malcolm Taylor | | #5

    It's a fun idea, but if your building assemblies are designed for your climate, they shouldn't experience widespread moisture of temperature problems. So what you are left with would be detecting spot issues caused by defective workmanship - and the chances of the sensors being in those spots is pretty low.

    1. Expert Member
      Zephyr7 | | #6

      Hey now Malcolm, no spoiling our fun with metrology systems here :-)

      I agree though, Murphy’s law (one of the one true laws of the universe, BTW) dictates that the leakiest assembly in your structure will always be the OTHER one, you know the one — it’s usually the one right next to the one you put the sensors in so you don’t detect it.

      To help combat this, you can go the “research grant” route and put a sensor in EVERY stud bay of EVERY wall assembly. Or you can be more realistic and pick the places most likely to show signs of trouble: corners, under windows, near eaves/soffits, etc. Its probably worth putting a few sensors in known non-trouble spots too for comparison purposes.

      As an aside, my R value measuring system design uses a 1 square foot plate with a number of temperature sensors arranged in a pattern between the the heating or cooling elements. The arrangement allows the sensor numbers to be averaged to arrive at a “whole plate surface” average temperature for use in R value calculations. The placement of the sensors is done so that some are in known hot/cold spots and others in between so that the differences average out. I’ll publish the design when I’m done and either post a Q+A here with some results or maybe write a guest blog article. If you do measure your wall conditions, you should do the same so that we can all learn from your results.


      1. Expert Member
        Malcolm Taylor | | #7


        I absolutely see why people using assemblies that theoretically could be problematic might put in sensors. Dan Kolbert used them to show cellulose filled double-stud walls appear to be safe.

        It just seems to me that a more fruitful approach for most of us is to make sure our assemblies have a level of safety inherent to them so you never need to worry about measuring the moisture levels.

        I do quite a few renovations (right now my head is covered in lumps from working in an 18" crawlspace). Invariably the problems I see come from simple, fundamental errors in construction. Get those right initially, and keep idiots from altering the structure over time, and monitoring becomes redundant.

        1. Matt F | | #8

          Malcom, your point is well made that sensors should not be needed unless doing something sketchy or cutting edge.

          Personally I am interested in the science of this and how our calculations compare with real world results. The only practical application I might use is to see if I can drop the temp set point in my conditioned attic without causing problems. Sensors would be positioned against the ccSPF beneath R30 rockwool at the peak. I still have R30 in most of the floor and actively condition the space. There would be some energy savings dropping the temp of the space.

        2. Expert Member
          Zephyr7 | | #9

          I agree, as these building methods are fairly well understood at this point. A “real” lab setup would likely involve different parts of the same wall being built slightly differently (like some exterior polyiso, another section with exterior EPS, another with exterior mineral wool, then comparing seasonal moisture level variations between the three to figure out which is best/driest).

          It’s a fun DIY project for little expense though. I look at it more as a “satisfy your curiosity” project then an “alert me to potential problems” type of project. As long as the walls are still built correctly, I don’t see any problem adding some sensors to watch it.

          My R value setup is more of a real lab setup, and I need to be able to publish results. As such, it’s setup to be “right” and hold up to any challenges to the data. Different goals. Ironically, I’ll probably have my setup ready for test runs — measuring the cold weather performance of polyiso samples — sometime around mid summer :-)

          BTW, apprentices are the answer to your head bumps. When I was running cabling crews in the 90s, our first-time guys were the “holy men” — they got to do all the drilling, usually in concrete walls. I told them they would be giving my sharpie marker magical powers that would make enable it to penetrate any material since I would be marking the locations I’d need holes drilled.


          1. Vlad Shpurik | | #13

            Good points on reliability and accuracy of these sensors. Although I don't have personal experience using BME280 sensors long term, all research that I've done so far points to their fairly short lifespan. I agree that moisture problems detection even 5 years down the road is out of the question and would be akin asking for problems. False sense of security would be worse than not having these sensors at all.

        3. Expert Member
          Malcolm Taylor | | #10


          For sure, do it out of curiosity. Similarly I'd like to put a small camera on my cat to see what it gets up to, although it will probably only lead to arguments between us.


          You are right of course. All I can say in mitigation is I've always tried to make sure I share the unpleasant tasks, not just do the fun ones.

          1. Expert Member
            Zephyr7 | | #11

            I’ve always like the quote “good leaders never ask others to do things they are unwilling to do themselves”. I get a lot of respect from my guys because they’ve all seen me get my hands dirty, and they all know I’ll give them a hand when needed.

            I remember helping my mechanical contractor ones with a stuck valve on an 8” glycol line. With all three of us leaning on it we got it loose. We also all got sprayed with some ethylene glycol. I remember saying “so it DOES taste sweet!” :-)

            To any other engineers out there: never be the guy all the contractors complain about when you’re not around. Work WITH your contractors, listen to them. Everyone ends up with a better result that way, and we’re all on the same team with the same goal of building a good building for our customers without breaking the budget.


    2. Mike Theis | | #12

      I worry about everything. Codes change , Materials change, People have good days and bad days. The weather changes. Tornadoes, thunder storms, floods and drought. Do the occupants run a humidifier in an already to wet house.
      Exterior insulation and poorly designed windows and doors are HARD to incorporate into a proper drainage plane. You can build stuff wrong for decades and not even know about it.
      I don't expect the sensors to catch everything. I want to know if the new wall assembly I am using is working or pushing the envelope a little to far.

    3. Vlad Shpurik | | #15

      I agree that properly designed and build wall shouldn't have moisture problems. I still think it would be interesting to confirm that theory matches reality, even if the sensors last only a few years.

      1. Expert Member
        Zephyr7 | | #16

        You could try a pair of parallel wires sandwiched in the framing somewhere as a type of robust moisture (but not humidity) sensor. Labs would use a pair of thin platinum wires (surprisingly not as expensive as you might think), since it is VERY nonreactive. A pair of stainless steel (ideally 316 alloy) would be good too, cheaper, and easier to get.

        To make a measurement, run a small current across the wires and use it to measure the resistance. Ideally you’d use an AC current (around 450 Hz works well, I’ve used that before for similar purposes in a sensor I built years ago), to avoid galvanic effects, but DC would work ok if you made VERY SHORT measurements OCCASIONALLY. If you run DC all the time, you’ll erode the wires. AC doesn’t have this problem, but is more difficult to use for this. What I did was to capacitively couple the AC measurement voltage to the sensing element, then take a bunch of A/D samples over a one-cycle period of the AC waveform and run an RMS calculation on those samples. It’s not that difficult to implement if you want to try it.


  6. Callum Hill | | #17

    Hi Vald, did you end up putting any sensors in your walls?


  7. Vlad Shpurik | | #18

    Unfortunately not, ran out of time and skipped this. In retrospect I think Bill's idea to add moisture sensors under the windows at the window sills would've been most valuable to implement. As far as the humidity sensors go, I figured that measuring temperature indoors and outdoors and keeping indoor humidity in check to ensure dew point is never reached at the sheathing would be good alternative.

    1. Callum Hill | | #20

      Thanks for the update Vlad.

  8. GBA Editor
    Kiley Jacques | | #19

    This is something high-performance builders Ben Bogie and Dan Kolbert do regularly and swear by. Take a look at this article, which includes talk on sensor placement in the walls: Why We Need Building Sensors.

    1. Callum Hill | | #21

      Thanks Kiley. I'll take a look at the article.

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