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Building Science

Why We Need Building Sensors

Understanding the payoff of diagnostic monitoring tools

The S-2 wireless sensor monitors temperature, humidity, and wood moisture content. This style of sensor with remote moisture probes is handy for situations where you want to monitor the wood moisture in a different location than the temperature and humidity.

My friend Jake has a great saying: “Trust but verify.” In terms of building, this means we can exercise our skills in material choices and how we assemble them to make something that we believe wholeheartedly will work. But unless we take some sort of step to actually measure and quantify performance, how will we ever be sure? How will we ever diagnose? How will we ever improve?

Sure, we can wait for something to fail and forensically explore what went wrong, or we can cross our fingers and hope, but I’d rather be a bit more pragmatic.

We’re at a crossroads of many streams of technology right now: Building and material sciences are progressing by the day, codes and client goals are elevating what we build, and electronics are becoming staggeringly more affordable and commonplace. In short, we’re assembling buildings that are far more advanced than at any time in history, and we have a flood of affordable user devices coming onto the market that allow us to measure and control their performance.

The need for information

I care about this opportunity for a few reasons. First and most important to me, we’re building houses that are highly insulated and super-airtight, so we need to be keenly aware of what’s happening with temperatures and humidity in the building’s components. People often correlate “low-energy” homes with the idea of paying less for their utilities to service that home. While this is true, it’s a side effect of the physics of what we’re doing: We are literally reducing the molecular energy flowing through the components of our buildings. We add insulation to slow the transfer of heat, we add air barriers to stop the physical movement of air and its energy, and we use vapor retarders to control the dynamics of water vapor. We are reducing and controlling energy on a much broader scale than just what we use to heat and cool our homes. By doing this, we allow ourselves to have a much greater ability to influence what happens to the guts of our buildings, but this can be a delicate balance. If one aspect falls out of balance, it can have less-than-desirable con- sequences—mold, air-quality issues, or even catastrophic rot from water and air leaks.

So how do we measure what’s happening in our buildings? In particular, how do we measure what’s happening inside our walls and roofs over time? We can’t expect clients to let us come back into their house every month for years to drill holes in their walls and take measurements, nor can we expect them to allow us in to download data from sensors. So we need a system that we can install during construction, inside of walls, roofs, and other hidden areas, that can be left there for years, requiring no maintenance, all while communicating wirelessly. Such systems used to be restricted to the realm of research projects due to their high cost or intrusiveness. But with the rapid growth of technology, analytical-grade wireless sensors are now affordable enough that any builder who cares can easily justify the cost as a measure of due diligence or an investment in their own education.

What are we measuring?

The variable with the farthest-reaching influence is temperature. The temperature of an assembly can be the proverbial canary in the coal mine for so many things happening in our houses. At the obvious end of the spectrum, there’s the risk of an unoccupied home freezing and the potentially disastrous consequences of that happening. Wouldn’t it be great if we could set a temperature limit that triggers a series of notifications (email, text, phone call) that alert us that something has gone wrong at a client’s home and we need to take action to resolve it or notify them that there’s trouble afoot? That alone is worth the price of admission.

Moving into less immediate criteria, temperature can tell us just how well the HVAC system is maintaining the indoor environment: Is everything holding at a uniform level or are there wide swings between the thermostat set point and the actual temperature in the space? We can also get a look at how one wall compares to another due to its location or type of insulation. And finally, we can use temperature to assess the risk of other phenomenon in the wall. If a wall is getting “wet” but only while it’s cold, then there’s much less risk of mold or rot setting in.

The second variable is wood moisture equivalent (WME). This is a measurement of how much water is in the wood that comprises the structure and decoration of the house. WME is expressed as a percentage of the wood that is water compared to the amount of wood that is, well, wood. For most climates and situations, we can expect to see WME somewhere between 6% and 20%; outside of this range, we run into issues. Early in the building process we can install sensors to monitor how our framing and sheathing are doing. If the wood structure of the building is too wet, it can lead to issues when insulation is installed or it can wreak havoc as it dries out with moisture building up behind vapor retarders or drywall. In the best case scenario, you might have plaster or joint compound that’s slow to cure; at the other extreme, you can end up with large amounts of water from condensation that causes mold, rot, damaged insulation, or failing drywall. With a sensor monitoring system, we can watch moisture levels in the interior trim and millwork as the building ages. We know that the WME of wood in the conditioned space should be between 6% and 12% depending on the settings of the HVAC system and the season. If we watch these numbers in the first year or two of the building’s life, we can tune the systems and settings to avoid issues with doors binding from swelling, trim and paint cracking, or wood flooring showing gaps or buckling.

The third major variable sensors allow us to watch is humidity, the measurement of how much water is in the air around us. In practice, this is expressed by our sensors in two forms: absolute humidity and relative humidity. Absolute humidity is a physical measurement of the mass of water in a given volume of air. If we take the absolute humidity measurement and cross that with the air temperature, we get relative humidity, which is the measurement we’re most familiar with and most likely to use. Relative humidity is a percentage expression of how much water is in the air compared to how much it can actually hold; as temperatures go down, the air can physically hold less moisture, and vice-versa.

This matters for a number of reasons. The first and often most relevant is comfort. The human body is most comfortable in a range of relative humidity from 50% to 60%, and if we can measure these conditions, we can tune the systems in the home to make sure the occupants aren’t experiencing any of the less-than-enjoyable side effects of being outside of this range (sinus issues, dry skin, nose bleeds, respiratory issues, sweating). The second reason we want to watch humidity is to ensure that the house itself isn’t at risk of any issues, most commonly from too much humidity. We can have issues with condensation on walls and windows resulting in mold growing, paint peeling, and wood components failing prematurely. Less noticeable but equally important is the buildup of moisture in our walls and roofs resulting again in mold and rot, which at best can cause unwanted odors and at worst can lead to health issues and catastrophic failure of the components.

The next thing to consider is dew point. Dew point is really a phenomenon expressed by the combination of temperature and humidity. We can all relate to the real-life experience of dew point as the phenomenon we see when we pour a cold beverage and get the resulting sweat on the glass. In scientific terms, dew point is the relationship between the temperature of a surface and the water in the air around it. When there’s a high enough level of humidity in the air and a lower temperature surface, the water in the air changes phase from a gas to a liquid and collects on that cooler surface. With a beverage, we can simply set it on a napkin or a coaster, but if this happens in a wall or roof over a long period of time, the results can be disastrous.

A final area of measurement that we’re implementing is indoor-air quality (IAQ). There are many particles and chemicals that can be measured in this area; to be frank, the research is still evolving and the sensors for some of them are cost- prohibitive. But a good starting point is to measure the carbon dioxide (CO2) levels in a home. We exhale CO2 as a by-product of breathing, and it needs to be removed via ventilation in order to provide a healthy environment for us to live well. CO2 is also a good analogue for other pollutants, as it is common for CO2 levels to rise at the same time that we see other pollutants rise.

The OmniSense G-7 air-quality monitor and gateway collects all of the data. It features a full suite of on-board sensors along with WiFi/LAN capability and a cellular SIM card for remote installations. Photo by Rodney Diaz.

Data collection

So what do we use to do this monitoring? For years, we relied on small HOBO data collectors (made by Onset) that we could use for short-term measurements that needed to be downloaded to a computer. These data collectors are still useful for diagnosing issues in existing buildings, but they don’t fit the needs of long-term remote monitoring. So what we’ve settled on is a system from the company OmniSense. The heart of the system is a wireless gateway (router) that collects the data from the sensors in the building and streams it live to a cloud-based host via a WiFi or LAN connection. This is the most visible part of the system, and we often install the gateway in the mechanical area or with the internet modem. OmniSense gateways range from $200 to $450.

In terms of the sensors, there’s something available to measure just about anything you could possibly want in a residential building, and they all communicate wirelessly with the gateway. You can monitor all the parameters we’ve discussed so far, along with energy consumption, wind speed, wind direction, rainfall, solar radiation, water flow, differential pressure of the building, sound-pressure levels, and a handful of others. The sensors are battery-powered by a small lithium-ion cell that should deliver anywhere from 5 to 20 years of service depending on the collection rate of the sensor and environmental conditions; they sell for just over $50 a sensor. We primarily use the S-11 sensors, which monitor temperature, relative and absolute humidity, and wood moisture content. The small (roughly 2-in. by 3-in. by 3-in.) boxes affix to a part of the structure with two stainless-steel screws. These screws are critical, as they not only secure the sensor but are the “pins” of the moisture meter, and their length can be used to vary the measurement depth inside the wood from which you want to take readings.

We have also used the S-2 sensors, which are functionally identical to the S-11 but measure wood moisture via screws attached to wires. These sensors are handy if you have a tight spot from which you want readings, but the sensor body can’t fit. The final one we deploy is the S-19 CO2 monitor, which we generally locate in the master bedroom in a concealed spot. The highest levels of CO2 are generally measured in the middle of the night in bedrooms, the areas of a house where we spend the most time. These sensors are more expensive, costing about $250 each.

Sensor placement

Placement depends on the project. In general, we bury one sensor at the sheathing plane of a north wall because it will see the least sun exposure, which also translates to drying (we all know the north side is the first to rot). We will usually install one sensor in the attic or roof assembly, and especially in any unvented roof sections. Inside the house, we’ll find a spot inside a closet or cabinet to tuck one away out of sight; this sensor gives us an interior base measurement to relate any of the assembly sensors to. If we’re seeing elevated moisture inside a wall, we can compare it to interior levels to quickly see if a change needs to be made to the interior environment or if the moisture is coming from somewhere else. The final option is to place one in a weather-protected location on the exterior of the building. This allows us to relate our data to the weather conditions the building is experiencing.

So what have we learned?

In short, both a bunch and not much. For the most part, we’ve found that our assemblies are performing as they should, but we have had to gain a much deeper understanding of how they’re doing it. The ability to quickly toggle between measurements, locations, and time periods, and have them graphed out instantly in front of our eyes is a tremendous learning experience. In the interest of full disclosure, we were able to catch a potential issue in an unvented roof assembly that we had concerns about from day one. It was a perfect storm of less-than-stellar insulation installation and a circuitous ERV duct run that led to some elevated humidity levels beyond what we’re comfortable with. It has cost us some time and money to correct, but that cost is much less than the potential damage that could have resulted if we hadn’t caught it through monitoring. That intervention right there is well worth the cost of implementing these tools.


-Ben Bogie is a lead carpenter with Kolbert Building in Portland, Maine. Main photo courtesy of the author.


  1. Jon R | | #1

    I've been impressed by what I've seen from this one:

    For example:

    1. Joe Braun | | #5

      Hi Jon R,

      Thanks for sharing the link to I like what he's done, but don't like the idea that I have to upload my data to his servers. Besides his site, do you know of any other sites that sell these type of sensors? The OmniSense is great, but I don't need wireless stuff. I am doing a new build and would like to direct wire everything in.

      The only other place I can think of is what they use at my work to monitor anything and everything (I work in a data center, if there is a data point, we have a sensor on it).

      1. Karl B (Zone 6A) | | #12

        Thanks to Ben Bogie for the excellent article, and to Jon R for the reference to WEL. The latter pointed me in the direction of the "1-wire" communications bus and ecosystem of sensors. It has some nice features (cheap, readily available, low voltage/power requirements, good range, flexible network requirements).

        To Joe's question, it would be relatively simple to re-create the basics of the WEL Server, as a DIY system: a Raspberry Pi (or similar) to act as the bus host, your choice of sensors/wire/topology, and some Python code to collect and display the data. It's not a turn-key solution like the WEL server, but could be a fun project.

  2. jkonst | | #2

    This is very informative, thanks for sharing. The written sensor placement description is helpful, and I'd be very interested to see a schematic showing "typical" sensor placement locations and quantities as well.

    Are there any PM2.5 sensor that tie into OmniSense or other systems? I understand there are questions about accuracy/longevity of consumer PM2.5 sensors, but I'm curious nonetheless. If OmniSense starts selling one, I shudder to imagine the cost if a single CO2 sensor is $300. I suppose in the end CO2 is probably correlated enough with PM2.5 to do the trick in most situations.

  3. Nate Reik | | #3

    I agree with jkonst on a schematic. Where in the roof sheathing would one put a sensor?

    Anyone have any input on non-internet based sensors? No good internet source for a part time place I'm thinking of...I'm trying to find a standalone way to log wood moisture content. Seems the Omni requires sensors, a gateway, at some point an internet connection, etc.
    I've got a few Lascar units going for temperatures, humidity indoors and out, as well as monitoring ground and slab temps in my project....but still searching for a wood moisture datalogging solution.

    1. John Michelotti | | #19

      IF you're game try to get tjhe spec's on that sensor, I bet it can be made to run off a Raspberry Pi, or Arduino. The latter is ideal for data collection. Go to to see the collection of sensors that are available. They don't have the WME sensor but most everything else.

      1. Nate Reik | | #20

        Right, my point exactly, the WME sensor is hard to find. Seems almost absurd that we have so many other sensors available, but not many choices for WME. There are tons of options for temperature and humidity dataloggers, and relatively inexpensive (<$200). Wood moisture datalogging? Not so much.

        The thing is, one can chose between a slew of moisture meters for about $25, and most of those work reasonably well. Adding datalogging to that shouldn't be too difficult, but apparently is TOO niche a market to have something out there.

        I'm trying to avoid making my own, especially when I want 2-4 sensors in 2-3 buildings, one of which is a couple hundred miles away. Been doing some research on this, and if I don't find a ready made solution soon, seems the best thing is going to be to to "implant" moisture meter "spikes" in sheathing in walls at the right spacing, and run wires to a connector somewhere outside the wall, and check manually with a handheld moisture meter when I think of it. Since the ones that work on resistance are working on the order of megaohms, adding a foot of wire shouldn't make any difference.

        1. John Michelotti | | #22

          Exactly my point, there is no magic to these things. If you stick a penny in water the resistance will change.
          What I was trying to say is that you can get the sensor without the gateway for about $50, then retrofit into whatever device you're using. This will mean dealing with whatever voltage the device uses, and amperage. Most of the sensors work using resistance.
          You might make a reasonable approximation by doing what you're suggesting.
          Do two tests, one with dry wood and one with wet wood. IF you don't care about being precise this may give you the warning you are looking for.
          I do think, though, any high readings of WME, absent a flood, would usually be preceded by high humidity content over time.
          Another idea would be to take two ambient moisture sensors, one near the suspected entry point of water/vapour/moisture, and one where you think you are venting that moisture. Knowing the delta between the two should tell you something.

          Last I would add that I think people get too hung up on the accuracy of these things. There will be a reading for dry and wet. I hear the PPM sensors are all over the map, and that goes for the pricier ones. When we turn the stove on our HEPA light goes from blue to red, for a candle blue to read.

          I'm pretty strongly correlated that the particle in the air go up when we have a flame. The HVAC guy where's a high end breather when he cleans my furnace. Do I need an accurate reading to tell me what I need to do, or don't do, to reduce particulates in my living space?
          Moisture alarms go off, time to take a trip. In most cases humid air will precede wet assemblies. Correct me if I am wrong, anyone. Thanks

          1. BlueSolar | | #26

            What's the state of the science on particulates and humans? I was wondering about this last week. Nature at its most pristine is awash in particulates. That's the environment in which humans evolved, so I wouldn't assume that it's necessarily healthy for humans to chronically breathe air stripped of all manner of particulates. It would be like living in a space station or something, forever.

            It also seems like it might condition people to have more problems in normal environments, outdoors, etc. by making them more sensitive and fragile. It reminds me of why I never went off dairy completely – doing so makes people very sensitive to trivial amounts of dairy in food, and I didn't want to be so fragile. Particulates seems a bit different in that it's just the default natural environment, and therefore unavoidable on Earth. Has anyone researched filtration-induced sensitivities to natural air? It also reminds me of the finding that kids who go to daycare end up with less allergy problems than kids who don't.

          2. John Michelotti | | #30

            To Blue Solar, the 2.5PPB has been arrived at through many studies. I don't know enough to comment on good vs bad particulates, other than the bad particulates are bad. The particulates coming off of your gas appliances are bad, they are not neutral. If there are metals or inorganic materials involved particulates are pathway to your lung and respiratory system, what is bad doesn't go away.
            The increased rates in asthma and allergies is very likely multifactorial and goes way beyond what we can accomplish in a GBA forum. (lower rates ofd nursing, homogenizing milk, plastics in pretty much everything) There is no simple answer to that. In poorer countries there are high rates of resp disease because they cook with wood. Gas wood be an improvement but the leftover combustion components are still there.
            Perhaps another way to frame this is that indoor air is far more concentrated that outdoor air.

        2. Jon R | | #23

          Maybe wood moisture content could be calculated from long term temperature and humidity data.

          1. BlueSolar | | #24

            Jon, how? Wouldn't you need a model of permeance or absorption? Where do we get such models?

            Are you talking about stud wood or sheathing? It seems like the model would have to be specific to one's wall configuration, layers, materials, etc. Or am I missing something obvious?

          2. John Michelotti | | #25

            That is what I think. If a piece of wood is in a high humidity enclosure, we can assume over time the wood will get wet. A lot of things work based on assumptions and algorithms.
            Its like a PTT (Partial Thromboplastin Time ) for blood coagulation. To really know all the qualitative factors requires a much higher level and more technical test. The question is what is actionable about any of it. (an argument certainly for another forum :). ) for managing clotting in a clinical setting it is good enough.
            I think you're spot on. With time and more importantly volume the prices of these things will come down. In the mean time we can use what we know and make progress right now.
            The world makes lots of assumptions about how things work to figure out what to do.

      2. Jon R | | #28

        I suppose that a wood moisture meter could be as simple as two screws in the wood, a series resistor and a high impedance voltmeter to determine ohms of the wood.

        1. Karl B (Zone 6A) | | #31

          You're on to something, Jon. The US Forest Service describes exactly the same methodology you describe, in this publication:

          Megaohm to gigaohm resistance measurement might require something more than a simple voltage divider (for use with an embedded ADC and 5V voltage reference, anyway), but it shouldn't be too hard. And really, a relative measurement of moisture content ought to be enough for monitoring: establish a baseline, and look for significant excursions from that baseline. Not sure what effect contact with borate-treated cellulose might have on the resistance over time.

          1. Expert Member
            Peter Engle | | #34

            I've used a similar setup myself. Two brass brads with wires soldered on. I embed the brads in the wood in question during construction and bring the wire leads to a convenient location for later measurement. I verify that everything is working with direct moisture meter measurements at installation. The wires have low enough impedance that they have little/no effect on the measured moisture content. Any time after construction, you just hook up the moisture meter to the wire leads and get a direct measurement of MC from the embedded sensors. Cheap and easy.

          2. Jon R | | #35

            > Two brass brads with wires soldered on

            I like it. Find out sooner rather than later that your roof sheathing is rotting and address it before it becomes really expensive.

  4. Jon R | | #4

    > CO2 is also a good analogue for other pollutants

    I suggest that it's a poor analogue for any pollutant not emitted directly by humans. Imagine coming home to a house where the ventilation has been off since CO2 was low in the unoccupied house - VOCs are likely to be high. Or compare CO2 level to PM2.5 while cooking (little correlation - you need to manually crank up the ventilation). The only useful correlation is "if CO2 is high, then ventilation is too low". But if CO2 is OK - don't conclude anything and continue to run at least the ASHRAE recommended CFM (much more when cooking).

  5. BlueSolar | | #6

    I'd love to be able to measure permeability more or less directly. I don't know if these moisture sensors are the best approach or if there are more complete sensors focused on the task.

    I don't believe most of the claims made about perm ratings of different materials, mostly because the experimental data is almost never available. I especially don't believe the claims about drywall painted with latex paint. There are far too many types and thicknesses of drywall, and far too many paints, for singular claims about "the" perm rating of the combination to have any validity. It's crazy how crude and unscientific people are about wall permeability characteristics – I feel like we're just spitballing it and hoping for the best.

    The focus on wood moisture sensors in the article reminds me that I don't understand why this site promotes wood framed homes so much. Wood is the worst building material we have, and should be our last resort. It's weak, it rots, it burns, it absorbs moisture, and it has inconsistent properties and dimensions from one batch to the next, or even from one stud to the next. We only use it in America because it was cheap and abundant. But wood these days is of interior quality, cut from young, small softwood trees from renewable forests and plantations. The industry steadily spirals toward the bare minimum quality wood or woodlike material, e.g. OSB. Any other building material or system is better than wood, so I don't know why the site keeps pushing wood. We've had the ability to build houses that don't burn down for centuries at this point, so it's weird that we keep building flammable buildings at all.

    1. Expert Member
      Malcolm Taylor | | #7


      - Sensors measure moisture not permeance, because it's moisture accumulation that causes problems in building assemblies. It's what you would like to know is happening In your walls or roof.

      - Can you give an example of a material where relying on the published perm ratings causes unforeseen difficulties indicating it is wrong?

      You have made it clear in any number of threads that you don't like wood frame construction. Fair enough, choose another system. But I'd pause and consider whether there might be a reason so many people in the building industry with a lot more experience than you, both practical and theoretical, think it is a good and appropriate method of construction. Use your professional training. The chances you have happened upon a new field of interest and somehow acquired special understanding the rest of us have missed is as you put it elsewhere, vanishingly thin.

      1. Expert Member
        Michael Maines | | #10

        Bluesolar, do you ever purchase the ASTM test method descriptions? I have a collection and find them helpful for understanding what is entailed. I have not purchased this one yet but it's on my list:

        In engineering school we did a lot of material testing so I have a decent understanding of how rigorous the tests are, and are not. In general, order-of-magnitude permeance testing should be good enough, if the assemblies are reasonably resilient. If an assembly is borderline and could get worse with typical future changes, I try to use a different assembly.

        Regarding wood, when you spend a few weeks testing stress, strain and other properties of lumber, you can see the typical variation. When you spend a career working on old houses, as I have, you see that wood is actually a pretty resilient material, as long as basic water and vapor management is considered, and when it fails, it's often not too difficult to repair. With the rise in synthetic surfaces it can be hard to see when wood framing is failing, and that's one of many reasons I prefer natural wood cladding and plastic-free interiors.

        1. BlueSolar | | #38

          Hi Michael, no I haven't purchased any ASTM test method descriptions. All that stuff should be open source – I don't like how they charge exorbitant prices for these things. They want $58 just for a PDF...

          A core issue here is variance in the wall assembly. People use different sheathing materials, and different thicknesses. They use different interior finish materials, and different thicknesses there as an well.

          For example, for drywall, what if I use USG Glass-Mat Mold Tough VHI Firecode X 5/8 inch panels?

          That would be my preference (VHI means Very High Impact). But what if I choose Glass-Mat Mold Tough AR Firecode X 5/8 inch? That's merely Abuse Resistant, not VHI. Different composition?

          Either one is probably thicker than the standards anticipate, since ½ inch is more common. And the standards are unlikely to account for glass mats. Do they account for the kind of material used in fire rated Type X drywall? Or mold resistant panels? Moreover, a lot of the specific materials and compositions used for mold resistance and abuse resistance will be proprietary and vary by manufacturer.

          That's just drywall. People might use plaster or magnesium oxide. And for exterior sheathing, they might use various thicknesses of OSB. Or they might use plywood, again in varying thicknesses. And I wouldn't assume that all OSB and all plywood are the same, across manufacturers. I have no idea, but who knows what they're using for glue and resins, and if their materials vary on the properties of interest when assessing wall moisture management and permeability. I would need to see the data from valid testing of these variations to have any confidence in a wooden wall.

          I'm not sure how the standards account for different insulation materials, which are wildly different. Some of them might not even have existed when the standards were written. They were wrong in their models of EPS moisture retention, which just illustrates how pre-scientific construction is. They don't really know much, not at the level of rigor that a lot of scientists would want to see. Their ways of knowing seem largely based on inferences from rumors, and personal experience. The time scales of buildings mean that there are severe limits on any human's ability to consolidate a lot of experiential knowledge of building performance, methods, etc., since most human careers last 30 years at most.

          1. Expert Member
            Michael Maines | | #40

            I agree that ASTM documents should be open source, but they are not, so on several occasions I have coughed up the $50-60 so I can understand what and how they are actually testing. I always learn more than enough to justify the cost, and don't have to guess or make assumptions.

            You have some good points about variations in materials, but it also sounds like you may not spend much time researching what manufacturers provide for information. I guess you don't have to believe what they publish but I often call manufacturers' technical departments and talk with their engineers, who can explain what's behind the numbers the marketing departments show.

  6. BlueSolar | | #8

    Getting the perm ratings wrong will matter if moisture management matters. The hallmark case is when moisture gets inside a wall and we want it to dry out in a given direction (to the interior or exterior). If we think an assembly or material is more permeable than it is then moisture will linger in walls beyond our models, potentially causing rot and mold.

    Another issue is the longevity of these purported perm levels. By its nature, it's hard to test the 20 or 40 year performance of retarders, barriers, etc. They can only model it, or perform some kind of accelerated test procedure like they do with physical corrosion tests of metals. Both can be wrong, kind of like how they recently discovered that EPS foam absorbs a lot less water than they believed. We're relying on the multidecadal performance of thin, perforated plastic membranes as retarders, for instance, which is strange. Especially in that we're relying on complementary tapes to have the same characteristics, and we have highly variable sheathing and other materials in the assembly. Probabilistically, I think it's smarter to just eliminate the possibility of moisture penetration with a much more robust barrier, much thicker than the thin fabric sheets we use now, than to try to manage subtle retardation and subtle directional drying with super-thin fabrics that may not retain their characteristics for 30 years.

  7. Jon R | | #9

    Note that a change in air-tightness is likely to have 100x more moisture effect than a change in permeability. So +1 on more proof that air-tightness is maintained over many decades. As an indirect indicator, wood moisture sensors can help with this.

    1. Expert Member
      Malcolm Taylor | | #11


      I agree with both you and BlueSolar that it's not a good idea to specify a marginal assembly, or to rely on a certain level of air-tightness being maintained over time.- although it does seem like position that is fairly unrelated to the complaint that moisture meters don't measure permeance.

      I'm also unconvinced the anything useful comes from a generalized attack on wood frame construction. Putting aside whether there are better alternatives, that's the way single family residential construction is done right now. The whole industry is geared to supporting it. If someone is looking at build a house in the near future there isn't much point hoping for some transformational change happening soon that will give them alternatives they may find more palatable.

  8. Jon R | | #13

    > order-of-magnitude permeance testing should be good enough

    IMO, the order-of-magnitude range in the use of Class I/II/III vapor retarder classifications is already too high - it often makes sense to talk about "low side of Class III". I wouldn't add to it with another order-of-magnitude variation in measurement, yielding a 100x range.

    1. BlueSolar | | #14

      Hi Jon, who are you replying to? Was there a deleted comment? Where are you getting these "order of magnitude" ranges? Is that the actual situation with permeance testing and vapor retarder classifications?

      1. Expert Member
        Michael Maines | | #15

        Jon was replying to my comment, #10, " In general, order-of-magnitude permeance testing should be good enough, if the assemblies are reasonably resilient. If an assembly is borderline and could get worse with typical future changes, I try to use a different assembly." While I agree that for borderline assemblies we often talk about the differences within each class, I stand by my comment--I prefer assemblies where the difference between 2 perms and 8 perms does not determine whether the assembly will fail.

  9. BlueSolar | | #16

    Hi Malcolm, my reply to your earlier question got nested wrong. It's comment #8.

    To this comment, I'll register my confusion. You think it's particularly difficult to build a robust house? Because wood frame is "the way it's done"? I think this might be an instance of the Northern bias or skew of GBA.

    Masonry homes are quite common in the Southwest. (A masonry home can in theory have wood framing, like the popular brick houses in Texas, but I mean pure masonry without wood framing.) Most new construction by production builders are wood frame with stucco exteriors, but not all. There are default-ICF communities in Phoenix built by production builders. As for custom homes, I don't know what method is #1, but the percentage of wood frame is much lower than it is with production builders. When my uncle built his house in Vail, AZ, it was concrete block. We built it ourselves, a typical Mexican extended family build effort, contracting out the electric.

    In Mexican and Mexican-American culture there's a long tradition of masonry homes, most notably adobe, but also rammed earth and even variations of mud-based bricks, blocks, and walls in poorer communities. In this era, concrete block and slump block are common, both with Mexicans and white people out here.

    And people build with steel frame all the time. Even though it's much less common than wood frame, in a nation of 330 million people even uncommon methods have sizeable markets – markets as large as the prevailing method in a much smaller country.

    And in green circles, ICF and SIPs seem incredibly common. I didn't realize how common until recently. I don't know why we would build with wood if we could just use ICF or SIP. ICF is much stronger, and I was surprised to hear that SIPs are also stronger than wood frame.

    Extant construction methods are junk, and rely on too many things going right. Wood frame houses can be good, but it requires all sorts of detailing and mitigations against the inherent unsuitability of wood as a material from which to build homes. There are so many things that have to be done with respect to air sealing, thermal bridging, vapor barriers, vapor retarders, vapor open this and that, drying to the exterior, drying to interior, special fasteners for fortified homes, hurricane clips, screws, all sorts of moisture management considerations that even affect one's choice of insulation materials, precise (and unscientific, unreliable) tuning of air, vapor, and moisture properties. You need to use all sorts of special tapes and goop to air seal or vapor retard or whatever. Tapes! We're building houses that rely on tape! And goop. That's nuts, just extremely strange. We don't normally build stuff that relies and tapes, goops, and thin sheets of some undocumented synthetic material to do important things.

    1. Trevor Lambert | | #17

      Neither ICF nor SIPs are green. ICF in particular is the opposite of green, whatever colour that might be (purple?). The fact that they are common in "green circles" is a sad testament to how little most people know about what the term means.

      1. BlueSolar | | #37

        Trevor, what's wrong with ICFs?

        1. Expert Member
          Michael Maines | | #39

          BlueSolar, we have discussed what's wrong with ICFs here many times. Why are you pretending you don't know why?

    2. Expert Member
      Malcolm Taylor | | #21


      I'd have a lot more sympathy for your position if you said instead: "I think extant construction methods are junk." Presenting your views on building as bald assertions as opposed to your opinion loses me.

    3. John Michelotti | | #29

      Concrete blocks (aka concrete masonry units) are your parents brick. We have thousands of building built here in Chicago using CMU's that are rife with water and moisture problems, some even structural. The older masonry buildings had triple wyeth walls that allowed for drying the wall assembly. The CMU's don't really work that way. They are built with modern trusses, that is they get wet they will rot and cause problems.
      EVen the older "well built" buildings when repaired or maintained with modern mortars start to have problems as they aren't really appropriate for the brick cladding that is used.
      Moisture may not be a problem in the southwest, but here in the midwest we need to deal with it.

      1. BlueSolar | | #36

        John are you talking about basement walls? We not only don't have much moisture most months, we don't have basements. Or crawl spaces. Well, "crawl space" in Arizona refers to a shallow attic space that isn't tall enough to stand in. (We don't have attics either, not the standing kind.)

        Slump block is a lot more common than concrete block. Also brick.

        In some communities, you're actually not allowed to build wood framed houses. There's a high-end community in Scottsdale that comes to mind, where by bylaw or HOA or something you must use masonry or ICF for the first floor. Only on the second floor are you allowed to use wood. Matt Risinger featured one of the builds there recently in a video on ICF construction.

  10. John Michelotti | | #18

    Thank for this article. I am a total amateur, and when I ask about this people look at me like I have four heads. I would also add that the following
    The sensors should be placed where we might predict moisture, not only, but in those locations fo sure.
    The sensors should be accessible through ports, in either door/window frames or in whatever places they are placed.
    To those who have concerns about the internet, or cost or whatever. There are literally 100s of sensors that can be purchased for Raspberry Pi's and Arduinos or other single board systems (SBC). Once you understand the components it is like anything else the combinations are endless. This analogous to hand and power tools. The essentially come with no instructions. It is your understanding of how you use them in context that makes them valuable.
    Last, if we hook these up to the internet, imagine how we might monitor a geographic area based on the data that is shared. During a flood, heatwave, cold wave. In the latter, this data would show where pipes are freezing in real time.
    During COVID times there is an IOS device mfg called Kinsa. Since you load the data to the cloud the are able to assemble maps to where fevers are occurring. This gets the contact tracers on track right away. I know, I know no one has to explain to me the darker side of this.
    We see time and again with these modern materials that if done poorly the often times turn to mush. Imagine if in a development of 100 homes we add 10 with sensors. IF they perform well, then we wasted a few bucks, but we know a good job was done.
    If they perform poorly, remediation can take place much sooner.
    I agree that the assemblies being built today (and masonry must be handled properly too) are of much higher risk, though had we loaded our old victorians with insulation and reduced energy I imagine we'd not be so nostalgic over the old material.
    I think these affordable sensors off enormous both macro and micro possibilities.

    Thanks everyone for this robust discussion.

  11. Expert Member
    Michael Maines | | #27

    In response to BlueSolar, comment 26: I recommend reading books and articles by the many professionals who spend their lives researching this stuff. We are reading this book for the BS + Beer Show Book Club and it's both dense with information and engaging to read:

  12. Nate Reik | | #32

    Found a solution to what I wanted: datalogging of wood moisture content without internet and a subscription service.

    $100 on Amazon, seems reasonable enough that I'll probably give one a shot.

    1. John Michelotti | | #33

      Good find, and good luck...

  13. BlueSolar | | #41

    Anyone know where we are in terms of sensor longevity at this point? I'd like temperature and moisture/humidity sensors that are good for 20 or 30 years. Is that realistic? The article mentions battery-powered sensors lasting 5+ years, but I think that's supposed to be the battery's constraint.

    Why batteries? My preference would be to just plug them in. Plugged-in power is a lot more reliable long-term. I wouldn't think we'd even need battery backup – if a humidity or temperature sensor is out for an hour because of a power outage, who cares? It doesn't really matter. It's not like a smoke alarm, doesn't need to function at all times.

    Batteries can last 30 years though, e.g. the lithium-thionyl-chloride (LiSOCl2) batteries they started using on gas and water meters in the 1980s. I just don't know why we need to create a dependency on batteries for devices that are permanently placed in our homes. It seems like we'd only need batteries for sensors in remote locations, like on a tree in a forest, or the "alternative data" spying sensors that a hedge fund might use to monitor the number of ore trucks leaving a copper mine or something.

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