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Why does the wood moisture equivalent value mirror the material temperature?

ethant | Posted in Energy Efficiency and Durability on

I’m tracking WME and Temp using Omnisense sensors on our newly constructed vented cathedral roof (16″ dense pack cellulose.)

Why is WME (wood moisture equiv.) mirroring material temp as shown in attached graph?

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Replies

  1. Expert Member
    Peter Yost | | #1

    HI Ethan -

    Great question! I have never used OmniSense equipment (use quite a bit of ONSET and Delmhorst though), so afraid we are going to have to get others to weigh in.

    There is of course temperature correction needed for moisture content (see this table from Delmhorst: https://www.delmhorst.com/temperature-correction-table).

    I am assuming you have checke in with OmniSense on this?

    I will ping some folks I know to see if they can weigh in.

    Peter

  2. user-6699028 | | #2

    Hi Ethan.

    I can speculate, but the speculation will be better if we know *exactly* where the sensor is placed and its local environment.

    I assume inside the house and inside the occupied and finished space on the top floor, mounted on the occupied surface of a cathedral ceiling down near the eave of the roof? Or mounted inside the ceiling?

    North side of the ceiling?.. or other compass point?

    In Vermont?.. or out in the California mountains above Sacramento?

    Sensor attached by screws into wood framing inside the densepacked cavity?... or into OSB? or into the gypsum board?

    Fastened above the densepack in the vent channel... or inside the densepack itself?

    How's the voltage on the battery? Above 3 volts.. or below? (I like to have my sensor all above 3 volts. Omnisense says no problem down to 2.8 volts.. but my results -sometimes- get unstable when it goes below 3.0.)

    Any details like these would be very helpful. I know these sound like fussy questions.. but you never know what might matter!

    Lew

  3. ethant | | #3

    Climate Zone 5A ; ~6000HDD (Hudson Valley, NY) ... Sensor mounted by screws to inside of exterior plywood sheathing (embedded in Dense Pack) at east facing ridge of 16" deep cathedral ceiling. 1 1/2" vent space above (2x4 on flat) with another layer of sheathing above. I just would assume that WME and temp would have a bit of an inverse relationship. These are pretty new, just installed, new construction. 3.3 V.

  4. user-6699028 | | #4

    Thanks Ethan. That's very helpful.

    So I think I can offer a theory, based on my own omnisense gear that's been measuring various locations in y own hose for the last 15 years or so.

    Our house was built in 1784. It's located in Portsmouth, NH. The sensor in question is screwed into the East-facing, wide pine boards of the roof sheathing in our (basically) unconditioned and "semi-vented" attic (one gable window cracked open about ¾" .) The omnisense sensor is modified slightly: I cut off the standoffs and cut away the plastic web that joins the standoffs so that air can circulate more freely around the sensor body. The sensor body is set on the shortened standoffs at about ¼" above the surface of the sheathing board. The sensor is located at 5 ft. above the floor, on the slanting roof. The peak of the roof is about 6 1/2 ft. above the floor. The attic contains the usual debris of 42 years of occupancy... cardboard boxes of books, old picture frames, Christmas decorations, extra sheet metal assembles from abandoned or not-yet-completed geekery, and so forth. The floor of the attic is not air sealed. But it is insulated with about 6" of ancient fiberglass batts and slightly less ancient cellulose infill, all below wide pine floor boards. The attic is unfinished with all wood surfaces (boards, not plywood or OSB) exposed to the attic air.

    With that background, you can look at the attached omnisense screenshots from this past week. The first shot shows the attic air temperature (as measured ¼" from the surface of the wood) along with the attic air dew point reported by that same sensor. What you see is that as the air temperature rises, the dew point rises in lock-step. That is a phenomenon that at fist shocked me years ago when I noticed it...and then made sense. As heat is added to the "system", surfaces all over the attic give up some of their moisture to the air, and therefore the dew point rises. When the air temperature falls, so too does the dew point. It might not behave in quite this way if the attic were better-ventilated.. but I've seen it enough in different attics to be sure that unless venting is *very* open, this pattern will persist.

    Now check out the second screenshot, which shows the attic air temperature, and the dew point, plotted against the wood moisture content. As in your case, the temperature and moisture content rise and fall together.

    So my theory is that:

    a. The omnisense sensor is measuring the air temperature close to the surface.. but it's not measuring the surface temperature.

    b. The actual surface temperature remains cooler (or hotter) than the air, by just a bit.

    c. The rising air dew point means that as slightly more humid air contacts the cooler surface, it releases some of it's moisture to the surface. Not much.. but enough to lower the electrical resistance of that surface by ... just enough.. that the omnisense sensor pins (screws) report a rise in moisture content.

    In your case, that theory (if correct) suggests something similar might be happening, ie:

    a. The dense packed cellulose is a terrific desiccant. Lots of adsorbent surface in there.

    b. Small changes in its temperature could make big rises and falls in the dew point of the air in proximity to the surface on which the omnisense is mounted..

    c. Perhaps that surface is slightly cooler than the air, so it wants to absorb a bit of that free water vapor and therefore report a higher moisture content.

    Would this theory square with what you observe?.. or am I missing something in either my own house, or yours?

    Respectfully,

    Lew

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